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kasan: docs: update SW_TAGS implementation details section 

Update the "Implementation details" section for SW_TAGS KASAN:

 - Clarify the introduction sentence.

 - Punctuation, readability, and other minor clean-ups.

Link: https://lkml.kernel.org/r/69b9b2e49d8cf789358fa24558be3fc0ce4ee32c.1615559068.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Andrey Konovalov 2021-04-29 23:00:30 -07:00 committed by Linus Torvalds
parent b8191d7d57
commit a6c18d4e76
1 changed files with 19 additions and 20 deletions
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39
Documentation/dev-tools/kasan.rst
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@ -244,38 +244,37 @@ quarantine (see mm/kasan/quarantine.c for implementation).
Software tag-based KASAN
~~~~~~~~~~~~~~~~~~~~~~~~
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 a software memory tagging approach to checking
access validity. It is currently only implemented for the 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).
to store a pointer tag in the top byte of kernel pointers. 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 software tag-based KASAN generates a random tag, tags
the allocated memory with this tag, and embeds this tag into the returned
On each memory allocation, software tag-based KASAN generates a random tag, tags
the allocated memory with this tag, and embeds the same 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 software tag-based KASAN prints a bug report.
before each memory access. These checks make sure that the tag of the memory
that is being accessed is equal to the tag of the pointer that is used to access
this 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
Software tag-based KASAN also has two instrumentation modes (outline, which
emits callbacks to check memory accesses; and inline, which performs the shadow
memory checks inline). With outline instrumentation mode, a bug report is
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.
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.
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
pointers with the 0xFF pointer tag are not checked). The value 0xFE is currently
reserved to tag freed memory regions.
Software tag-based KASAN currently only supports tagging of
kmem_cache_alloc/kmalloc and page_alloc memory.
Software tag-based KASAN currently only supports tagging of slab and page_alloc
memory.
Hardware tag-based KASAN
~~~~~~~~~~~~~~~~~~~~~~~~