From: Andrey Konovalov <andreyknvl@xxxxxxxxxx> Subject: 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@xxxxxxxxxx Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx> Reviewed-by: Marco Elver <elver@xxxxxxxxxx> Cc: Alexander Potapenko <glider@xxxxxxxxxx> Cc: Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- Documentation/dev-tools/kasan.rst | 39 +++++++++++++--------------- 1 file changed, 19 insertions(+), 20 deletions(-) --- a/Documentation/dev-tools/kasan.rst~kasan-docs-update-sw_tags-implementation-details-section +++ a/Documentation/dev-tools/kasan.rst @@ -244,38 +244,37 @@ quarantine (see mm/kasan/quarantine.c fo 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 ~~~~~~~~~~~~~~~~~~~~~~~~ _
