[PATCH v9 00/44] kasan: add hardware tag-based mode for arm64

From: Andrey Konovalov
Date: Tue Nov 10 2020 - 17:11:09 EST

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This patchset adds a new hardware tag-based mode to KASAN [1]. The new mode
is similar to the existing software tag-based KASAN, but relies on arm64
Memory Tagging Extension (MTE) [2] to perform memory and pointer tagging
(instead of shadow memory and compiler instrumentation).

This patchset is co-developed by
Vincenzo Frascino <vincenzo.frascino@xxxxxxx>.

This patchset is available here:

https://github.com/xairy/linux/tree/up-kasan-mte-v9

and has also been uploaded to the Linux kernel Gerrit instance:

https://linux-review.googlesource.com/c/linux/kernel/git/torvalds/linux/+/3319

For testing in QEMU hardware tag-based KASAN requires:

1. QEMU built from master [4] (use "-machine virt,mte=on -cpu max" arguments
to run).
2. GCC version 10.

[1] https://www.kernel.org/doc/html/latest/dev-tools/kasan.html
[2] https://community.arm.com/developer/ip-products/processors/b/processors-ip-blog/posts/enhancing-memory-safety
[3] git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux for-next/mte
[4] https://github.com/qemu/qemu

====== Overview

The underlying ideas of the approach used by hardware tag-based KASAN are:

1. By relying on the Top Byte Ignore (TBI) arm64 CPU feature, pointer tags
are stored in the top byte of each kernel pointer.

2. With the Memory Tagging Extension (MTE) arm64 CPU feature, memory tags
for kernel memory allocations are stored in a dedicated memory not
accessible via normal instuctions.

3. On each memory allocation, a random tag is generated, embedded it into
the returned pointer, and the corresponding memory is tagged with the
same tag value.

4. With MTE the CPU performs a check on each memory access to make sure
that the pointer tag matches the memory tag.

5. On a tag mismatch the CPU generates a tag fault, and a KASAN report is
printed.

Same as other KASAN modes, hardware tag-based KASAN is intended as a
debugging feature at this point.

====== Rationale

There are two main reasons for this new hardware tag-based mode:

1. Previously implemented software tag-based KASAN is being successfully
used on dogfood testing devices due to its low memory overhead (as
initially planned). The new hardware mode keeps the same low memory
overhead, and is expected to have significantly lower performance
impact, due to the tag checks being performed by the hardware.
Therefore the new mode can be used as a better alternative in dogfood
testing for hardware that supports MTE.

2. The new mode lays the groundwork for the planned in-kernel MTE-based
memory corruption mitigation to be used in production.

====== Technical details

>From the implementation perspective, hardware tag-based KASAN is almost
identical to the software mode. The key difference is using MTE for
assigning and checking tags.

Compared to the software mode, the hardware mode uses 4 bits per tag, as
dictated by MTE. Pointer tags are stored in bits [56:60), the top 4 bits
have the normal value 0xF. Having less distict tags increases the
probablity of false negatives (from ~1/256 to ~1/16) in certain cases.

Only synchronous exceptions are set up and used by hardware tag-based KASAN.

====== Benchmarks

Note: all measurements have been performed with software emulation of Memory
Tagging Extension, performance numbers for hardware tag-based KASAN on the
actual hardware are expected to be better.

Boot time [1]:
* 2.8 sec for clean kernel
* 5.7 sec for hardware tag-based KASAN
* 11.8 sec for software tag-based KASAN
* 11.6 sec for generic KASAN

Slab memory usage after boot [2]:
* 7.0 kb for clean kernel
* 9.7 kb for hardware tag-based KASAN
* 9.7 kb for software tag-based KASAN
* 41.3 kb for generic KASAN

Measurements have been performed with:
* defconfig-based configs
* Manually built QEMU master
* QEMU arguments: -machine virt,mte=on -cpu max
* CONFIG_KASAN_STACK_ENABLE disabled
* CONFIG_KASAN_INLINE enabled
* clang-10 as the compiler and gcc-10 as the assembler

[1] Time before the ext4 driver is initialized.
[2] Measured as `cat /proc/meminfo | grep Slab`.

====== Notes

The cover letter for software tag-based KASAN patchset can be found here:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0116523cfffa62aeb5aa3b85ce7419f3dae0c1b8

====== History

Changes v8->v9:
(Andrey:)
- Drop __init for kasan_init_hw_tags.
- Split out mte_enable() from mte_init_tags().
- Fix incorrect commit author.
- Revert addr_has_metadata() change done in a previous version, otherwise
KASAN fails to print metadata for page_alloc allocations.
- Split kasan_init_hw_tags() into kasan_init_hw_tags_cpu() that is called
for each CPU in cpu_enable_mte(), and kasan_init_hw_tags() that is called
for boot CPU in smp_prepare_boot_cpu().
- Move kunit_kasan_expectation struct definition under CONFIG_KASAN.
(Vincenzo:)
- Address Catalin's comments for "arm64: mte: Reset the page tag in
page->flags".
- New patch "kasan, mm: untag page address in free_reserved_area".

Changes v7->v8:
(Andrey:)
- Rebased onto 5.10-rc2+.
- Enable in-kernel MTE via kasan_init_hw_tags() instead of doing it
directly from cpu_enable_mte(). This changes mte_init_tags() to both
init RRND and enable in-kernel MTE in sync mode.
- Put the patches back into a logical order.
- Rename KASAN_SHADOW_GRANULE_SIZE to KASAN_GRANULE_SIZE in
lib/test_kasan_module.c.
- Rename kasan_init_tags() to kasan_init_sw_tags() and mark as __init.

Changes v6->v7:
(Andrey:)
- Rebase onto 5.10-rc2.
- Fix pgd_t not defined build warning on s390.
- Mark kasan_init_depth() as static.
- Don't use mte_*() functions directly in report_hw_tags.c
- Add hw_ prefix to KASAN-level definitions of arch_*() tag helpers.
- Add missing <sched/task_stack.h> include to report_generic.h.

Changes v5->v6:
(Vincenzo:)
- Re-based on 5.10-rc1.
- Modified the flow of the mte fault handler in order to address an issue
with word at a time routines that would affect Android init process.
- Dropped Reviewed-by from the involved patches.
(Andrey:)
- Properly use #if CONFIG_KASAN_STACK instead of #ifdef
CONFIG_KASAN_STACK_ENABLE.
- Expand CONFIG_KASAN checks in arch/arm64/kernel/kaslr.c and
arch/arm64/kernel/module.c.
- Don't select CONFIG_CONSTRUCTORS for HW_TAGS mode.
- Check PageSlab() in addr_has_metadata().

Changes v4->v5:
(Vincenzo:)
- Reset the tag associated by the kernel to a page when this is changed by
the user.
- Add a kselftest to verify that GCR_EL1 is preserved during context
switch.
- Squashed the TBI patch.
- Addressed some review comments.
- Verified the series with LTP.
(Andrey:)
- Put core arm64 patches first as requested by Catalin.

Changes v3->v4:
(Vincenzo:)
- Introduced mte-def.h that contains MTE definitions.
- Moved __MTE_PREAMBLE in mte.h.
- Clarified which API is part of mte-kasan.h.
- Removed tsk argument from mte_set_kernel_gcr().
- Addressed several nitpicks pointed out during review.
(Andrey:)
- Move include <asm/kasan.h> in include/linux/kasan.h to avoid build
failures.
- Don't move "select SLUB_DEBUG if SLUB" back and forth between patches
in KASAN Kconfig.
- Add arm64 prefix to "kasan: don't allow SW_TAGS with ARM64_MTE" commit.
- Don't add braces when defining KASAN_GRANULE_SIZE.
- Make KASAN_HW_TAGS compatible with init_on_alloc/free.

Changes v2->v3:
(Vincenzo:)
- Use ARM64_ASM_PREAMBLE for asm macros.
- Rename mte-helper.h to mte-kasan.h. The new header is meant to contain
only macros and prototypes directly used in KASAN. The rest is defined
in mte.h.
- Update mte_get_mem_tag()/mte_get_random_tag() to used directly asm
volatile() macros instead of calling library functions.
- Harden mte_assign_mem_tag_range() to prevent an infinite loop in case of
unaligned size.
- Made sure that report_tag_fault() is executed only once.
- Simplify the mte code in __cpu_setup.
- Remove kprobes.h from mte.c includes.
- General cleanup of the code.
(Andrey:)
- Use READ/WRITE_ONCE when accessing reported in do_tag_recovery().
- Move .unreq mte_tcr under CONFIG_ARM64_MTE to avoid build errors when MTE
is not enabled.
- Improve mm/kasan/shadow.c comment header.
- Clarify what is a memory granule in "kasan: rename KASAN_SHADOW_* to
KASAN_GRANULE_" commit description.
- Rename (report_)tags_sw/hw.c to to (report_)sw/hw_tags.c and drop
unnecessary rename commit.
- Adopt 100 lines limit for some mm/kasan/ changes.
- Align arguments for stack_trace_save() call in mm/slub.c.
- Restore comment before kasan_init_tags().
- Remove GNU headers from all mm/kasan/ files.
- Simplify check_invalid_free() implementation tag-based modes.
- Drop subsequently removed report_tag_fault() implementation.
- Add KASAN_GRANULE_PAGE and use instead of PAGE_SIZE * KASAN_GRANULE_SIZE.
- Move kasan_enable/disable_current() declarations to simplify
include/linux/kasan.h.
- Drop dependency on CONFIG_SLUB_DEBUG.
- Clarify the purpose of CONFIG_STACKTRACE in KASAN Kconfig.

Changes v1->v2:
- Rebase onto v10 of the user MTE patchset.
- Only enable in-kernel MTE when KASAN_HW_TAGS is enabled.
- Add a layer of arch-level indirection, so KASAN doesn't call MTE helpers
directly (this will be useful in case more architectures will add support
for HW_TAGS).
- Don't do arm64_skip_faulting_instruction() on MTE fault, disable MTE
instead.
- Don't allow software tags with MTE via arch/arm64/Kconfig instead of
lib/Kconfig.kasan.
- Rename mm/kasan/tags.c to tags_sw.c and mte.c to tags_hw.c, and do the
same for report_*.c files.
- Reword HW_TAGS Kconfig help text to make it less MTE specific.
- Reword and clarify Documentation.
- Drop unnecessary is_el1_mte_sync_tag_check_fault().
- Change report_tag_fault() to only call kasan_report() once HW_TAGS is
introduced.
- Rename arch/arm64/include/asm/mte_asm.h to mte-helpers.h and move all
MTE-related defines and some helper functions there.
- Change mm/kasan/kasan.h to include mte-def.h instead of mte.h.
- Add WARN_ON() on unaligned size to mte_set_mem_tag_range().
- Implement ldg/irg MTE routines as inline assembly.
- Remove smp_wmb() from mte_set_mem_tag_range().
- Drop __must_check from mte_set_mem_tag_range() as KASAN has no use for
the return value.
- Drop zero size check from mte_assign_mem_tag_range().
- Drop unnecessary include <asm/kasan.h> from low-level arm64 code.
- Move enabling TBI1 into __cpu_setup().
- Drop stale comment about callee-saved register from
arch/arm64/kernel/entry.S.
- Mark gcr_kernel_excl as __ro_after_init.
- Use GENMASK() in mte_init_tags().

Andrey Konovalov (35):
kasan: drop unnecessary GPL text from comment headers
kasan: KASAN_VMALLOC depends on KASAN_GENERIC
kasan: group vmalloc code
s390/kasan: include asm/page.h from asm/kasan.h
kasan: shadow declarations only for software modes
kasan: rename (un)poison_shadow to (un)poison_memory
kasan: rename KASAN_SHADOW_* to KASAN_GRANULE_*
kasan: only build init.c for software modes
kasan: split out shadow.c from common.c
kasan: define KASAN_GRANULE_PAGE
kasan: rename report and tags files
kasan: don't duplicate config dependencies
kasan: hide invalid free check implementation
kasan: decode stack frame only with KASAN_STACK_ENABLE
kasan, arm64: only init shadow for software modes
kasan, arm64: only use kasan_depth for software modes
kasan, arm64: move initialization message
kasan, arm64: rename kasan_init_tags and mark as __init
kasan: rename addr_has_shadow to addr_has_metadata
kasan: rename print_shadow_for_address to print_memory_metadata
kasan: kasan_non_canonical_hook only for software modes
kasan: rename SHADOW layout macros to META
kasan: separate metadata_fetch_row for each mode
kasan, arm64: don't allow SW_TAGS with ARM64_MTE
kasan: introduce CONFIG_KASAN_HW_TAGS
arm64: kasan: Align allocations for HW_TAGS
arm64: kasan: Add arch layer for memory tagging helpers
kasan: define KASAN_GRANULE_SIZE for HW_TAGS
kasan, x86, s390: update undef CONFIG_KASAN
kasan, arm64: expand CONFIG_KASAN checks
kasan, arm64: implement HW_TAGS runtime
kasan, arm64: print report from tag fault handler
kasan, mm: reset tags when accessing metadata
kasan, arm64: enable CONFIG_KASAN_HW_TAGS
kasan: add documentation for hardware tag-based mode

Vincenzo Frascino (9):
arm64: Enable armv8.5-a asm-arch option
arm64: mte: Add in-kernel MTE helpers
arm64: mte: Reset the page tag in page->flags
arm64: mte: Add in-kernel tag fault handler
arm64: kasan: Allow enabling in-kernel MTE
arm64: mte: Convert gcr_user into an exclude mask
arm64: mte: Switch GCR_EL1 in kernel entry and exit
kasan, mm: untag page address in free_reserved_area
kselftest/arm64: Check GCR_EL1 after context switch

Documentation/dev-tools/kasan.rst | 80 ++-
arch/arm64/Kconfig | 9 +-
arch/arm64/Makefile | 7 +-
arch/arm64/include/asm/assembler.h | 2 +-
arch/arm64/include/asm/cache.h | 3 +
arch/arm64/include/asm/esr.h | 1 +
arch/arm64/include/asm/kasan.h | 5 +-
arch/arm64/include/asm/memory.h | 15 +-
arch/arm64/include/asm/mte-def.h | 14 +
arch/arm64/include/asm/mte-kasan.h | 67 +++
arch/arm64/include/asm/mte.h | 22 +-
arch/arm64/include/asm/processor.h | 2 +-
arch/arm64/include/asm/string.h | 5 +-
arch/arm64/include/asm/uaccess.h | 23 +
arch/arm64/kernel/asm-offsets.c | 3 +
arch/arm64/kernel/cpufeature.c | 3 +
arch/arm64/kernel/entry.S | 41 ++
arch/arm64/kernel/head.S | 2 +-
arch/arm64/kernel/hibernate.c | 5 +
arch/arm64/kernel/image-vars.h | 2 +-
arch/arm64/kernel/kaslr.c | 3 +-
arch/arm64/kernel/module.c | 6 +-
arch/arm64/kernel/mte.c | 118 +++-
arch/arm64/kernel/setup.c | 2 +-
arch/arm64/kernel/smp.c | 2 +
arch/arm64/lib/mte.S | 16 +
arch/arm64/mm/copypage.c | 1 +
arch/arm64/mm/fault.c | 59 ++
arch/arm64/mm/kasan_init.c | 19 +-
arch/arm64/mm/mteswap.c | 9 +
arch/arm64/mm/proc.S | 23 +-
arch/arm64/mm/ptdump.c | 6 +-
arch/s390/boot/string.c | 1 +
arch/s390/include/asm/kasan.h | 2 +
arch/x86/boot/compressed/misc.h | 1 +
include/linux/kasan-checks.h | 2 +-
include/linux/kasan.h | 125 ++--
include/linux/mm.h | 2 +-
include/linux/moduleloader.h | 3 +-
include/linux/page-flags-layout.h | 2 +-
include/linux/sched.h | 2 +-
include/linux/string.h | 2 +-
init/init_task.c | 2 +-
kernel/fork.c | 4 +-
lib/Kconfig.kasan | 62 +-
lib/test_kasan.c | 2 +-
lib/test_kasan_module.c | 2 +-
mm/kasan/Makefile | 25 +-
mm/kasan/common.c | 560 +-----------------
mm/kasan/generic.c | 40 +-
mm/kasan/generic_report.c | 165 ------
mm/kasan/hw_tags.c | 80 +++
mm/kasan/init.c | 17 +-
mm/kasan/kasan.h | 72 ++-
mm/kasan/quarantine.c | 10 -
mm/kasan/report.c | 259 ++------
mm/kasan/report_generic.c | 327 ++++++++++
mm/kasan/report_hw_tags.c | 42 ++
mm/kasan/{tags_report.c => report_sw_tags.c} | 14 +-
mm/kasan/shadow.c | 503 ++++++++++++++++
mm/kasan/{tags.c => sw_tags.c} | 24 +-
mm/page_alloc.c | 9 +-
mm/page_poison.c | 2 +-
mm/ptdump.c | 13 +-
mm/slab_common.c | 2 +-
mm/slub.c | 29 +-
scripts/Makefile.lib | 2 +
tools/testing/selftests/arm64/mte/Makefile | 2 +-
.../arm64/mte/check_gcr_el1_cswitch.c | 152 +++++
69 files changed, 1990 insertions(+), 1148 deletions(-)
create mode 100644 arch/arm64/include/asm/mte-def.h
create mode 100644 arch/arm64/include/asm/mte-kasan.h
delete mode 100644 mm/kasan/generic_report.c
create mode 100644 mm/kasan/hw_tags.c
create mode 100644 mm/kasan/report_generic.c
create mode 100644 mm/kasan/report_hw_tags.c
rename mm/kasan/{tags_report.c => report_sw_tags.c} (87%)
create mode 100644 mm/kasan/shadow.c
rename mm/kasan/{tags.c => sw_tags.c} (92%)
create mode 100644 tools/testing/selftests/arm64/mte/check_gcr_el1_cswitch.c

--
2.29.2.222.g5d2a92d10f8-goog

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