Re: [PATCH v4 1/3] kasan: support backing vmalloc space with real shadow memory

From: Christophe Leroy
Date: Fri Aug 16 2019 - 03:47:11 EST

Le 15/08/2019 Ã 02:16, Daniel Axtens a ÃcritÂ:
Hook into vmalloc and vmap, and dynamically allocate real shadow
memory to back the mappings.

Most mappings in vmalloc space are small, requiring less than a full
page of shadow space. Allocating a full shadow page per mapping would
therefore be wasteful. Furthermore, to ensure that different mappings
use different shadow pages, mappings would have to be aligned to

Instead, share backing space across multiple mappings. Allocate
a backing page the first time a mapping in vmalloc space uses a
particular page of the shadow region. Keep this page around
regardless of whether the mapping is later freed - in the mean time
the page could have become shared by another vmalloc mapping.

This can in theory lead to unbounded memory growth, but the vmalloc
allocator is pretty good at reusing addresses, so the practical memory
usage grows at first but then stays fairly stable.

I guess people having gigabytes of memory don't mind, but I'm concerned about tiny targets with very little amount of memory. I have boards with as little as 32Mbytes of RAM. The shadow region for the linear space already takes one eighth of the RAM. I'd rather avoid keeping unused shadow pages busy.

Each page of shadow memory represent 8 pages of real memory. Could we use page_ref to count how many pieces of a shadow page are used so that we can free it when the ref count decreases to 0.

This requires architecture support to actually use: arches must stop
mapping the read-only zero page over portion of the shadow region that
covers the vmalloc space and instead leave it unmapped.

Why 'must' ? Couldn't we switch back and forth from the zero page to real page on demand ?

If the zero page is not mapped for unused vmalloc space, bad memory accesses will Oops on the shadow memory access instead of Oopsing on the real bad access, making it more difficult to locate and identify the issue.

This allows KASAN with VMAP_STACK, and will be needed for architectures
that do not have a separate module space (e.g. powerpc64, which I am
currently working on). It also allows relaxing the module alignment
back to PAGE_SIZE.

Why 'needed' ? powerpc32 doesn't have a separate module space and doesn't need that.

Acked-by: Vasily Gorbik <gor@xxxxxxxxxxxxx>
Signed-off-by: Daniel Axtens <dja@xxxxxxxxxx>
[Mark: rework shadow allocation]
Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>


v2: let kasan_unpoison_shadow deal with ranges that do not use a
full shadow byte.

v3: relax module alignment
rename to kasan_populate_vmalloc which is a much better name
deal with concurrency correctly

v4: Integrate Mark's rework
Poision pages on vfree
Handle allocation failures. I've tested this by inserting artificial
failures and using test_vmalloc to stress it. I haven't handled the
per-cpu case: it looked like it would require a messy hacking-up of
the function to deal with an OOM failure case in a debug feature.

Documentation/dev-tools/kasan.rst | 60 +++++++++++++++++++++++++++
include/linux/kasan.h | 24 +++++++++++
include/linux/moduleloader.h | 2 +-
include/linux/vmalloc.h | 12 ++++++
lib/Kconfig.kasan | 16 ++++++++
lib/test_kasan.c | 26 ++++++++++++
mm/kasan/common.c | 67 +++++++++++++++++++++++++++++++
mm/kasan/generic_report.c | 3 ++
mm/kasan/kasan.h | 1 +
mm/vmalloc.c | 28 ++++++++++++-
10 files changed, 237 insertions(+), 2 deletions(-)

diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
index b72d07d70239..35fda484a672 100644
--- a/Documentation/dev-tools/kasan.rst
+++ b/Documentation/dev-tools/kasan.rst
@@ -215,3 +215,63 @@ brk handler is used to print bug reports.
A potential expansion of this mode is a hardware tag-based mode, which would
use hardware memory tagging support instead of compiler instrumentation and
manual shadow memory manipulation.
+What memory accesses are sanitised by KASAN?
+The kernel maps memory in a number of different parts of the address
+space. This poses something of a problem for KASAN, which requires
+that all addresses accessed by instrumented code have a valid shadow
+The range of kernel virtual addresses is large: there is not enough
+real memory to support a real shadow region for every address that
+could be accessed by the kernel.
+By default
+By default, architectures only map real memory over the shadow region
+for the linear mapping (and potentially other small areas). For all
+other areas - such as vmalloc and vmemmap space - a single read-only
+page is mapped over the shadow area. This read-only shadow page
+declares all memory accesses as permitted.
+This presents a problem for modules: they do not live in the linear
+mapping, but in a dedicated module space. By hooking in to the module
+allocator, KASAN can temporarily map real shadow memory to cover
+them. This allows detection of invalid accesses to module globals, for
+This also creates an incompatibility with ``VMAP_STACK``: if the stack
+lives in vmalloc space, it will be shadowed by the read-only page, and
+the kernel will fault when trying to set up the shadow data for stack
+With ``CONFIG_KASAN_VMALLOC``, KASAN can cover vmalloc space at the
+cost of greater memory usage. Currently this is only supported on x86.
+This works by hooking into vmalloc and vmap, and dynamically
+allocating real shadow memory to back the mappings.
+Most mappings in vmalloc space are small, requiring less than a full
+page of shadow space. Allocating a full shadow page per mapping would
+therefore be wasteful. Furthermore, to ensure that different mappings
+use different shadow pages, mappings would have to be aligned to
+Instead, we share backing space across multiple mappings. We allocate
+a backing page the first time a mapping in vmalloc space uses a
+particular page of the shadow region. We keep this page around
+regardless of whether the mapping is later freed - in the mean time
+this page could have become shared by another vmalloc mapping.
+This can in theory lead to unbounded memory growth, but the vmalloc
+allocator is pretty good at reusing addresses, so the practical memory
+usage grows at first but then stays fairly stable.
+This allows ``VMAP_STACK`` support on x86, and enables support of
+architectures that do not have a fixed module region.

That's wrong, powerpc32 doesn't have a fixed module region and is already supported.

diff --git a/include/linux/kasan.h b/include/linux/kasan.h
index cc8a03cc9674..d666748cd378 100644
--- a/include/linux/kasan.h
+++ b/include/linux/kasan.h
@@ -70,8 +70,18 @@ struct kasan_cache {
int free_meta_offset;
+ * These functions provide a special case to support backing module
+ * allocations with real shadow memory. With KASAN vmalloc, the special
+ * case is unnecessary, as the work is handled in the generic case.
+ */
int kasan_module_alloc(void *addr, size_t size);
void kasan_free_shadow(const struct vm_struct *vm);
+static inline int kasan_module_alloc(void *addr, size_t size) { return 0; }
+static inline void kasan_free_shadow(const struct vm_struct *vm) {}
int kasan_add_zero_shadow(void *start, unsigned long size);
void kasan_remove_zero_shadow(void *start, unsigned long size);
@@ -194,4 +204,18 @@ static inline void *kasan_reset_tag(const void *addr)
+int kasan_populate_vmalloc(unsigned long requested_size,
+ struct vm_struct *area);
+void kasan_free_vmalloc(void *start, unsigned long size);
+static inline int kasan_populate_vmalloc(unsigned long requested_size,
+ struct vm_struct *area)
+ return 0;
+static inline void kasan_free_vmalloc(void *start, unsigned long size) {}
#endif /* LINUX_KASAN_H */
diff --git a/include/linux/moduleloader.h b/include/linux/moduleloader.h
index 5229c18025e9..ca92aea8a6bd 100644
--- a/include/linux/moduleloader.h
+++ b/include/linux/moduleloader.h
@@ -91,7 +91,7 @@ void module_arch_cleanup(struct module *mod);
/* Any cleanup before freeing mod->module_init */
void module_arch_freeing_init(struct module *mod);
+#if defined(CONFIG_KASAN) && !defined(CONFIG_KASAN_VMALLOC)
#include <linux/kasan.h>
diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
index 9b21d0047710..cdc7a60f7d81 100644
--- a/include/linux/vmalloc.h
+++ b/include/linux/vmalloc.h
@@ -21,6 +21,18 @@ struct notifier_block; /* in notifier.h */
#define VM_UNINITIALIZED 0x00000020 /* vm_struct is not fully initialized */
#define VM_NO_GUARD 0x00000040 /* don't add guard page */
#define VM_KASAN 0x00000080 /* has allocated kasan shadow memory */
+ * VM_KASAN is used slighly differently depending on CONFIG_KASAN_VMALLOC.
+ *
+ * If IS_ENABLED(CONFIG_KASAN_VMALLOC), VM_KASAN is set on a vm_struct after
+ * shadow memory has been mapped. It's used to handle allocation errors so that
+ * we don't try to poision shadow on free if it was never allocated.
+ *
+ * Otherwise, VM_KASAN is set for kasan_module_alloc() allocations and used to
+ * determine which allocations need the module shadow freed.
+ */
* Memory with VM_FLUSH_RESET_PERMS cannot be freed in an interrupt or with
* vfree_atomic().
diff --git a/lib/Kconfig.kasan b/lib/Kconfig.kasan
index 4fafba1a923b..a320dc2e9317 100644
--- a/lib/Kconfig.kasan
+++ b/lib/Kconfig.kasan
@@ -6,6 +6,9 @@ config HAVE_ARCH_KASAN
+ bool
def_bool $(cc-option, -fsanitize=kernel-address)
@@ -135,6 +138,19 @@ config KASAN_S390_4_LEVEL_PAGING
to 3TB of RAM with KASan enabled). This options allows to force
4-level paging instead.
+ bool "Back mappings in vmalloc space with real shadow memory"
+ help
+ By default, the shadow region for vmalloc space is the read-only
+ zero page. This means that KASAN cannot detect errors involving
+ vmalloc space.
+ Enabling this option will hook in to vmap/vmalloc and back those
+ mappings with real shadow memory allocated on demand. This allows
+ for KASAN to detect more sorts of errors (and to support vmapped
+ stacks), but at the cost of higher memory usage.
tristate "Module for testing KASAN for bug detection"
depends on m && KASAN
diff --git a/lib/test_kasan.c b/lib/test_kasan.c
index b63b367a94e8..d375246f5f96 100644
--- a/lib/test_kasan.c
+++ b/lib/test_kasan.c

Could we put the testing part in a separate patch ?

@@ -18,6 +18,7 @@
#include <linux/slab.h>
#include <linux/string.h>
#include <linux/uaccess.h>
+#include <linux/vmalloc.h>
* Note: test functions are marked noinline so that their names appear in
@@ -709,6 +710,30 @@ static noinline void __init kmalloc_double_kzfree(void)
+static noinline void __init vmalloc_oob(void)
+ void *area;
+ pr_info("vmalloc out-of-bounds\n");
+ /*
+ * We have to be careful not to hit the guard page.
+ * The MMU will catch that and crash us.
+ */
+ area = vmalloc(3000);
+ if (!area) {
+ pr_err("Allocation failed\n");
+ return;
+ }
+ ((volatile char *)area)[3100];
+ vfree(area);
+static void __init vmalloc_oob(void) {}
static int __init kmalloc_tests_init(void)
@@ -752,6 +777,7 @@ static int __init kmalloc_tests_init(void)
+ vmalloc_oob();
diff --git a/mm/kasan/common.c b/mm/kasan/common.c
index 2277b82902d8..b8374e3773cf 100644
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -568,6 +568,7 @@ void kasan_kfree_large(void *ptr, unsigned long ip)
/* The object will be poisoned by page_alloc. */
int kasan_module_alloc(void *addr, size_t size)
void *ret;
@@ -603,6 +604,7 @@ void kasan_free_shadow(const struct vm_struct *vm)
if (vm->flags & VM_KASAN)
extern void __kasan_report(unsigned long addr, size_t size, bool is_write, unsigned long ip);
@@ -722,3 +724,68 @@ static int __init kasan_memhotplug_init(void)
+static int kasan_populate_vmalloc_pte(pte_t *ptep, unsigned long addr,
+ void *unused)
+ unsigned long page;
+ pte_t pte;
+ if (likely(!pte_none(*ptep)))
+ return 0;

Prior to this, the zero shadow area should be mapped, and the test should be:

if (likely(pte_pfn(*ptep) != PHYS_PFN(__pa(kasan_early_shadow_page))))
return 0;

+ page = __get_free_page(GFP_KERNEL);
+ if (!page)
+ return -ENOMEM;
+ memset((void *)page, KASAN_VMALLOC_INVALID, PAGE_SIZE);
+ pte = pfn_pte(PFN_DOWN(__pa(page)), PAGE_KERNEL);
+ /*
+ * Ensure poisoning is visible before the shadow is made visible
+ * to other CPUs.
+ */
+ smp_wmb();
+ spin_lock(&init_mm.page_table_lock);
+ if (likely(pte_none(*ptep))) {
+ set_pte_at(&init_mm, addr, ptep, pte);
+ page = 0;
+ }
+ spin_unlock(&init_mm.page_table_lock);
+ if (page)
+ free_page(page);
+ return 0;
+int kasan_populate_vmalloc(unsigned long requested_size, struct vm_struct *area)
+ unsigned long shadow_start, shadow_end;
+ int ret;
+ shadow_start = (unsigned long)kasan_mem_to_shadow(area->addr);
+ shadow_start = ALIGN_DOWN(shadow_start, PAGE_SIZE);
+ shadow_end = (unsigned long)kasan_mem_to_shadow(
+ area->addr + area->size);
+ shadow_end = ALIGN(shadow_end, PAGE_SIZE);
+ ret = apply_to_page_range(&init_mm, shadow_start,
+ shadow_end - shadow_start,
+ kasan_populate_vmalloc_pte, NULL);
+ if (ret)
+ return ret;
+ kasan_unpoison_shadow(area->addr, requested_size);
+ area->flags |= VM_KASAN;
+ return 0;
+void kasan_free_vmalloc(void *start, unsigned long size)
+ size = round_up(size, KASAN_SHADOW_SCALE_SIZE);
+ kasan_poison_shadow(start, size, KASAN_VMALLOC_INVALID);
diff --git a/mm/kasan/generic_report.c b/mm/kasan/generic_report.c
index 36c645939bc9..2d97efd4954f 100644
--- a/mm/kasan/generic_report.c
+++ b/mm/kasan/generic_report.c
@@ -86,6 +86,9 @@ static const char *get_shadow_bug_type(struct kasan_access_info *info)
bug_type = "alloca-out-of-bounds";
+ bug_type = "vmalloc-out-of-bounds";
+ break;
return bug_type;
diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
index 014f19e76247..8b1f2fbc780b 100644
--- a/mm/kasan/kasan.h
+++ b/mm/kasan/kasan.h
@@ -25,6 +25,7 @@
#define KASAN_GLOBAL_REDZONE 0xFA /* redzone for global variable */
+#define KASAN_VMALLOC_INVALID 0xF9 /* unallocated space in vmapped page */
* Stack redzone shadow values
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 4fa8d84599b0..c20a7e663004 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -2056,6 +2056,22 @@ static struct vm_struct *__get_vm_area_node(unsigned long size,
setup_vmalloc_vm(area, va, flags, caller);
+ /*
+ * For KASAN, if we are in vmalloc space, we need to cover the shadow
+ * area with real memory. If we come here through VM_ALLOC, this is
+ * done by a higher level function that has access to the true size,
+ * which might not be a full page.
+ *
+ * We assume module space comes via VM_ALLOC path.
+ */
+ if (is_vmalloc_addr(area->addr) && !(area->flags & VM_ALLOC)) {
+ if (kasan_populate_vmalloc(area->size, area)) {
+ unmap_vmap_area(va);
+ kfree(area);
+ return NULL;
+ }
+ }
return area;
@@ -2233,6 +2249,9 @@ static void __vunmap(const void *addr, int deallocate_pages)
debug_check_no_locks_freed(area->addr, get_vm_area_size(area));
debug_check_no_obj_freed(area->addr, get_vm_area_size(area));
+ if (area->flags & VM_KASAN)
+ kasan_free_vmalloc(area->addr, area->size);
vm_remove_mappings(area, deallocate_pages);
if (deallocate_pages) {
@@ -2483,6 +2502,9 @@ void *__vmalloc_node_range(unsigned long size, unsigned long align,
if (!addr)
return NULL;
+ if (kasan_populate_vmalloc(real_size, area))
+ return NULL;
* In this function, newly allocated vm_struct has VM_UNINITIALIZED
* flag. It means that vm_struct is not fully initialized.
@@ -3324,10 +3346,14 @@ struct vm_struct **pcpu_get_vm_areas(const unsigned long *offsets,
/* insert all vm's */
- for (area = 0; area < nr_vms; area++)
+ for (area = 0; area < nr_vms; area++) {
setup_vmalloc_vm(vms[area], vas[area], VM_ALLOC,
+ /* assume success here */
+ kasan_populate_vmalloc(sizes[area], vms[area]);
+ }
return vms;