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SubjectRe: [PATCH v3 39/39] kasan: add documentation for hardware tag-based mode
On Fri, Sep 25, 2020 at 12:50AM +0200, Andrey Konovalov wrote:
> Add documentation for hardware tag-based KASAN mode and also add some
> clarifications for software tag-based mode.
>
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>

Reviewed-by: Marco Elver <elver@google.com>

> ---
> Change-Id: Ib46cb444cfdee44054628940a82f5139e10d0258
> ---
> Documentation/dev-tools/kasan.rst | 78 ++++++++++++++++++++++---------
> 1 file changed, 57 insertions(+), 21 deletions(-)
>
> diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
> index a3030fc6afe5..d2d47c82a7b9 100644
> --- a/Documentation/dev-tools/kasan.rst
> +++ b/Documentation/dev-tools/kasan.rst
> @@ -5,12 +5,14 @@ Overview
> --------
>
> KernelAddressSANitizer (KASAN) is a dynamic memory error detector designed to
> -find out-of-bound and use-after-free bugs. KASAN has two modes: generic KASAN
> -(similar to userspace ASan) and software tag-based KASAN (similar to userspace
> -HWASan).
> +find out-of-bound and use-after-free bugs. KASAN has three modes:
> +1. generic KASAN (similar to userspace ASan),
> +2. software tag-based KASAN (similar to userspace HWASan),
> +3. hardware tag-based KASAN (based on hardware memory tagging).
>
> -KASAN uses compile-time instrumentation to insert validity checks before every
> -memory access, and therefore requires a compiler version that supports that.
> +Software KASAN modes (1 and 2) use compile-time instrumentation to insert
> +validity checks before every memory access, and therefore require a compiler
> +version that supports that.
>
> Generic KASAN is supported in both GCC and Clang. With GCC it requires version
> 8.3.0 or later. With Clang it requires version 7.0.0 or later, but detection of
> @@ -19,7 +21,7 @@ out-of-bounds accesses for global variables is only supported since Clang 11.
> Tag-based KASAN is only supported in Clang and requires version 7.0.0 or later.
>
> Currently generic KASAN is supported for the x86_64, arm64, xtensa, s390 and
> -riscv architectures, and tag-based KASAN is supported only for arm64.
> +riscv architectures, and tag-based KASAN modes are supported only for arm64.
>
> Usage
> -----
> @@ -28,14 +30,16 @@ To enable KASAN configure kernel with::
>
> CONFIG_KASAN = y
>
> -and choose between CONFIG_KASAN_GENERIC (to enable generic KASAN) and
> -CONFIG_KASAN_SW_TAGS (to enable software tag-based KASAN).
> +and choose between CONFIG_KASAN_GENERIC (to enable generic KASAN),
> +CONFIG_KASAN_SW_TAGS (to enable software tag-based KASAN), and
> +CONFIG_KASAN_HW_TAGS (to enable hardware tag-based KASAN).
>
> -You also need to choose between CONFIG_KASAN_OUTLINE and CONFIG_KASAN_INLINE.
> -Outline and inline are compiler instrumentation types. The former produces
> -smaller binary while the latter is 1.1 - 2 times faster.
> +For software modes, you also need to choose between CONFIG_KASAN_OUTLINE and
> +CONFIG_KASAN_INLINE. Outline and inline are compiler instrumentation types.
> +The former produces smaller binary while the latter is 1.1 - 2 times faster.
>
> -Both KASAN modes work with both SLUB and SLAB memory allocators.
> +Both software KASAN modes work with both SLUB and SLAB memory allocators,
> +hardware tag-based KASAN currently only support SLUB.
> For better bug detection and nicer reporting, enable CONFIG_STACKTRACE.
>
> To augment reports with last allocation and freeing stack of the physical page,
> @@ -196,17 +200,24 @@ and the second to last.
> Software tag-based KASAN
> ~~~~~~~~~~~~~~~~~~~~~~~~
>
> -Tag-based KASAN uses the Top Byte Ignore (TBI) feature of modern arm64 CPUs to
> -store a pointer tag in the top byte of kernel pointers. Like generic KASAN it
> -uses shadow memory to store memory tags associated with each 16-byte memory
> +Software tag-based KASAN requires software memory tagging support in the form
> +of HWASan-like compiler instrumentation (see HWASan documentation for details).
> +
> +Software tag-based KASAN is currently only implemented for arm64 architecture.
> +
> +Software tag-based KASAN uses the Top Byte Ignore (TBI) feature of arm64 CPUs
> +to store a pointer tag in the top byte of kernel pointers. Like generic KASAN
> +it uses shadow memory to store memory tags associated with each 16-byte memory
> cell (therefore it dedicates 1/16th of the kernel memory for shadow memory).
>
> -On each memory allocation tag-based KASAN generates a random tag, tags the
> -allocated memory with this tag, and embeds this tag into the returned pointer.
> +On each memory allocation software tag-based KASAN generates a random tag, tags
> +the allocated memory with this tag, and embeds this tag into the returned
> +pointer.
> +
> Software tag-based KASAN uses compile-time instrumentation to insert checks
> before each memory access. These checks make sure that tag of the memory that
> is being accessed is equal to tag of the pointer that is used to access this
> -memory. In case of a tag mismatch tag-based KASAN prints a bug report.
> +memory. In case of a tag mismatch software tag-based KASAN prints a bug report.
>
> Software tag-based KASAN also has two instrumentation modes (outline, that
> emits callbacks to check memory accesses; and inline, that performs the shadow
> @@ -215,9 +226,34 @@ simply printed from the function that performs the access check. With inline
> instrumentation a brk instruction is emitted by the compiler, and a dedicated
> 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.
> +Software tag-based KASAN uses 0xFF as a match-all pointer tag (accesses through
> +pointers with 0xFF pointer tag aren't checked). The value 0xFE is currently
> +reserved to tag freed memory regions.
> +
> +Software tag-based KASAN currently only supports tagging of slab memory.
> +
> +Hardware tag-based KASAN
> +~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +Hardware tag-based KASAN is similar to the software mode in concept, but uses
> +hardware memory tagging support instead of compiler instrumentation and
> +shadow memory.
> +
> +Hardware tag-based KASAN is currently only implemented for arm64 architecture
> +and based on both arm64 Memory Tagging Extension (MTE) introduced in ARMv8.5
> +Instruction Set Architecture, and Top Byte Ignore (TBI).
> +
> +Special arm64 instructions are used to assign memory tags for each allocation.
> +Same tags are assigned to pointers to those allocations. On every memory
> +access, hardware makes sure that tag of the memory that is being accessed is
> +equal to tag of the pointer that is used to access this memory. In case of a
> +tag mismatch a fault is generated and a report is printed.
> +
> +Hardware tag-based KASAN uses 0xFF as a match-all pointer tag (accesses through
> +pointers with 0xFF pointer tag aren't checked). The value 0xFE is currently
> +reserved to tag freed memory regions.
> +
> +Hardware tag-based KASAN currently only supports tagging of slab memory.
>
> What memory accesses are sanitised by KASAN?
> --------------------------------------------
> --
> 2.28.0.681.g6f77f65b4e-goog
>

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