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It's recommended to have slub's user tracking enabled with CONFIG_KASAN,
because:
a) User tracking disables slab merging which improves
detecting out-of-bounds accesses.
b) User tracking metadata acts as redzone which also improves
detecting out-of-bounds accesses.
c) User tracking provides additional information about object.
This information helps to understand bugs.
Currently it is not enabled by default. Besides recompiling the kernel
with KASAN and reinstalling it, user also have to change the boot cmdline,
which is not very handy.
Enable slub user tracking by default with KASAN=y, since there is no good
reason to not do this.
[akpm@linux-foundation.org: little fixes, per David]
Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
173 lines
8.2 KiB
Plaintext
173 lines
8.2 KiB
Plaintext
KernelAddressSanitizer (KASAN)
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==============================
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0. Overview
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===========
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KernelAddressSANitizer (KASAN) is a dynamic memory error detector. It provides
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a fast and comprehensive solution for finding use-after-free and out-of-bounds
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bugs.
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KASAN uses compile-time instrumentation for checking every memory access,
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therefore you will need a GCC version 4.9.2 or later. GCC 5.0 or later is
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required for detection of out-of-bounds accesses to stack or global variables.
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Currently KASAN is supported only for x86_64 architecture and requires the
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kernel to be built with the SLUB allocator.
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1. Usage
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========
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To enable KASAN configure kernel with:
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CONFIG_KASAN = y
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and choose between CONFIG_KASAN_OUTLINE and CONFIG_KASAN_INLINE. Outline and
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inline are compiler instrumentation types. The former produces smaller binary
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the latter is 1.1 - 2 times faster. Inline instrumentation requires a GCC
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version 5.0 or later.
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Currently KASAN works only with the SLUB memory allocator.
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For better bug detection and nicer reporting, enable CONFIG_STACKTRACE.
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To disable instrumentation for specific files or directories, add a line
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similar to the following to the respective kernel Makefile:
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For a single file (e.g. main.o):
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KASAN_SANITIZE_main.o := n
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For all files in one directory:
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KASAN_SANITIZE := n
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1.1 Error reports
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=================
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A typical out of bounds access report looks like this:
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==================================================================
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BUG: AddressSanitizer: out of bounds access in kmalloc_oob_right+0x65/0x75 [test_kasan] at addr ffff8800693bc5d3
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Write of size 1 by task modprobe/1689
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=============================================================================
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BUG kmalloc-128 (Not tainted): kasan error
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-----------------------------------------------------------------------------
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Disabling lock debugging due to kernel taint
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INFO: Allocated in kmalloc_oob_right+0x3d/0x75 [test_kasan] age=0 cpu=0 pid=1689
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__slab_alloc+0x4b4/0x4f0
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kmem_cache_alloc_trace+0x10b/0x190
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kmalloc_oob_right+0x3d/0x75 [test_kasan]
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init_module+0x9/0x47 [test_kasan]
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do_one_initcall+0x99/0x200
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load_module+0x2cb3/0x3b20
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SyS_finit_module+0x76/0x80
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system_call_fastpath+0x12/0x17
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INFO: Slab 0xffffea0001a4ef00 objects=17 used=7 fp=0xffff8800693bd728 flags=0x100000000004080
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INFO: Object 0xffff8800693bc558 @offset=1368 fp=0xffff8800693bc720
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Bytes b4 ffff8800693bc548: 00 00 00 00 00 00 00 00 5a 5a 5a 5a 5a 5a 5a 5a ........ZZZZZZZZ
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Object ffff8800693bc558: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
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Object ffff8800693bc568: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
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Object ffff8800693bc578: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
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Object ffff8800693bc588: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
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Object ffff8800693bc598: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
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Object ffff8800693bc5a8: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
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Object ffff8800693bc5b8: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
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Object ffff8800693bc5c8: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b a5 kkkkkkkkkkkkkkk.
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Redzone ffff8800693bc5d8: cc cc cc cc cc cc cc cc ........
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Padding ffff8800693bc718: 5a 5a 5a 5a 5a 5a 5a 5a ZZZZZZZZ
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CPU: 0 PID: 1689 Comm: modprobe Tainted: G B 3.18.0-rc1-mm1+ #98
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Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.7.5-0-ge51488c-20140602_164612-nilsson.home.kraxel.org 04/01/2014
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ffff8800693bc000 0000000000000000 ffff8800693bc558 ffff88006923bb78
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ffffffff81cc68ae 00000000000000f3 ffff88006d407600 ffff88006923bba8
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ffffffff811fd848 ffff88006d407600 ffffea0001a4ef00 ffff8800693bc558
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Call Trace:
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[<ffffffff81cc68ae>] dump_stack+0x46/0x58
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[<ffffffff811fd848>] print_trailer+0xf8/0x160
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[<ffffffffa00026a7>] ? kmem_cache_oob+0xc3/0xc3 [test_kasan]
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[<ffffffff811ff0f5>] object_err+0x35/0x40
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[<ffffffffa0002065>] ? kmalloc_oob_right+0x65/0x75 [test_kasan]
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[<ffffffff8120b9fa>] kasan_report_error+0x38a/0x3f0
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[<ffffffff8120a79f>] ? kasan_poison_shadow+0x2f/0x40
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[<ffffffff8120b344>] ? kasan_unpoison_shadow+0x14/0x40
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[<ffffffff8120a79f>] ? kasan_poison_shadow+0x2f/0x40
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[<ffffffffa00026a7>] ? kmem_cache_oob+0xc3/0xc3 [test_kasan]
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[<ffffffff8120a995>] __asan_store1+0x75/0xb0
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[<ffffffffa0002601>] ? kmem_cache_oob+0x1d/0xc3 [test_kasan]
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[<ffffffffa0002065>] ? kmalloc_oob_right+0x65/0x75 [test_kasan]
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[<ffffffffa0002065>] kmalloc_oob_right+0x65/0x75 [test_kasan]
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[<ffffffffa00026b0>] init_module+0x9/0x47 [test_kasan]
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[<ffffffff810002d9>] do_one_initcall+0x99/0x200
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[<ffffffff811e4e5c>] ? __vunmap+0xec/0x160
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[<ffffffff81114f63>] load_module+0x2cb3/0x3b20
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[<ffffffff8110fd70>] ? m_show+0x240/0x240
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[<ffffffff81115f06>] SyS_finit_module+0x76/0x80
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[<ffffffff81cd3129>] system_call_fastpath+0x12/0x17
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Memory state around the buggy address:
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ffff8800693bc300: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
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ffff8800693bc380: fc fc 00 00 00 00 00 00 00 00 00 00 00 00 00 fc
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ffff8800693bc400: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
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ffff8800693bc480: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
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ffff8800693bc500: fc fc fc fc fc fc fc fc fc fc fc 00 00 00 00 00
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>ffff8800693bc580: 00 00 00 00 00 00 00 00 00 00 03 fc fc fc fc fc
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^
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ffff8800693bc600: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
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ffff8800693bc680: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
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ffff8800693bc700: fc fc fc fc fb fb fb fb fb fb fb fb fb fb fb fb
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ffff8800693bc780: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
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ffff8800693bc800: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
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==================================================================
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The header of the report discribe what kind of bug happened and what kind of
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access caused it. It's followed by the description of the accessed slub object
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(see 'SLUB Debug output' section in Documentation/vm/slub.txt for details) and
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the description of the accessed memory page.
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In the last section the report shows memory state around the accessed address.
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Reading this part requires some understanding of how KASAN works.
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The state of each 8 aligned bytes of memory is encoded in one shadow byte.
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Those 8 bytes can be accessible, partially accessible, freed or be a redzone.
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We use the following encoding for each shadow byte: 0 means that all 8 bytes
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of the corresponding memory region are accessible; number N (1 <= N <= 7) means
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that the first N bytes are accessible, and other (8 - N) bytes are not;
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any negative value indicates that the entire 8-byte word is inaccessible.
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We use different negative values to distinguish between different kinds of
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inaccessible memory like redzones or freed memory (see mm/kasan/kasan.h).
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In the report above the arrows point to the shadow byte 03, which means that
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the accessed address is partially accessible.
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2. Implementation details
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=========================
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From a high level, our approach to memory error detection is similar to that
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of kmemcheck: use shadow memory to record whether each byte of memory is safe
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to access, and use compile-time instrumentation to check shadow memory on each
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memory access.
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AddressSanitizer dedicates 1/8 of kernel memory to its shadow memory
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(e.g. 16TB to cover 128TB on x86_64) and uses direct mapping with a scale and
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offset to translate a memory address to its corresponding shadow address.
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Here is the function which translates an address to its corresponding shadow
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address:
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static inline void *kasan_mem_to_shadow(const void *addr)
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{
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return ((unsigned long)addr >> KASAN_SHADOW_SCALE_SHIFT)
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+ KASAN_SHADOW_OFFSET;
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}
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where KASAN_SHADOW_SCALE_SHIFT = 3.
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Compile-time instrumentation used for checking memory accesses. Compiler inserts
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function calls (__asan_load*(addr), __asan_store*(addr)) before each memory
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access of size 1, 2, 4, 8 or 16. These functions check whether memory access is
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valid or not by checking corresponding shadow memory.
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GCC 5.0 has possibility to perform inline instrumentation. Instead of making
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function calls GCC directly inserts the code to check the shadow memory.
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This option significantly enlarges kernel but it gives x1.1-x2 performance
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boost over outline instrumented kernel.
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