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Andrey Konovalov0295fd52015-11-05 18:51:06 -08001KernelAddressSanitizer (KASAN)
2==============================
Andrey Ryabinin0b24bec2015-02-13 14:39:17 -08003
40. Overview
5===========
6
Andrey Konovalov0295fd52015-11-05 18:51:06 -08007KernelAddressSANitizer (KASAN) is a dynamic memory error detector. It provides
Andrey Ryabinin0b24bec2015-02-13 14:39:17 -08008a fast and comprehensive solution for finding use-after-free and out-of-bounds
9bugs.
10
Andrey Konovalov0295fd52015-11-05 18:51:06 -080011KASAN uses compile-time instrumentation for checking every memory access,
12therefore you will need a GCC version 4.9.2 or later. GCC 5.0 or later is
13required for detection of out-of-bounds accesses to stack or global variables.
Andrey Ryabinin0b24bec2015-02-13 14:39:17 -080014
Andrey Konovalov0295fd52015-11-05 18:51:06 -080015Currently KASAN is supported only for x86_64 architecture and requires the
16kernel to be built with the SLUB allocator.
Andrey Ryabinin0b24bec2015-02-13 14:39:17 -080017
181. Usage
Andrey Konovalov0295fd52015-11-05 18:51:06 -080019========
Andrey Ryabinin0b24bec2015-02-13 14:39:17 -080020
21To enable KASAN configure kernel with:
22
23 CONFIG_KASAN = y
24
Andrey Konovalov0295fd52015-11-05 18:51:06 -080025and choose between CONFIG_KASAN_OUTLINE and CONFIG_KASAN_INLINE. Outline and
26inline are compiler instrumentation types. The former produces smaller binary
27the latter is 1.1 - 2 times faster. Inline instrumentation requires a GCC
28version 5.0 or later.
Andrey Ryabinin0b24bec2015-02-13 14:39:17 -080029
30Currently KASAN works only with the SLUB memory allocator.
Andrey Ryabinin89d3c872015-11-05 18:51:23 -080031For better bug detection and nicer reporting, enable CONFIG_STACKTRACE.
Andrey Ryabinin0b24bec2015-02-13 14:39:17 -080032
33To disable instrumentation for specific files or directories, add a line
34similar to the following to the respective kernel Makefile:
35
36 For a single file (e.g. main.o):
37 KASAN_SANITIZE_main.o := n
38
39 For all files in one directory:
40 KASAN_SANITIZE := n
41
421.1 Error reports
Andrey Konovalov0295fd52015-11-05 18:51:06 -080043=================
Andrey Ryabinin0b24bec2015-02-13 14:39:17 -080044
45A typical out of bounds access report looks like this:
46
47==================================================================
48BUG: AddressSanitizer: out of bounds access in kmalloc_oob_right+0x65/0x75 [test_kasan] at addr ffff8800693bc5d3
49Write of size 1 by task modprobe/1689
50=============================================================================
51BUG kmalloc-128 (Not tainted): kasan error
52-----------------------------------------------------------------------------
53
54Disabling lock debugging due to kernel taint
55INFO: Allocated in kmalloc_oob_right+0x3d/0x75 [test_kasan] age=0 cpu=0 pid=1689
56 __slab_alloc+0x4b4/0x4f0
57 kmem_cache_alloc_trace+0x10b/0x190
58 kmalloc_oob_right+0x3d/0x75 [test_kasan]
59 init_module+0x9/0x47 [test_kasan]
60 do_one_initcall+0x99/0x200
61 load_module+0x2cb3/0x3b20
62 SyS_finit_module+0x76/0x80
63 system_call_fastpath+0x12/0x17
64INFO: Slab 0xffffea0001a4ef00 objects=17 used=7 fp=0xffff8800693bd728 flags=0x100000000004080
65INFO: Object 0xffff8800693bc558 @offset=1368 fp=0xffff8800693bc720
66
67Bytes b4 ffff8800693bc548: 00 00 00 00 00 00 00 00 5a 5a 5a 5a 5a 5a 5a 5a ........ZZZZZZZZ
68Object ffff8800693bc558: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
69Object ffff8800693bc568: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
70Object ffff8800693bc578: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
71Object ffff8800693bc588: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
72Object ffff8800693bc598: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
73Object ffff8800693bc5a8: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
74Object ffff8800693bc5b8: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
75Object ffff8800693bc5c8: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b a5 kkkkkkkkkkkkkkk.
76Redzone ffff8800693bc5d8: cc cc cc cc cc cc cc cc ........
77Padding ffff8800693bc718: 5a 5a 5a 5a 5a 5a 5a 5a ZZZZZZZZ
78CPU: 0 PID: 1689 Comm: modprobe Tainted: G B 3.18.0-rc1-mm1+ #98
79Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.7.5-0-ge51488c-20140602_164612-nilsson.home.kraxel.org 04/01/2014
80 ffff8800693bc000 0000000000000000 ffff8800693bc558 ffff88006923bb78
81 ffffffff81cc68ae 00000000000000f3 ffff88006d407600 ffff88006923bba8
82 ffffffff811fd848 ffff88006d407600 ffffea0001a4ef00 ffff8800693bc558
83Call Trace:
84 [<ffffffff81cc68ae>] dump_stack+0x46/0x58
85 [<ffffffff811fd848>] print_trailer+0xf8/0x160
86 [<ffffffffa00026a7>] ? kmem_cache_oob+0xc3/0xc3 [test_kasan]
87 [<ffffffff811ff0f5>] object_err+0x35/0x40
88 [<ffffffffa0002065>] ? kmalloc_oob_right+0x65/0x75 [test_kasan]
89 [<ffffffff8120b9fa>] kasan_report_error+0x38a/0x3f0
90 [<ffffffff8120a79f>] ? kasan_poison_shadow+0x2f/0x40
91 [<ffffffff8120b344>] ? kasan_unpoison_shadow+0x14/0x40
92 [<ffffffff8120a79f>] ? kasan_poison_shadow+0x2f/0x40
93 [<ffffffffa00026a7>] ? kmem_cache_oob+0xc3/0xc3 [test_kasan]
94 [<ffffffff8120a995>] __asan_store1+0x75/0xb0
95 [<ffffffffa0002601>] ? kmem_cache_oob+0x1d/0xc3 [test_kasan]
96 [<ffffffffa0002065>] ? kmalloc_oob_right+0x65/0x75 [test_kasan]
97 [<ffffffffa0002065>] kmalloc_oob_right+0x65/0x75 [test_kasan]
98 [<ffffffffa00026b0>] init_module+0x9/0x47 [test_kasan]
99 [<ffffffff810002d9>] do_one_initcall+0x99/0x200
100 [<ffffffff811e4e5c>] ? __vunmap+0xec/0x160
101 [<ffffffff81114f63>] load_module+0x2cb3/0x3b20
102 [<ffffffff8110fd70>] ? m_show+0x240/0x240
103 [<ffffffff81115f06>] SyS_finit_module+0x76/0x80
104 [<ffffffff81cd3129>] system_call_fastpath+0x12/0x17
105Memory state around the buggy address:
106 ffff8800693bc300: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
107 ffff8800693bc380: fc fc 00 00 00 00 00 00 00 00 00 00 00 00 00 fc
108 ffff8800693bc400: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
109 ffff8800693bc480: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
110 ffff8800693bc500: fc fc fc fc fc fc fc fc fc fc fc 00 00 00 00 00
111>ffff8800693bc580: 00 00 00 00 00 00 00 00 00 00 03 fc fc fc fc fc
112 ^
113 ffff8800693bc600: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
114 ffff8800693bc680: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
115 ffff8800693bc700: fc fc fc fc fb fb fb fb fb fb fb fb fb fb fb fb
116 ffff8800693bc780: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
117 ffff8800693bc800: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
118==================================================================
119
Andrey Konovalov0295fd52015-11-05 18:51:06 -0800120The header of the report discribe what kind of bug happened and what kind of
121access caused it. It's followed by the description of the accessed slub object
122(see 'SLUB Debug output' section in Documentation/vm/slub.txt for details) and
123the description of the accessed memory page.
Andrey Ryabinin0b24bec2015-02-13 14:39:17 -0800124
125In the last section the report shows memory state around the accessed address.
Andrey Konovalov0295fd52015-11-05 18:51:06 -0800126Reading this part requires some understanding of how KASAN works.
Andrey Ryabinin0b24bec2015-02-13 14:39:17 -0800127
Andrey Konovalov0295fd52015-11-05 18:51:06 -0800128The state of each 8 aligned bytes of memory is encoded in one shadow byte.
129Those 8 bytes can be accessible, partially accessible, freed or be a redzone.
Andrey Ryabinin0b24bec2015-02-13 14:39:17 -0800130We use the following encoding for each shadow byte: 0 means that all 8 bytes
131of the corresponding memory region are accessible; number N (1 <= N <= 7) means
132that the first N bytes are accessible, and other (8 - N) bytes are not;
133any negative value indicates that the entire 8-byte word is inaccessible.
134We use different negative values to distinguish between different kinds of
135inaccessible memory like redzones or freed memory (see mm/kasan/kasan.h).
136
137In the report above the arrows point to the shadow byte 03, which means that
138the accessed address is partially accessible.
139
140
1412. Implementation details
Andrey Konovalov0295fd52015-11-05 18:51:06 -0800142=========================
Andrey Ryabinin0b24bec2015-02-13 14:39:17 -0800143
144From a high level, our approach to memory error detection is similar to that
145of kmemcheck: use shadow memory to record whether each byte of memory is safe
146to access, and use compile-time instrumentation to check shadow memory on each
147memory access.
148
149AddressSanitizer dedicates 1/8 of kernel memory to its shadow memory
150(e.g. 16TB to cover 128TB on x86_64) and uses direct mapping with a scale and
151offset to translate a memory address to its corresponding shadow address.
152
Wang Longf66fa082015-04-23 07:47:02 +0800153Here is the function which translates an address to its corresponding shadow
Andrey Ryabinin0b24bec2015-02-13 14:39:17 -0800154address:
155
156static inline void *kasan_mem_to_shadow(const void *addr)
157{
158 return ((unsigned long)addr >> KASAN_SHADOW_SCALE_SHIFT)
159 + KASAN_SHADOW_OFFSET;
160}
161
162where KASAN_SHADOW_SCALE_SHIFT = 3.
163
164Compile-time instrumentation used for checking memory accesses. Compiler inserts
165function calls (__asan_load*(addr), __asan_store*(addr)) before each memory
166access of size 1, 2, 4, 8 or 16. These functions check whether memory access is
167valid or not by checking corresponding shadow memory.
168
169GCC 5.0 has possibility to perform inline instrumentation. Instead of making
170function calls GCC directly inserts the code to check the shadow memory.
171This option significantly enlarges kernel but it gives x1.1-x2 performance
172boost over outline instrumented kernel.