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Subject[PATCH v3 00/39] kasan: add hardware tag-based mode for arm64
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 <>.

This patchset is available here:

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

This patchset is based on the v10 of the user MTE patchset [3].

This patchset essentially consists of four parts:

1. Rework KASAN code to allow easier integration of the hardware tag-based
2. Introduce config option for the new mode.
3. Introduce core in-kernel MTE routines.
4. Combine the previous parts together to implement the new mode.

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.

[3] git:// for-next/mte

====== 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

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
* 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:

====== History

Changes v2->v3:
- 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.
- 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.
- Move kasan_enable/disable_current() declarations to simplify
- 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
- Don't allow software tags with MTE via arch/arm64/Kconfig instead of
- 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
- 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
- Mark gcr_kernel_excl as __ro_after_init.
- Use GENMASK() in mte_init_tags().

Andrey Konovalov (32):
kasan: drop unnecessary GPL text from comment headers
kasan: group vmalloc code
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: 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: don't allow SW_TAGS with ARM64_MTE
kasan: introduce CONFIG_KASAN_HW_TAGS
arm64: kasan: Add arch layer for memory tagging helpers
arm64: kasan: Align allocations for HW_TAGS
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, slub: reset tags when accessing metadata
kasan, arm64: enable CONFIG_KASAN_HW_TAGS
kasan: add documentation for hardware tag-based mode

Vincenzo Frascino (7):
arm64: Enable armv8.5-a asm-arch option
arm64: mte: Add in-kernel MTE helpers
arm64: mte: Add in-kernel tag fault handler
arm64: kasan: Enable in-kernel MTE
arm64: mte: Convert gcr_user into an exclude mask
arm64: mte: Switch GCR_EL1 in kernel entry and exit
arm64: kasan: Enable TBI EL1

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 | 8 +-
arch/arm64/include/asm/memory.h | 14 +-
arch/arm64/include/asm/mte-kasan.h | 66 +++
arch/arm64/include/asm/mte.h | 19 +-
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 | 10 +
arch/arm64/kernel/entry.S | 47 ++
arch/arm64/kernel/head.S | 2 +-
arch/arm64/kernel/image-vars.h | 2 +-
arch/arm64/kernel/mte.c | 85 ++-
arch/arm64/kernel/setup.c | 5 +-
arch/arm64/lib/mte.S | 19 +
arch/arm64/mm/dump.c | 6 +-
arch/arm64/mm/fault.c | 52 +-
arch/arm64/mm/kasan_init.c | 22 +-
arch/arm64/mm/proc.S | 24 +-
arch/s390/boot/string.c | 1 +
arch/x86/boot/compressed/misc.h | 1 +
include/linux/kasan-checks.h | 2 +-
include/linux/kasan.h | 104 ++--
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 | 66 ++-
lib/test_kasan.c | 2 +-
mm/kasan/Makefile | 25 +-
mm/kasan/common.c | 560 +------------------
mm/kasan/generic.c | 38 +-
mm/kasan/generic_report.c | 165 ------
mm/kasan/hw_tags.c | 70 +++
mm/kasan/init.c | 17 +-
mm/kasan/kasan.h | 64 ++-
mm/kasan/quarantine.c | 10 -
mm/kasan/report.c | 259 ++-------
mm/kasan/report_generic.c | 326 +++++++++++
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} | 18 +-
mm/page_poison.c | 2 +-
mm/ptdump.c | 13 +-
mm/slab_common.c | 2 +-
mm/slub.c | 25 +-
scripts/Makefile.lib | 2 +
57 files changed, 1727 insertions(+), 1137 deletions(-)
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} (94%)


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