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diff --git a/Documentation/kasan.txt b/Documentation/kasan.txt index 0d32355a4c34..aa1e0c91e368 100644 --- a/Documentation/kasan.txt +++ b/Documentation/kasan.txt @@ -1,36 +1,34 @@ -Kernel address sanitizer -================ +KernelAddressSanitizer (KASAN) +============================== 0. Overview =========== -Kernel Address sanitizer (KASan) is a dynamic memory error detector. It provides +KernelAddressSANitizer (KASAN) is a dynamic memory error detector. It provides a fast and comprehensive solution for finding use-after-free and out-of-bounds bugs. -KASan uses compile-time instrumentation for checking every memory access, -therefore you will need a gcc version of 4.9.2 or later. KASan could detect out -of bounds accesses to stack or global variables, but only if gcc 5.0 or later was -used to built the kernel. +KASAN uses compile-time instrumentation for checking every memory access, +therefore you will need a GCC version 4.9.2 or later. GCC 5.0 or later is +required for detection of out-of-bounds accesses to stack or global variables. -Currently KASan is supported only for x86_64 architecture and requires that the -kernel be built with the SLUB allocator. +Currently KASAN is supported only for x86_64 architecture and requires the +kernel to be built with the SLUB allocator. 1. Usage -========= +======== To enable KASAN configure kernel with: CONFIG_KASAN = y -and choose between CONFIG_KASAN_OUTLINE and CONFIG_KASAN_INLINE. Outline/inline -is compiler instrumentation types. The former produces smaller binary the -latter is 1.1 - 2 times faster. Inline instrumentation requires a gcc version -of 5.0 or later. +and choose between CONFIG_KASAN_OUTLINE and CONFIG_KASAN_INLINE. Outline and +inline are compiler instrumentation types. The former produces smaller binary +the latter is 1.1 - 2 times faster. Inline instrumentation requires a GCC +version 5.0 or later. Currently KASAN works only with the SLUB memory allocator. -For better bug detection and nicer report, enable CONFIG_STACKTRACE and put -at least 'slub_debug=U' in the boot cmdline. +For better bug detection and nicer reporting, enable CONFIG_STACKTRACE. To disable instrumentation for specific files or directories, add a line similar to the following to the respective kernel Makefile: @@ -42,7 +40,7 @@ similar to the following to the respective kernel Makefile: KASAN_SANITIZE := n 1.1 Error reports -========== +================= A typical out of bounds access report looks like this: @@ -119,14 +117,16 @@ Memory state around the buggy address: ffff8800693bc800: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb ================================================================== -First sections describe slub object where bad access happened. -See 'SLUB Debug output' section in Documentation/vm/slub.txt for details. +The header of the report discribe what kind of bug happened and what kind of +access caused it. It's followed by the description of the accessed slub object +(see 'SLUB Debug output' section in Documentation/vm/slub.txt for details) and +the description of the accessed memory page. In the last section the report shows memory state around the accessed address. -Reading this part requires some more understanding of how KASAN works. +Reading this part requires some understanding of how KASAN works. -Each 8 bytes of memory are encoded in one shadow byte as accessible, -partially accessible, freed or they can be part of a redzone. +The state of each 8 aligned bytes of memory is encoded in one shadow byte. +Those 8 bytes can be accessible, partially accessible, freed or be a redzone. We use the following encoding for each shadow byte: 0 means that all 8 bytes of the corresponding memory region are accessible; number N (1 <= N <= 7) means that the first N bytes are accessible, and other (8 - N) bytes are not; @@ -139,7 +139,7 @@ the accessed address is partially accessible. 2. Implementation details -======================== +========================= From a high level, our approach to memory error detection is similar to that of kmemcheck: use shadow memory to record whether each byte of memory is safe |