Re: [PATCH v3 01/10] mm: add Kernel Electric-Fence infrastructure

[Andrey Konovalov]

On Mon, Sep 21, 2020 at 3:26 PM Marco Elver <elver@xxxxxxxxxx> wrote:
>
> From: Alexander Potapenko <glider@xxxxxxxxxx>
>
> This adds the Kernel Electric-Fence (KFENCE) infrastructure. KFENCE is a
> low-overhead sampling-based memory safety error detector of heap
> use-after-free, invalid-free, and out-of-bounds access errors.
>
> KFENCE is designed to be enabled in production kernels, and has near
> zero performance overhead. Compared to KASAN, KFENCE trades performance
> for precision. The main motivation behind KFENCE's design, is that with
> enough total uptime KFENCE will detect bugs in code paths not typically
> exercised by non-production test workloads. One way to quickly achieve a
> large enough total uptime is when the tool is deployed across a large
> fleet of machines.
>
> KFENCE objects each reside on a dedicated page, at either the left or
> right page boundaries. The pages to the left and right of the object
> page are "guard pages", whose attributes are changed to a protected
> state, and cause page faults on any attempted access to them. Such page
> faults are then intercepted by KFENCE, which handles the fault
> gracefully by reporting a memory access error. To detect out-of-bounds
> writes to memory within the object's page itself, KFENCE also uses
> pattern-based redzones. The following figure illustrates the page
> layout:
>
>   ---+-----------+-----------+-----------+-----------+-----------+---
>      | xxxxxxxxx | O :       | xxxxxxxxx |       : O | xxxxxxxxx |
>      | xxxxxxxxx | B :       | xxxxxxxxx |       : B | xxxxxxxxx |
>      | x GUARD x | J : RED-  | x GUARD x | RED-  : J | x GUARD x |
>      | xxxxxxxxx | E :  ZONE | xxxxxxxxx |  ZONE : E | xxxxxxxxx |
>      | xxxxxxxxx | C :       | xxxxxxxxx |       : C | xxxxxxxxx |
>      | xxxxxxxxx | T :       | xxxxxxxxx |       : T | xxxxxxxxx |
>   ---+-----------+-----------+-----------+-----------+-----------+---
>
> Guarded allocations are set up based on a sample interval (can be set
> via kfence.sample_interval). After expiration of the sample interval, a
> guarded allocation from the KFENCE object pool is returned to the main
> allocator (SLAB or SLUB). At this point, the timer is reset, and the
> next allocation is set up after the expiration of the interval.
>
> To enable/disable a KFENCE allocation through the main allocator's
> fast-path without overhead, KFENCE relies on static branches via the
> static keys infrastructure. The static branch is toggled to redirect the
> allocation to KFENCE. To date, we have verified by running synthetic
> benchmarks (sysbench I/O workloads) that a kernel compiled with KFENCE
> is performance-neutral compared to the non-KFENCE baseline.
>
> For more details, see Documentation/dev-tools/kfence.rst (added later in
> the series).
>
> Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Co-developed-by: Marco Elver <elver@xxxxxxxxxx>
> Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
> Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx>
> ---
> v3:
> * Reports by SeongJae Park:
>   * Remove reference to Documentation/dev-tools/kfence.rst.
>   * Remove redundant braces.
>   * Use CONFIG_KFENCE_NUM_OBJECTS instead of ARRAY_SIZE(...).
>   * Align some comments.
> * Add figure from Documentation/dev-tools/kfence.rst added later in
>   series to patch description.
>
> v2:
> * Add missing __printf attribute to seq_con_printf, and fix new warning.
>   [reported by kernel test robot <lkp@xxxxxxxxx>]
> * Fix up some comments [reported by Jonathan Cameron].
> * Remove 2 cases of redundant stack variable initialization
>   [reported by Jonathan Cameron].
> * Fix printf format [reported by kernel test robot <lkp@xxxxxxxxx>].
> * Print (in kfence-#nn) after address, to more clearly establish link
>   between first and second stacktrace [reported by Andrey Konovalov].
> * Make choice between KASAN and KFENCE clearer in Kconfig help text
>   [suggested by Dave Hansen].
> * Document CONFIG_KFENCE_SAMPLE_INTERVAL=0.
> * Shorten memory corruption report line length.
> * Make /sys/module/kfence/parameters/sample_interval root-writable for
>   all builds (to enable debugging, automatic dynamic tweaking).
> * Reports by Dmitry Vyukov:
>   * Do not store negative size for right-located objects
>   * Only cache-align addresses of right-located objects.
>   * Run toggle_allocation_gate() after KFENCE is enabled.
>   * Add empty line between allocation and free stacks.
>   * Add comment about SLAB_TYPESAFE_BY_RCU.
>   * Also skip internals for allocation/free stacks.
>   * s/KFENCE_FAULT_INJECTION/KFENCE_STRESS_TEST_FAULTS/ as FAULT_INJECTION
>     is already overloaded in different contexts.
>   * Parenthesis for macro variable.
>   * Lower max of KFENCE_NUM_OBJECTS config variable.
> ---
>  MAINTAINERS            |  11 +
>  include/linux/kfence.h | 174 ++++++++++
>  init/main.c            |   2 +
>  lib/Kconfig.debug      |   1 +
>  lib/Kconfig.kfence     |  63 ++++
>  mm/Makefile            |   1 +
>  mm/kfence/Makefile     |   3 +
>  mm/kfence/core.c       | 733 +++++++++++++++++++++++++++++++++++++++++
>  mm/kfence/kfence.h     | 102 ++++++
>  mm/kfence/report.c     | 219 ++++++++++++
>  10 files changed, 1309 insertions(+)
>  create mode 100644 include/linux/kfence.h
>  create mode 100644 lib/Kconfig.kfence
>  create mode 100644 mm/kfence/Makefile
>  create mode 100644 mm/kfence/core.c
>  create mode 100644 mm/kfence/kfence.h
>  create mode 100644 mm/kfence/report.c
>
> diff --git a/MAINTAINERS b/MAINTAINERS
> index b5cfab015bd6..863899ed9a29 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -9673,6 +9673,17 @@ F:       include/linux/keyctl.h
>  F:     include/uapi/linux/keyctl.h
>  F:     security/keys/
>
> +KFENCE
> +M:     Alexander Potapenko <glider@xxxxxxxxxx>
> +M:     Marco Elver <elver@xxxxxxxxxx>
> +R:     Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> +L:     kasan-dev@xxxxxxxxxxxxxxxx
> +S:     Maintained
> +F:     Documentation/dev-tools/kfence.rst
> +F:     include/linux/kfence.h
> +F:     lib/Kconfig.kfence
> +F:     mm/kfence/
> +
>  KFIFO
>  M:     Stefani Seibold <stefani@xxxxxxxxxxx>
>  S:     Maintained
> diff --git a/include/linux/kfence.h b/include/linux/kfence.h
> new file mode 100644
> index 000000000000..8128ba7b5e90
> --- /dev/null
> +++ b/include/linux/kfence.h
> @@ -0,0 +1,174 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef _LINUX_KFENCE_H
> +#define _LINUX_KFENCE_H
> +
> +#include <linux/mm.h>
> +#include <linux/percpu.h>
> +#include <linux/static_key.h>
> +#include <linux/types.h>
> +
> +#ifdef CONFIG_KFENCE
> +
> +/*
> + * We allocate an even number of pages, as it simplifies calculations to map
> + * address to metadata indices; effectively, the very first page serves as an
> + * extended guard page, but otherwise has no special purpose.
> + */
> +#define KFENCE_POOL_SIZE ((CONFIG_KFENCE_NUM_OBJECTS + 1) * 2 * PAGE_SIZE)
> +#ifdef CONFIG_HAVE_ARCH_KFENCE_STATIC_POOL
> +extern char __kfence_pool[KFENCE_POOL_SIZE];
> +#else
> +extern char *__kfence_pool;
> +#endif
> +
> +extern struct static_key_false kfence_allocation_key;
> +
> +/**
> + * is_kfence_address() - check if an address belongs to KFENCE pool
> + * @addr: address to check
> + *
> + * Return: true or false depending on whether the address is within the KFENCE
> + * object range.
> + *
> + * KFENCE objects live in a separate page range and are not to be intermixed
> + * with regular heap objects (e.g. KFENCE objects must never be added to the
> + * allocator freelists). Failing to do so may and will result in heap
> + * corruptions, therefore is_kfence_address() must be used to check whether
> + * an object requires specific handling.
> + */
> +static __always_inline bool is_kfence_address(const void *addr)
> +{
> +       return unlikely((char *)addr >= __kfence_pool &&
> +                       (char *)addr < __kfence_pool + KFENCE_POOL_SIZE);
> +}
> +
> +/**
> + * kfence_init() - perform KFENCE initialization at boot time
> + */
> +void kfence_init(void);
> +
> +/**
> + * kfence_shutdown_cache() - handle shutdown_cache() for KFENCE objects
> + * @s: cache being shut down
> + *
> + * Return: true on success, false if any leftover objects persist.
> + *
> + * Before shutting down a cache, one must ensure there are no remaining objects
> + * allocated from it. KFENCE objects are not referenced from the cache, so
> + * kfence_shutdown_cache() takes care of them.
> + */
> +bool __must_check kfence_shutdown_cache(struct kmem_cache *s);
> +
> +/*
> + * Allocate a KFENCE object. Allocators must not call this function directly,
> + * use kfence_alloc() instead.
> + */
> +void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags);
> +
> +/**
> + * kfence_alloc() - allocate a KFENCE object with a low probability
> + * @s:     struct kmem_cache with object requirements
> + * @size:  exact size of the object to allocate (can be less than @s->size
> + *         e.g. for kmalloc caches)
> + * @flags: GFP flags
> + *
> + * Return:
> + * * NULL     - must proceed with allocating as usual,
> + * * non-NULL - pointer to a KFENCE object.
> + *
> + * kfence_alloc() should be inserted into the heap allocation fast path,
> + * allowing it to transparently return KFENCE-allocated objects with a low
> + * probability using a static branch (the probability is controlled by the
> + * kfence.sample_interval boot parameter).
> + */
> +static __always_inline void *kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
> +{
> +       return static_branch_unlikely(&kfence_allocation_key) ? __kfence_alloc(s, size, flags) :
> +                                                                     NULL;
> +}
> +
> +/**
> + * kfence_ksize() - get actual amount of memory allocated for a KFENCE object
> + * @addr: pointer to a heap object
> + *
> + * Return:
> + * * 0     - not a KFENCE object, must call __ksize() instead,
> + * * non-0 - this many bytes can be accessed without causing a memory error.
> + *
> + * kfence_ksize() returns the number of bytes requested for a KFENCE object at
> + * allocation time. This number may be less than the object size of the
> + * corresponding struct kmem_cache.
> + */
> +size_t kfence_ksize(const void *addr);
> +
> +/**
> + * kfence_object_start() - find the beginning of a KFENCE object
> + * @addr - address within a KFENCE-allocated object
> + *
> + * Return: address of the beginning of the object.
> + *
> + * SL[AU]B-allocated objects are laid out within a page one by one, so it is
> + * easy to calculate the beginning of an object given a pointer inside it and
> + * the object size. The same is not true for KFENCE, which places a single
> + * object at either end of the page. This helper function is used to find the
> + * beginning of a KFENCE-allocated object.
> + */
> +void *kfence_object_start(const void *addr);
> +
> +/*
> + * Release a KFENCE-allocated object to KFENCE pool. Allocators must not call
> + * this function directly, use kfence_free() instead.
> + */
> +void __kfence_free(void *addr);
> +
> +/**
> + * kfence_free() - try to release an arbitrary heap object to KFENCE pool
> + * @addr: object to be freed
> + *
> + * Return:
> + * * false - object doesn't belong to KFENCE pool and was ignored,
> + * * true  - object was released to KFENCE pool.
> + *
> + * Release a KFENCE object and mark it as freed. May be called on any object,
> + * even non-KFENCE objects, to simplify integration of the hooks into the
> + * allocator's free codepath. The allocator must check the return value to
> + * determine if it was a KFENCE object or not.
> + */
> +static __always_inline __must_check bool kfence_free(void *addr)
> +{
> +       if (!is_kfence_address(addr))
> +               return false;
> +       __kfence_free(addr);
> +       return true;
> +}
> +
> +/**
> + * kfence_handle_page_fault() - perform page fault handling for KFENCE pages
> + * @addr: faulting address
> + *
> + * Return:
> + * * false - address outside KFENCE pool,
> + * * true  - page fault handled by KFENCE, no additional handling required.
> + *
> + * A page fault inside KFENCE pool indicates a memory error, such as an
> + * out-of-bounds access, a use-after-free or an invalid memory access. In these
> + * cases KFENCE prints an error message and marks the offending page as
> + * present, so that the kernel can proceed.
> + */
> +bool __must_check kfence_handle_page_fault(unsigned long addr);
> +
> +#else /* CONFIG_KFENCE */
> +
> +static inline bool is_kfence_address(const void *addr) { return false; }
> +static inline void kfence_init(void) { }
> +static inline bool __must_check kfence_shutdown_cache(struct kmem_cache *s) { return true; }
> +static inline void *kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags) { return NULL; }
> +static inline size_t kfence_ksize(const void *addr) { return 0; }
> +static inline void *kfence_object_start(const void *addr) { return NULL; }
> +static inline bool __must_check kfence_free(void *addr) { return false; }
> +static inline bool __must_check kfence_handle_page_fault(unsigned long addr) { return false; }
> +
> +#endif
> +
> +#endif /* _LINUX_KFENCE_H */
> diff --git a/init/main.c b/init/main.c
> index ae78fb68d231..ec7de9dc1ed8 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -39,6 +39,7 @@
>  #include <linux/security.h>
>  #include <linux/smp.h>
>  #include <linux/profile.h>
> +#include <linux/kfence.h>
>  #include <linux/rcupdate.h>
>  #include <linux/moduleparam.h>
>  #include <linux/kallsyms.h>
> @@ -942,6 +943,7 @@ asmlinkage __visible void __init __no_sanitize_address start_kernel(void)
>         hrtimers_init();
>         softirq_init();
>         timekeeping_init();
> +       kfence_init();
>
>         /*
>          * For best initial stack canary entropy, prepare it after:
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index e068c3c7189a..d09c6a306532 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -880,6 +880,7 @@ config DEBUG_STACKOVERFLOW
>           If in doubt, say "N".
>
>  source "lib/Kconfig.kasan"
> +source "lib/Kconfig.kfence"
>
>  endmenu # "Memory Debugging"
>
> diff --git a/lib/Kconfig.kfence b/lib/Kconfig.kfence
> new file mode 100644
> index 000000000000..4c2ea1c722de
> --- /dev/null
> +++ b/lib/Kconfig.kfence
> @@ -0,0 +1,63 @@
> +# SPDX-License-Identifier: GPL-2.0-only
> +
> +config HAVE_ARCH_KFENCE
> +       bool
> +
> +config HAVE_ARCH_KFENCE_STATIC_POOL
> +       bool
> +       help
> +         If the architecture supports using the static pool.
> +
> +menuconfig KFENCE
> +       bool "KFENCE: low-overhead sampling-based memory safety error detector"
> +       depends on HAVE_ARCH_KFENCE && !KASAN && (SLAB || SLUB)
> +       depends on JUMP_LABEL # To ensure performance, require jump labels
> +       select STACKTRACE
> +       help
> +         KFENCE is low-overhead sampling-based detector for heap out-of-bounds
> +         access, use-after-free, and invalid-free errors. KFENCE is designed
> +         to have negligible cost to permit enabling it in production
> +         environments.
> +
> +         Note that, KFENCE is not a substitute for explicit testing with tools
> +         such as KASAN. KFENCE can detect a subset of bugs that KASAN can
> +         detect, albeit at very different performance profiles. If you can
> +         afford to use KASAN, continue using KASAN, for example in test
> +         environments. If your kernel targets production use, and cannot
> +         enable KASAN due to its cost, consider using KFENCE.
> +
> +if KFENCE
> +
> +config KFENCE_SAMPLE_INTERVAL
> +       int "Default sample interval in milliseconds"
> +       default 100
> +       help
> +         The KFENCE sample interval determines the frequency with which heap
> +         allocations will be guarded by KFENCE. May be overridden via boot
> +         parameter "kfence.sample_interval".
> +
> +         Set this to 0 to disable KFENCE by default, in which case only
> +         setting "kfence.sample_interval" to a non-zero value enables KFENCE.
> +
> +config KFENCE_NUM_OBJECTS
> +       int "Number of guarded objects available"
> +       default 255
> +       range 1 16383
> +       help
> +         The number of guarded objects available. For each KFENCE object, 2
> +         pages are required; with one containing the object and two adjacent
> +         ones used as guard pages.
> +
> +config KFENCE_STRESS_TEST_FAULTS
> +       int "Stress testing of fault handling and error reporting"
> +       default 0
> +       depends on EXPERT
> +       help
> +         The inverse probability with which to randomly protect KFENCE object
> +         pages, resulting in spurious use-after-frees. The main purpose of
> +         this option is to stress test KFENCE with concurrent error reports
> +         and allocations/frees. A value of 0 disables stress testing logic.
> +
> +         The option is only to test KFENCE; set to 0 if you are unsure.
> +
> +endif # KFENCE
> diff --git a/mm/Makefile b/mm/Makefile
> index d5649f1c12c0..afdf1ae0900b 100644
> --- a/mm/Makefile
> +++ b/mm/Makefile
> @@ -81,6 +81,7 @@ obj-$(CONFIG_PAGE_POISONING) += page_poison.o
>  obj-$(CONFIG_SLAB) += slab.o
>  obj-$(CONFIG_SLUB) += slub.o
>  obj-$(CONFIG_KASAN)    += kasan/
> +obj-$(CONFIG_KFENCE) += kfence/
>  obj-$(CONFIG_FAILSLAB) += failslab.o
>  obj-$(CONFIG_MEMORY_HOTPLUG) += memory_hotplug.o
>  obj-$(CONFIG_MEMTEST)          += memtest.o
> diff --git a/mm/kfence/Makefile b/mm/kfence/Makefile
> new file mode 100644
> index 000000000000..d991e9a349f0
> --- /dev/null
> +++ b/mm/kfence/Makefile
> @@ -0,0 +1,3 @@
> +# SPDX-License-Identifier: GPL-2.0
> +
> +obj-$(CONFIG_KFENCE) := core.o report.o
> diff --git a/mm/kfence/core.c b/mm/kfence/core.c
> new file mode 100644
> index 000000000000..4af407837830
> --- /dev/null
> +++ b/mm/kfence/core.c
> @@ -0,0 +1,733 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#define pr_fmt(fmt) "kfence: " fmt
> +
> +#include <linux/atomic.h>
> +#include <linux/bug.h>
> +#include <linux/debugfs.h>
> +#include <linux/kcsan-checks.h>
> +#include <linux/kfence.h>
> +#include <linux/list.h>
> +#include <linux/lockdep.h>
> +#include <linux/moduleparam.h>
> +#include <linux/random.h>
> +#include <linux/rcupdate.h>
> +#include <linux/seq_file.h>
> +#include <linux/slab.h>
> +#include <linux/spinlock.h>
> +#include <linux/string.h>
> +
> +#include <asm/kfence.h>
> +
> +#include "kfence.h"
> +
> +/* Disables KFENCE on the first warning assuming an irrecoverable error. */
> +#define KFENCE_WARN_ON(cond)                                                   \
> +       ({                                                                     \
> +               const bool __cond = WARN_ON(cond);                             \
> +               if (unlikely(__cond))                                          \
> +                       WRITE_ONCE(kfence_enabled, false);                     \
> +               __cond;                                                        \
> +       })
> +
> +#ifndef CONFIG_KFENCE_STRESS_TEST_FAULTS /* Only defined with CONFIG_EXPERT. */
> +#define CONFIG_KFENCE_STRESS_TEST_FAULTS 0
> +#endif
> +
> +/* === Data ================================================================= */
> +
> +static unsigned long kfence_sample_interval __read_mostly = CONFIG_KFENCE_SAMPLE_INTERVAL;
> +
> +#ifdef MODULE_PARAM_PREFIX
> +#undef MODULE_PARAM_PREFIX
> +#endif
> +#define MODULE_PARAM_PREFIX "kfence."
> +module_param_named(sample_interval, kfence_sample_interval, ulong, 0600);
> +
> +static bool kfence_enabled __read_mostly;
> +
> +/*
> + * The pool of pages used for guard pages and objects. If supported, allocated
> + * statically, so that is_kfence_address() avoids a pointer load, and simply
> + * compares against a constant address. Assume that if KFENCE is compiled into
> + * the kernel, it is usually enabled, and the space is to be allocated one way
> + * or another.
> + */
> +#ifdef CONFIG_HAVE_ARCH_KFENCE_STATIC_POOL
> +char __kfence_pool[KFENCE_POOL_SIZE] __aligned(KFENCE_POOL_ALIGNMENT);
> +#else
> +char *__kfence_pool __read_mostly;
> +#endif
> +EXPORT_SYMBOL(__kfence_pool); /* Export for test modules. */
> +
> +/*
> + * Per-object metadata, with one-to-one mapping of object metadata to
> + * backing pages (in __kfence_pool).
> + */
> +static_assert(CONFIG_KFENCE_NUM_OBJECTS > 0);
> +struct kfence_metadata kfence_metadata[CONFIG_KFENCE_NUM_OBJECTS];
> +
> +/* Freelist with available objects. */
> +static struct list_head kfence_freelist = LIST_HEAD_INIT(kfence_freelist);
> +static DEFINE_RAW_SPINLOCK(kfence_freelist_lock); /* Lock protecting freelist. */
> +
> +/* The static key to set up a KFENCE allocation. */
> +DEFINE_STATIC_KEY_FALSE(kfence_allocation_key);
> +
> +/* Gates the allocation, ensuring only one succeeds in a given period. */
> +static atomic_t allocation_gate = ATOMIC_INIT(1);
> +
> +/* Wait queue to wake up allocation-gate timer task. */
> +static DECLARE_WAIT_QUEUE_HEAD(allocation_wait);
> +
> +/* Statistics counters for debugfs. */
> +enum kfence_counter_id {
> +       KFENCE_COUNTER_ALLOCATED,
> +       KFENCE_COUNTER_ALLOCS,
> +       KFENCE_COUNTER_FREES,
> +       KFENCE_COUNTER_BUGS,
> +       KFENCE_COUNTER_COUNT,
> +};
> +static atomic_long_t counters[KFENCE_COUNTER_COUNT];
> +static const char *const counter_names[] = {
> +       [KFENCE_COUNTER_ALLOCATED]      = "currently allocated",
> +       [KFENCE_COUNTER_ALLOCS]         = "total allocations",
> +       [KFENCE_COUNTER_FREES]          = "total frees",
> +       [KFENCE_COUNTER_BUGS]           = "total bugs",
> +};
> +static_assert(ARRAY_SIZE(counter_names) == KFENCE_COUNTER_COUNT);
> +
> +/* === Internals ============================================================ */
> +
> +static bool kfence_protect(unsigned long addr)
> +{
> +       return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), true));
> +}
> +
> +static bool kfence_unprotect(unsigned long addr)
> +{
> +       return !KFENCE_WARN_ON(!kfence_protect_page(ALIGN_DOWN(addr, PAGE_SIZE), false));
> +}
> +
> +static inline struct kfence_metadata *addr_to_metadata(unsigned long addr)
> +{
> +       long index;
> +
> +       /* The checks do not affect performance; only called from slow-paths. */
> +
> +       if (!is_kfence_address((void *)addr))
> +               return NULL;
> +
> +       /*
> +        * May be an invalid index if called with an address at the edge of
> +        * __kfence_pool, in which case we would report an "invalid access"
> +        * error.
> +        */
> +       index = (addr - (unsigned long)__kfence_pool) / (PAGE_SIZE * 2) - 1;

Why do we subtract 1 here? We do have the metadata entry reserved for something?

> +       if (index < 0 || index >= CONFIG_KFENCE_NUM_OBJECTS)
> +               return NULL;
> +
> +       return &kfence_metadata[index];
> +}
> +
> +static inline unsigned long metadata_to_pageaddr(const struct kfence_metadata *meta)
> +{
> +       unsigned long offset = (meta - kfence_metadata + 1) * PAGE_SIZE * 2;
> +       unsigned long pageaddr = (unsigned long)&__kfence_pool[offset];
> +
> +       /* The checks do not affect performance; only called from slow-paths. */
> +
> +       /* Only call with a pointer into kfence_metadata. */
> +       if (KFENCE_WARN_ON(meta < kfence_metadata ||
> +                          meta >= kfence_metadata + CONFIG_KFENCE_NUM_OBJECTS))
> +               return 0;
> +
> +       /*
> +        * This metadata object only ever maps to 1 page; verify the calculation
> +        * happens and that the stored address was not corrupted.
> +        */
> +       if (KFENCE_WARN_ON(ALIGN_DOWN(meta->addr, PAGE_SIZE) != pageaddr))
> +               return 0;
> +
> +       return pageaddr;
> +}
> +
> +/*
> + * Update the object's metadata state, including updating the alloc/free stacks
> + * depending on the state transition.
> + */
> +static noinline void metadata_update_state(struct kfence_metadata *meta,
> +                                          enum kfence_object_state next)
> +{
> +       unsigned long *entries = next == KFENCE_OBJECT_FREED ? meta->free_stack : meta->alloc_stack;
> +       /*
> +        * Skip over 1 (this) functions; noinline ensures we do not accidentally
> +        * skip over the caller by never inlining.
> +        */
> +       const int nentries = stack_trace_save(entries, KFENCE_STACK_DEPTH, 1);
> +
> +       lockdep_assert_held(&meta->lock);
> +
> +       if (next == KFENCE_OBJECT_FREED)
> +               meta->num_free_stack = nentries;
> +       else
> +               meta->num_alloc_stack = nentries;
> +
> +       /*
> +        * Pairs with READ_ONCE() in
> +        *      kfence_shutdown_cache(),
> +        *      kfence_handle_page_fault().
> +        */
> +       WRITE_ONCE(meta->state, next);
> +}
> +
> +/* Write canary byte to @addr. */
> +static inline bool set_canary_byte(u8 *addr)
> +{
> +       *addr = KFENCE_CANARY_PATTERN(addr);
> +       return true;
> +}
> +
> +/* Check canary byte at @addr. */
> +static inline bool check_canary_byte(u8 *addr)
> +{
> +       if (*addr == KFENCE_CANARY_PATTERN(addr))
> +               return true;
> +
> +       atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
> +       kfence_report_error((unsigned long)addr, addr_to_metadata((unsigned long)addr),
> +                           KFENCE_ERROR_CORRUPTION);
> +       return false;
> +}
> +
> +static inline void for_each_canary(const struct kfence_metadata *meta, bool (*fn)(u8 *))
> +{
> +       unsigned long addr;
> +
> +       lockdep_assert_held(&meta->lock);
> +
> +       for (addr = ALIGN_DOWN(meta->addr, PAGE_SIZE); addr < meta->addr; addr++) {
> +               if (!fn((u8 *)addr))
> +                       break;
> +       }
> +
> +       for (addr = meta->addr + meta->size; addr < PAGE_ALIGN(meta->addr); addr++) {
> +               if (!fn((u8 *)addr))
> +                       break;
> +       }
> +}
> +
> +static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t gfp)
> +{
> +       struct kfence_metadata *meta = NULL;
> +       unsigned long flags;
> +       void *addr;
> +
> +       /* Try to obtain a free object. */
> +       raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
> +       if (!list_empty(&kfence_freelist)) {
> +               meta = list_entry(kfence_freelist.next, struct kfence_metadata, list);
> +               list_del_init(&meta->list);
> +       }
> +       raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
> +       if (!meta)
> +               return NULL;
> +
> +       if (unlikely(!raw_spin_trylock_irqsave(&meta->lock, flags))) {
> +               /*
> +                * This is extremely unlikely -- we are reporting on a
> +                * use-after-free, which locked meta->lock, and the reporting
> +                * code via printk calls kmalloc() which ends up in
> +                * kfence_alloc() and tries to grab the same object that we're
> +                * reporting on. While it has never been observed, lockdep does
> +                * report that there is a possibility of deadlock. Fix it by
> +                * using trylock and bailing out gracefully.
> +                */
> +               raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
> +               /* Put the object back on the freelist. */
> +               list_add_tail(&meta->list, &kfence_freelist);
> +               raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
> +
> +               return NULL;
> +       }
> +
> +       meta->addr = metadata_to_pageaddr(meta);
> +       /* Unprotect if we're reusing this page. */
> +       if (meta->state == KFENCE_OBJECT_FREED)
> +               kfence_unprotect(meta->addr);
> +
> +       /*
> +        * Note: for allocations made before RNG initialization, will always
> +        * return zero. We still benefit from enabling KFENCE as early as
> +        * possible, even when the RNG is not yet available, as this will allow
> +        * KFENCE to detect bugs due to earlier allocations. The only downside
> +        * is that the out-of-bounds accesses detected are deterministic for
> +        * such allocations.
> +        */
> +       if (prandom_u32_max(2)) {
> +               /* Allocate on the "right" side, re-calculate address. */
> +               meta->addr += PAGE_SIZE - size;
> +               meta->addr = ALIGN_DOWN(meta->addr, cache->align);
> +       }
> +
> +       /* Update remaining metadata. */
> +       metadata_update_state(meta, KFENCE_OBJECT_ALLOCATED);
> +       /* Pairs with READ_ONCE() in kfence_shutdown_cache(). */
> +       WRITE_ONCE(meta->cache, cache);
> +       meta->size = size;
> +       for_each_canary(meta, set_canary_byte);
> +       virt_to_page(meta->addr)->slab_cache = cache;
> +
> +       raw_spin_unlock_irqrestore(&meta->lock, flags);
> +
> +       /* Memory initialization. */
> +
> +       /*
> +        * We check slab_want_init_on_alloc() ourselves, rather than letting
> +        * SL*B do the initialization, as otherwise we might overwrite KFENCE's
> +        * redzone.
> +        */
> +       addr = (void *)meta->addr;
> +       if (unlikely(slab_want_init_on_alloc(gfp, cache)))
> +               memzero_explicit(addr, size);
> +       if (cache->ctor)
> +               cache->ctor(addr);
> +
> +       if (CONFIG_KFENCE_STRESS_TEST_FAULTS && !prandom_u32_max(CONFIG_KFENCE_STRESS_TEST_FAULTS))
> +               kfence_protect(meta->addr); /* Random "faults" by protecting the object. */
> +
> +       atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCATED]);
> +       atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCS]);
> +
> +       return addr;
> +}
> +
> +static void kfence_guarded_free(void *addr, struct kfence_metadata *meta)
> +{
> +       struct kcsan_scoped_access assert_page_exclusive;
> +       unsigned long flags;
> +
> +       raw_spin_lock_irqsave(&meta->lock, flags);
> +
> +       if (meta->state != KFENCE_OBJECT_ALLOCATED || meta->addr != (unsigned long)addr) {
> +               /* Invalid or double-free, bail out. */
> +               atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
> +               kfence_report_error((unsigned long)addr, meta, KFENCE_ERROR_INVALID_FREE);
> +               raw_spin_unlock_irqrestore(&meta->lock, flags);
> +               return;
> +       }
> +
> +       /* Detect racy use-after-free, or incorrect reallocation of this page by KFENCE. */
> +       kcsan_begin_scoped_access((void *)ALIGN_DOWN((unsigned long)addr, PAGE_SIZE), PAGE_SIZE,
> +                                 KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT,
> +                                 &assert_page_exclusive);
> +
> +       if (CONFIG_KFENCE_STRESS_TEST_FAULTS)
> +               kfence_unprotect((unsigned long)addr); /* To check canary bytes. */
> +
> +       /* Restore page protection if there was an OOB access. */
> +       if (meta->unprotected_page) {
> +               kfence_protect(meta->unprotected_page);
> +               meta->unprotected_page = 0;
> +       }
> +
> +       /* Check canary bytes for memory corruption. */
> +       for_each_canary(meta, check_canary_byte);
> +
> +       /*
> +        * Clear memory if init-on-free is set. While we protect the page, the
> +        * data is still there, and after a use-after-free is detected, we
> +        * unprotect the page, so the data is still accessible.
> +        */
> +       if (unlikely(slab_want_init_on_free(meta->cache)))
> +               memzero_explicit(addr, meta->size);
> +
> +       /* Mark the object as freed. */
> +       metadata_update_state(meta, KFENCE_OBJECT_FREED);
> +
> +       raw_spin_unlock_irqrestore(&meta->lock, flags);
> +
> +       /* Protect to detect use-after-frees. */
> +       kfence_protect((unsigned long)addr);
> +
> +       /* Add it to the tail of the freelist for reuse. */
> +       raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
> +       KFENCE_WARN_ON(!list_empty(&meta->list));
> +       list_add_tail(&meta->list, &kfence_freelist);
> +       kcsan_end_scoped_access(&assert_page_exclusive);
> +       raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
> +
> +       atomic_long_dec(&counters[KFENCE_COUNTER_ALLOCATED]);
> +       atomic_long_inc(&counters[KFENCE_COUNTER_FREES]);
> +}
> +
> +static void rcu_guarded_free(struct rcu_head *h)
> +{
> +       struct kfence_metadata *meta = container_of(h, struct kfence_metadata, rcu_head);
> +
> +       kfence_guarded_free((void *)meta->addr, meta);
> +}
> +
> +static bool __init kfence_initialize_pool(void)
> +{
> +       unsigned long addr;
> +       struct page *pages;
> +       int i;
> +
> +       if (!arch_kfence_initialize_pool())
> +               return false;
> +
> +       addr = (unsigned long)__kfence_pool;
> +       pages = virt_to_page(addr);
> +
> +       /*
> +        * Set up object pages: they must have PG_slab set, to avoid freeing
> +        * these as real pages.
> +        *
> +        * We also want to avoid inserting kfence_free() in the kfree()
> +        * fast-path in SLUB, and therefore need to ensure kfree() correctly
> +        * enters __slab_free() slow-path.
> +        */
> +       for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) {
> +               if (!i || (i % 2))
> +                       continue;
> +
> +               __SetPageSlab(&pages[i]);
> +       }
> +
> +       /*
> +        * Protect the first 2 pages. The first page is mostly unnecessary, and
> +        * merely serves as an extended guard page. However, adding one
> +        * additional page in the beginning gives us an even number of pages,
> +        * which simplifies the mapping of address to metadata index.
> +        */
> +       for (i = 0; i < 2; i++) {
> +               if (unlikely(!kfence_protect(addr)))
> +                       return false;
> +
> +               addr += PAGE_SIZE;
> +       }
> +
> +       for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
> +               struct kfence_metadata *meta = &kfence_metadata[i];
> +
> +               /* Initialize metadata. */
> +               INIT_LIST_HEAD(&meta->list);
> +               raw_spin_lock_init(&meta->lock);
> +               meta->state = KFENCE_OBJECT_UNUSED;
> +               meta->addr = addr; /* Initialize for validation in metadata_to_pageaddr(). */
> +               list_add_tail(&meta->list, &kfence_freelist);
> +
> +               /* Protect the right redzone. */
> +               if (unlikely(!kfence_protect(addr + PAGE_SIZE)))
> +                       return false;
> +
> +               addr += 2 * PAGE_SIZE;
> +       }
> +
> +       return true;
> +}
> +
> +/* === DebugFS Interface ==================================================== */
> +
> +static int stats_show(struct seq_file *seq, void *v)
> +{
> +       int i;
> +
> +       seq_printf(seq, "enabled: %i\n", READ_ONCE(kfence_enabled));
> +       for (i = 0; i < KFENCE_COUNTER_COUNT; i++)
> +               seq_printf(seq, "%s: %ld\n", counter_names[i], atomic_long_read(&counters[i]));
> +
> +       return 0;
> +}
> +DEFINE_SHOW_ATTRIBUTE(stats);
> +
> +/*
> + * debugfs seq_file operations for /sys/kernel/debug/kfence/objects.
> + * start_object() and next_object() return the object index + 1, because NULL is used
> + * to stop iteration.
> + */
> +static void *start_object(struct seq_file *seq, loff_t *pos)
> +{
> +       if (*pos < CONFIG_KFENCE_NUM_OBJECTS)
> +               return (void *)((long)*pos + 1);
> +       return NULL;
> +}
> +
> +static void stop_object(struct seq_file *seq, void *v)
> +{
> +}
> +
> +static void *next_object(struct seq_file *seq, void *v, loff_t *pos)
> +{
> +       ++*pos;
> +       if (*pos < CONFIG_KFENCE_NUM_OBJECTS)
> +               return (void *)((long)*pos + 1);
> +       return NULL;
> +}
> +
> +static int show_object(struct seq_file *seq, void *v)
> +{
> +       struct kfence_metadata *meta = &kfence_metadata[(long)v - 1];
> +       unsigned long flags;
> +
> +       raw_spin_lock_irqsave(&meta->lock, flags);
> +       kfence_print_object(seq, meta);
> +       raw_spin_unlock_irqrestore(&meta->lock, flags);
> +       seq_puts(seq, "---------------------------------\n");
> +
> +       return 0;
> +}
> +
> +static const struct seq_operations object_seqops = {
> +       .start = start_object,
> +       .next = next_object,
> +       .stop = stop_object,
> +       .show = show_object,
> +};
> +
> +static int open_objects(struct inode *inode, struct file *file)
> +{
> +       return seq_open(file, &object_seqops);
> +}
> +
> +static const struct file_operations objects_fops = {
> +       .open = open_objects,
> +       .read = seq_read,
> +       .llseek = seq_lseek,
> +};
> +
> +static int __init kfence_debugfs_init(void)
> +{
> +       struct dentry *kfence_dir = debugfs_create_dir("kfence", NULL);
> +
> +       debugfs_create_file("stats", 0444, kfence_dir, NULL, &stats_fops);
> +       debugfs_create_file("objects", 0400, kfence_dir, NULL, &objects_fops);
> +       return 0;
> +}
> +
> +late_initcall(kfence_debugfs_init);
> +
> +/* === Allocation Gate Timer ================================================ */
> +
> +/*
> + * Set up delayed work, which will enable and disable the static key. We need to
> + * use a work queue (rather than a simple timer), since enabling and disabling a
> + * static key cannot be done from an interrupt.
> + */
> +static struct delayed_work kfence_timer;
> +static void toggle_allocation_gate(struct work_struct *work)
> +{
> +       if (!READ_ONCE(kfence_enabled))
> +               return;
> +
> +       /* Enable static key, and await allocation to happen. */
> +       atomic_set(&allocation_gate, 0);
> +       static_branch_enable(&kfence_allocation_key);
> +       wait_event(allocation_wait, atomic_read(&allocation_gate) != 0);
> +
> +       /* Disable static key and reset timer. */
> +       static_branch_disable(&kfence_allocation_key);
> +       schedule_delayed_work(&kfence_timer, msecs_to_jiffies(kfence_sample_interval));
> +}
> +static DECLARE_DELAYED_WORK(kfence_timer, toggle_allocation_gate);
> +
> +/* === Public interface ===================================================== */
> +
> +void __init kfence_init(void)
> +{
> +       /* Setting kfence_sample_interval to 0 on boot disables KFENCE. */
> +       if (!kfence_sample_interval)
> +               return;
> +
> +       if (!kfence_initialize_pool()) {
> +               pr_err("%s failed\n", __func__);
> +               return;
> +       }
> +
> +       WRITE_ONCE(kfence_enabled, true);
> +       schedule_delayed_work(&kfence_timer, 0);
> +       pr_info("initialized - using %lu bytes for %d objects", KFENCE_POOL_SIZE,
> +               CONFIG_KFENCE_NUM_OBJECTS);
> +       if (IS_ENABLED(CONFIG_DEBUG_KERNEL))
> +               pr_cont(" at 0x%px-0x%px\n", (void *)__kfence_pool,
> +                       (void *)(__kfence_pool + KFENCE_POOL_SIZE));
> +       else
> +               pr_cont("\n");
> +}
> +
> +bool kfence_shutdown_cache(struct kmem_cache *s)
> +{
> +       unsigned long flags;
> +       struct kfence_metadata *meta;
> +       int i;
> +
> +       for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
> +               bool in_use;
> +
> +               meta = &kfence_metadata[i];
> +
> +               /*
> +                * If we observe some inconsistent cache and state pair where we
> +                * should have returned false here, cache destruction is racing
> +                * with either kmem_cache_alloc() or kmem_cache_free(). Taking
> +                * the lock will not help, as different critical section
> +                * serialization will have the same outcome.
> +                */
> +               if (READ_ONCE(meta->cache) != s ||
> +                   READ_ONCE(meta->state) != KFENCE_OBJECT_ALLOCATED)
> +                       continue;
> +
> +               raw_spin_lock_irqsave(&meta->lock, flags);
> +               in_use = meta->cache == s && meta->state == KFENCE_OBJECT_ALLOCATED;
> +               raw_spin_unlock_irqrestore(&meta->lock, flags);
> +
> +               if (in_use)
> +                       return false;
> +       }
> +
> +       for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
> +               meta = &kfence_metadata[i];
> +
> +               /* See above. */
> +               if (READ_ONCE(meta->cache) != s || READ_ONCE(meta->state) != KFENCE_OBJECT_FREED)
> +                       continue;
> +
> +               raw_spin_lock_irqsave(&meta->lock, flags);
> +               if (meta->cache == s && meta->state == KFENCE_OBJECT_FREED)
> +                       meta->cache = NULL;
> +               raw_spin_unlock_irqrestore(&meta->lock, flags);
> +       }
> +
> +       return true;
> +}
> +
> +void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
> +{
> +       /*
> +        * allocation_gate only needs to become non-zero, so it doesn't make
> +        * sense to continue writing to it and pay the associated contention
> +        * cost, in case we have a large number of concurrent allocations.
> +        */
> +       if (atomic_read(&allocation_gate) || atomic_inc_return(&allocation_gate) > 1)
> +               return NULL;
> +       wake_up(&allocation_wait);
> +
> +       if (!READ_ONCE(kfence_enabled))
> +               return NULL;
> +
> +       if (size > PAGE_SIZE)
> +               return NULL;
> +
> +       return kfence_guarded_alloc(s, size, flags);
> +}
> +
> +size_t kfence_ksize(const void *addr)
> +{
> +       const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
> +
> +       /*
> +        * Read locklessly -- if there is a race with __kfence_alloc(), this is
> +        * either a use-after-free or invalid access.
> +        */
> +       return meta ? meta->size : 0;
> +}
> +
> +void *kfence_object_start(const void *addr)
> +{
> +       const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
> +
> +       /*
> +        * Read locklessly -- if there is a race with __kfence_alloc(), this is
> +        * either a use-after-free or invalid access.
> +        */
> +       return meta ? (void *)meta->addr : NULL;
> +}
> +
> +void __kfence_free(void *addr)
> +{
> +       struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
> +
> +       /*
> +        * If the objects of the cache are SLAB_TYPESAFE_BY_RCU, defer freeing
> +        * the object, as the object page may be recycled for other-typed
> +        * objects once it has been freed.
> +        */
> +       if (unlikely(meta->cache->flags & SLAB_TYPESAFE_BY_RCU))
> +               call_rcu(&meta->rcu_head, rcu_guarded_free);
> +       else
> +               kfence_guarded_free(addr, meta);
> +}
> +
> +bool kfence_handle_page_fault(unsigned long addr)
> +{
> +       const int page_index = (addr - (unsigned long)__kfence_pool) / PAGE_SIZE;
> +       struct kfence_metadata *to_report = NULL;
> +       enum kfence_error_type error_type;
> +       unsigned long flags;
> +
> +       if (!is_kfence_address((void *)addr))
> +               return false;
> +
> +       if (!READ_ONCE(kfence_enabled)) /* If disabled at runtime ... */
> +               return kfence_unprotect(addr); /* ... unprotect and proceed. */
> +
> +       atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
> +
> +       if (page_index % 2) {
> +               /* This is a redzone, report a buffer overflow. */
> +               struct kfence_metadata *meta;
> +               int distance = 0;
> +
> +               meta = addr_to_metadata(addr - PAGE_SIZE);
> +               if (meta && READ_ONCE(meta->state) == KFENCE_OBJECT_ALLOCATED) {
> +                       to_report = meta;
> +                       /* Data race ok; distance calculation approximate. */
> +                       distance = addr - data_race(meta->addr + meta->size);
> +               }
> +
> +               meta = addr_to_metadata(addr + PAGE_SIZE);
> +               if (meta && READ_ONCE(meta->state) == KFENCE_OBJECT_ALLOCATED) {
> +                       /* Data race ok; distance calculation approximate. */
> +                       if (!to_report || distance > data_race(meta->addr) - addr)
> +                               to_report = meta;
> +               }
> +
> +               if (!to_report)
> +                       goto out;
> +
> +               raw_spin_lock_irqsave(&to_report->lock, flags);
> +               to_report->unprotected_page = addr;
> +               error_type = KFENCE_ERROR_OOB;
> +
> +               /*
> +                * If the object was freed before we took the look we can still
> +                * report this as an OOB -- the report will simply show the
> +                * stacktrace of the free as well.
> +                */
> +       } else {
> +               to_report = addr_to_metadata(addr);
> +               if (!to_report)
> +                       goto out;
> +
> +               raw_spin_lock_irqsave(&to_report->lock, flags);
> +               error_type = KFENCE_ERROR_UAF;
> +               /*
> +                * We may race with __kfence_alloc(), and it is possible that a
> +                * freed object may be reallocated. We simply report this as a
> +                * use-after-free, with the stack trace showing the place where
> +                * the object was re-allocated.
> +                */
> +       }
> +
> +out:
> +       if (to_report) {
> +               kfence_report_error(addr, to_report, error_type);
> +               raw_spin_unlock_irqrestore(&to_report->lock, flags);
> +       } else {
> +               /* This may be a UAF or OOB access, but we can't be sure. */
> +               kfence_report_error(addr, NULL, KFENCE_ERROR_INVALID);
> +       }
> +
> +       return kfence_unprotect(addr); /* Unprotect and let access proceed. */
> +}
> diff --git a/mm/kfence/kfence.h b/mm/kfence/kfence.h
> new file mode 100644
> index 000000000000..2f606a3f58b6
> --- /dev/null
> +++ b/mm/kfence/kfence.h
> @@ -0,0 +1,102 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef MM_KFENCE_KFENCE_H
> +#define MM_KFENCE_KFENCE_H
> +
> +#include <linux/mm.h>
> +#include <linux/slab.h>
> +#include <linux/spinlock.h>
> +#include <linux/types.h>
> +
> +#include "../slab.h" /* for struct kmem_cache */
> +
> +/* For non-debug builds, avoid leaking kernel pointers into dmesg. */
> +#ifdef CONFIG_DEBUG_KERNEL
> +#define PTR_FMT "%px"
> +#else
> +#define PTR_FMT "%p"
> +#endif
> +
> +/*
> + * Get the canary byte pattern for @addr. Use a pattern that varies based on the
> + * lower 3 bits of the address, to detect memory corruptions with higher
> + * probability, where similar constants are used.
> + */
> +#define KFENCE_CANARY_PATTERN(addr) ((u8)0xaa ^ (u8)((unsigned long)(addr) & 0x7))
> +
> +/* Maximum stack depth for reports. */
> +#define KFENCE_STACK_DEPTH 64
> +
> +/* KFENCE object states. */
> +enum kfence_object_state {
> +       KFENCE_OBJECT_UNUSED,           /* Object is unused. */
> +       KFENCE_OBJECT_ALLOCATED,        /* Object is currently allocated. */
> +       KFENCE_OBJECT_FREED,            /* Object was allocated, and then freed. */
> +};
> +
> +/* KFENCE metadata per guarded allocation. */
> +struct kfence_metadata {
> +       struct list_head list;          /* Freelist node; access under kfence_freelist_lock. */
> +       struct rcu_head rcu_head;       /* For delayed freeing. */
> +
> +       /*
> +        * Lock protecting below data; to ensure consistency of the below data,
> +        * since the following may execute concurrently: __kfence_alloc(),
> +        * __kfence_free(), kfence_handle_page_fault(). However, note that we
> +        * cannot grab the same metadata off the freelist twice, and multiple
> +        * __kfence_alloc() cannot run concurrently on the same metadata.
> +        */
> +       raw_spinlock_t lock;
> +
> +       /* The current state of the object; see above. */
> +       enum kfence_object_state state;
> +
> +       /*
> +        * Allocated object address; cannot be calculated from size, because of
> +        * alignment requirements.
> +        *
> +        * Invariant: ALIGN_DOWN(addr, PAGE_SIZE) is constant.
> +        */
> +       unsigned long addr;
> +
> +       /*
> +        * The size of the original allocation.
> +        */
> +       size_t size;
> +
> +       /*
> +        * The kmem_cache cache of the last allocation; NULL if never allocated
> +        * or the cache has already been destroyed.
> +        */
> +       struct kmem_cache *cache;
> +
> +       /*
> +        * In case of an invalid access, the page that was unprotected; we
> +        * optimistically only store address.
> +        */
> +       unsigned long unprotected_page;
> +
> +       /* Allocation and free stack information. */
> +       int num_alloc_stack;
> +       int num_free_stack;
> +       unsigned long alloc_stack[KFENCE_STACK_DEPTH];
> +       unsigned long free_stack[KFENCE_STACK_DEPTH];

It was a concious decision to not use stackdepot, right? Perhaps it
makes sense to document the reason somewhere.


> +};
> +
> +extern struct kfence_metadata kfence_metadata[CONFIG_KFENCE_NUM_OBJECTS];
> +
> +/* KFENCE error types for report generation. */
> +enum kfence_error_type {
> +       KFENCE_ERROR_OOB,               /* Detected a out-of-bounds access. */
> +       KFENCE_ERROR_UAF,               /* Detected a use-after-free access. */
> +       KFENCE_ERROR_CORRUPTION,        /* Detected a memory corruption on free. */
> +       KFENCE_ERROR_INVALID,           /* Invalid access of unknown type. */
> +       KFENCE_ERROR_INVALID_FREE,      /* Invalid free. */
> +};
> +
> +void kfence_report_error(unsigned long address, const struct kfence_metadata *meta,
> +                        enum kfence_error_type type);
> +
> +void kfence_print_object(struct seq_file *seq, const struct kfence_metadata *meta);
> +
> +#endif /* MM_KFENCE_KFENCE_H */
> diff --git a/mm/kfence/report.c b/mm/kfence/report.c
> new file mode 100644
> index 000000000000..0375867e85b3
> --- /dev/null
> +++ b/mm/kfence/report.c
> @@ -0,0 +1,219 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <stdarg.h>
> +
> +#include <linux/kernel.h>
> +#include <linux/lockdep.h>
> +#include <linux/printk.h>
> +#include <linux/seq_file.h>
> +#include <linux/stacktrace.h>
> +#include <linux/string.h>
> +
> +#include <asm/kfence.h>
> +
> +#include "kfence.h"
> +
> +/* Helper function to either print to a seq_file or to console. */
> +__printf(2, 3)
> +static void seq_con_printf(struct seq_file *seq, const char *fmt, ...)
> +{
> +       va_list args;
> +
> +       va_start(args, fmt);
> +       if (seq)
> +               seq_vprintf(seq, fmt, args);
> +       else
> +               vprintk(fmt, args);
> +       va_end(args);
> +}
> +
> +/*
> + * Get the number of stack entries to skip get out of MM internals. @type is
> + * optional, and if set to NULL, assumes an allocation or free stack.
> + */
> +static int get_stack_skipnr(const unsigned long stack_entries[], int num_entries,
> +                           const enum kfence_error_type *type)
> +{
> +       char buf[64];
> +       int skipnr, fallback = 0;
> +       bool is_access_fault = false;
> +
> +       if (type) {
> +               /* Depending on error type, find different stack entries. */
> +               switch (*type) {
> +               case KFENCE_ERROR_UAF:
> +               case KFENCE_ERROR_OOB:
> +               case KFENCE_ERROR_INVALID:
> +                       is_access_fault = true;
> +                       break;
> +               case KFENCE_ERROR_CORRUPTION:
> +               case KFENCE_ERROR_INVALID_FREE:
> +                       break;
> +               }
> +       }
> +
> +       for (skipnr = 0; skipnr < num_entries; skipnr++) {
> +               int len = scnprintf(buf, sizeof(buf), "%ps", (void *)stack_entries[skipnr]);
> +
> +               if (is_access_fault) {
> +                       if (!strncmp(buf, KFENCE_SKIP_ARCH_FAULT_HANDLER, len))
> +                               goto found;
> +               } else {
> +                       if (str_has_prefix(buf, "kfence_") || str_has_prefix(buf, "__kfence_"))
> +                               fallback = skipnr + 1; /* In case of tail calls into kfence. */
> +
> +                       /* Also the *_bulk() variants by only checking prefixes. */
> +                       if (str_has_prefix(buf, "kfree") ||
> +                           str_has_prefix(buf, "kmem_cache_free") ||
> +                           str_has_prefix(buf, "__kmalloc") ||
> +                           str_has_prefix(buf, "kmem_cache_alloc"))
> +                               goto found;
> +               }
> +       }
> +       if (fallback < num_entries)
> +               return fallback;
> +found:
> +       skipnr++;
> +       return skipnr < num_entries ? skipnr : 0;
> +}
> +
> +static void kfence_print_stack(struct seq_file *seq, const struct kfence_metadata *meta,
> +                              bool show_alloc)
> +{
> +       const unsigned long *entries = show_alloc ? meta->alloc_stack : meta->free_stack;
> +       const int nentries = show_alloc ? meta->num_alloc_stack : meta->num_free_stack;
> +
> +       if (nentries) {
> +               /* Skip allocation/free internals stack. */
> +               int i = get_stack_skipnr(entries, nentries, NULL);
> +
> +               /* stack_trace_seq_print() does not exist; open code our own. */
> +               for (; i < nentries; i++)
> +                       seq_con_printf(seq, " %pS\n", (void *)entries[i]);
> +       } else {
> +               seq_con_printf(seq, " no %s stack\n", show_alloc ? "allocation" : "deallocation");
> +       }
> +}
> +
> +void kfence_print_object(struct seq_file *seq, const struct kfence_metadata *meta)
> +{
> +       const int size = abs(meta->size);
> +       const unsigned long start = meta->addr;
> +       const struct kmem_cache *const cache = meta->cache;
> +
> +       lockdep_assert_held(&meta->lock);
> +
> +       if (meta->state == KFENCE_OBJECT_UNUSED) {
> +               seq_con_printf(seq, "kfence-#%zd unused\n", meta - kfence_metadata);
> +               return;
> +       }
> +
> +       seq_con_printf(seq,
> +                      "kfence-#%zd [0x" PTR_FMT "-0x" PTR_FMT
> +                      ", size=%d, cache=%s] allocated in:\n",
> +                      meta - kfence_metadata, (void *)start, (void *)(start + size - 1), size,
> +                      (cache && cache->name) ? cache->name : "<destroyed>");
> +       kfence_print_stack(seq, meta, true);
> +
> +       if (meta->state == KFENCE_OBJECT_FREED) {
> +               seq_con_printf(seq, "\nfreed in:\n");
> +               kfence_print_stack(seq, meta, false);
> +       }
> +}
> +
> +/*
> + * Show bytes at @addr that are different from the expected canary values, up to
> + * @max_bytes.
> + */
> +static void print_diff_canary(const u8 *addr, size_t max_bytes)
> +{
> +       const u8 *max_addr = min((const u8 *)PAGE_ALIGN((unsigned long)addr), addr + max_bytes);
> +
> +       pr_cont("[");
> +       for (; addr < max_addr; addr++) {
> +               if (*addr == KFENCE_CANARY_PATTERN(addr))
> +                       pr_cont(" .");
> +               else if (IS_ENABLED(CONFIG_DEBUG_KERNEL))
> +                       pr_cont(" 0x%02x", *addr);
> +               else /* Do not leak kernel memory in non-debug builds. */
> +                       pr_cont(" !");
> +       }
> +       pr_cont(" ]");
> +}
> +
> +void kfence_report_error(unsigned long address, const struct kfence_metadata *meta,
> +                        enum kfence_error_type type)
> +{
> +       unsigned long stack_entries[KFENCE_STACK_DEPTH] = { 0 };
> +       int num_stack_entries = stack_trace_save(stack_entries, KFENCE_STACK_DEPTH, 1);
> +       int skipnr = get_stack_skipnr(stack_entries, num_stack_entries, &type);
> +       const ptrdiff_t object_index = meta ? meta - kfence_metadata : -1;
> +
> +       /* Require non-NULL meta, except if KFENCE_ERROR_INVALID. */
> +       if (WARN_ON(type != KFENCE_ERROR_INVALID && !meta))
> +               return;
> +
> +       if (meta)
> +               lockdep_assert_held(&meta->lock);
> +       /*
> +        * Because we may generate reports in printk-unfriendly parts of the
> +        * kernel, such as scheduler code, the use of printk() could deadlock.
> +        * Until such time that all printing code here is safe in all parts of
> +        * the kernel, accept the risk, and just get our message out (given the
> +        * system might already behave unpredictably due to the memory error).
> +        * As such, also disable lockdep to hide warnings, and avoid disabling
> +        * lockdep for the rest of the kernel.
> +        */
> +       lockdep_off();
> +
> +       pr_err("==================================================================\n");
> +       /* Print report header. */
> +       switch (type) {
> +       case KFENCE_ERROR_OOB:
> +               pr_err("BUG: KFENCE: out-of-bounds in %pS\n\n", (void *)stack_entries[skipnr]);
> +               pr_err("Out-of-bounds access at 0x" PTR_FMT " (%s of kfence-#%zd):\n",
> +                      (void *)address, address < meta->addr ? "left" : "right", object_index);
> +               break;
> +       case KFENCE_ERROR_UAF:
> +               pr_err("BUG: KFENCE: use-after-free in %pS\n\n", (void *)stack_entries[skipnr]);
> +               pr_err("Use-after-free access at 0x" PTR_FMT " (in kfence-#%zd):\n",
> +                      (void *)address, object_index);
> +               break;
> +       case KFENCE_ERROR_CORRUPTION:
> +               pr_err("BUG: KFENCE: memory corruption in %pS\n\n", (void *)stack_entries[skipnr]);
> +               pr_err("Corrupted memory at 0x" PTR_FMT " ", (void *)address);
> +               print_diff_canary((u8 *)address, 16);
> +               pr_cont(" (in kfence-#%zd):\n", object_index);
> +               break;
> +       case KFENCE_ERROR_INVALID:
> +               pr_err("BUG: KFENCE: invalid access in %pS\n\n", (void *)stack_entries[skipnr]);
> +               pr_err("Invalid access at 0x" PTR_FMT ":\n", (void *)address);
> +               break;
> +       case KFENCE_ERROR_INVALID_FREE:
> +               pr_err("BUG: KFENCE: invalid free in %pS\n\n", (void *)stack_entries[skipnr]);
> +               pr_err("Invalid free of 0x" PTR_FMT " (in kfence-#%zd):\n", (void *)address,
> +                      object_index);
> +               break;
> +       }
> +
> +       /* Print stack trace and object info. */
> +       stack_trace_print(stack_entries + skipnr, num_stack_entries - skipnr, 0);
> +
> +       if (meta) {
> +               pr_err("\n");
> +               kfence_print_object(NULL, meta);
> +       }
> +
> +       /* Print report footer. */
> +       pr_err("\n");
> +       dump_stack_print_info(KERN_DEFAULT);
> +       pr_err("==================================================================\n");
> +
> +       lockdep_on();
> +
> +       if (panic_on_warn)
> +               panic("panic_on_warn set ...\n");
> +
> +       /* We encountered a memory unsafety error, taint the kernel! */
> +       add_taint(TAINT_WARN, LOCKDEP_STILL_OK);
> +}
> --
> 2.28.0.681.g6f77f65b4e-goog
>