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1KernelAddressSanitizer (KASAN)
2==============================
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3
40. Overview
5===========
6
0295fd5d 7KernelAddressSANitizer (KASAN) is a dynamic memory error detector. It provides
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8a fast and comprehensive solution for finding use-after-free and out-of-bounds
9bugs.
10
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11KASAN 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.
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7ed2f9e6 15Currently KASAN is supported only for x86_64 architecture.
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16
171. Usage
0295fd5d 18========
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19
20To enable KASAN configure kernel with:
21
22 CONFIG_KASAN = y
23
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24and choose between CONFIG_KASAN_OUTLINE and CONFIG_KASAN_INLINE. Outline and
25inline are compiler instrumentation types. The former produces smaller binary
26the latter is 1.1 - 2 times faster. Inline instrumentation requires a GCC
27version 5.0 or later.
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7ed2f9e6 29KASAN works with both SLUB and SLAB memory allocators.
89d3c87e 30For better bug detection and nicer reporting, enable CONFIG_STACKTRACE.
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31
32To disable instrumentation for specific files or directories, add a line
33similar to the following to the respective kernel Makefile:
34
35 For a single file (e.g. main.o):
36 KASAN_SANITIZE_main.o := n
37
38 For all files in one directory:
39 KASAN_SANITIZE := n
40
411.1 Error reports
0295fd5d 42=================
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43
44A typical out of bounds access report looks like this:
45
46==================================================================
47BUG: AddressSanitizer: out of bounds access in kmalloc_oob_right+0x65/0x75 [test_kasan] at addr ffff8800693bc5d3
48Write of size 1 by task modprobe/1689
49=============================================================================
50BUG kmalloc-128 (Not tainted): kasan error
51-----------------------------------------------------------------------------
52
53Disabling lock debugging due to kernel taint
54INFO: Allocated in kmalloc_oob_right+0x3d/0x75 [test_kasan] age=0 cpu=0 pid=1689
55 __slab_alloc+0x4b4/0x4f0
56 kmem_cache_alloc_trace+0x10b/0x190
57 kmalloc_oob_right+0x3d/0x75 [test_kasan]
58 init_module+0x9/0x47 [test_kasan]
59 do_one_initcall+0x99/0x200
60 load_module+0x2cb3/0x3b20
61 SyS_finit_module+0x76/0x80
62 system_call_fastpath+0x12/0x17
63INFO: Slab 0xffffea0001a4ef00 objects=17 used=7 fp=0xffff8800693bd728 flags=0x100000000004080
64INFO: Object 0xffff8800693bc558 @offset=1368 fp=0xffff8800693bc720
65
66Bytes b4 ffff8800693bc548: 00 00 00 00 00 00 00 00 5a 5a 5a 5a 5a 5a 5a 5a ........ZZZZZZZZ
67Object ffff8800693bc558: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
68Object ffff8800693bc568: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
69Object ffff8800693bc578: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
70Object ffff8800693bc588: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
71Object ffff8800693bc598: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
72Object ffff8800693bc5a8: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
73Object ffff8800693bc5b8: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
74Object ffff8800693bc5c8: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b a5 kkkkkkkkkkkkkkk.
75Redzone ffff8800693bc5d8: cc cc cc cc cc cc cc cc ........
76Padding ffff8800693bc718: 5a 5a 5a 5a 5a 5a 5a 5a ZZZZZZZZ
77CPU: 0 PID: 1689 Comm: modprobe Tainted: G B 3.18.0-rc1-mm1+ #98
78Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.7.5-0-ge51488c-20140602_164612-nilsson.home.kraxel.org 04/01/2014
79 ffff8800693bc000 0000000000000000 ffff8800693bc558 ffff88006923bb78
80 ffffffff81cc68ae 00000000000000f3 ffff88006d407600 ffff88006923bba8
81 ffffffff811fd848 ffff88006d407600 ffffea0001a4ef00 ffff8800693bc558
82Call Trace:
83 [<ffffffff81cc68ae>] dump_stack+0x46/0x58
84 [<ffffffff811fd848>] print_trailer+0xf8/0x160
85 [<ffffffffa00026a7>] ? kmem_cache_oob+0xc3/0xc3 [test_kasan]
86 [<ffffffff811ff0f5>] object_err+0x35/0x40
87 [<ffffffffa0002065>] ? kmalloc_oob_right+0x65/0x75 [test_kasan]
88 [<ffffffff8120b9fa>] kasan_report_error+0x38a/0x3f0
89 [<ffffffff8120a79f>] ? kasan_poison_shadow+0x2f/0x40
90 [<ffffffff8120b344>] ? kasan_unpoison_shadow+0x14/0x40
91 [<ffffffff8120a79f>] ? kasan_poison_shadow+0x2f/0x40
92 [<ffffffffa00026a7>] ? kmem_cache_oob+0xc3/0xc3 [test_kasan]
93 [<ffffffff8120a995>] __asan_store1+0x75/0xb0
94 [<ffffffffa0002601>] ? kmem_cache_oob+0x1d/0xc3 [test_kasan]
95 [<ffffffffa0002065>] ? kmalloc_oob_right+0x65/0x75 [test_kasan]
96 [<ffffffffa0002065>] kmalloc_oob_right+0x65/0x75 [test_kasan]
97 [<ffffffffa00026b0>] init_module+0x9/0x47 [test_kasan]
98 [<ffffffff810002d9>] do_one_initcall+0x99/0x200
99 [<ffffffff811e4e5c>] ? __vunmap+0xec/0x160
100 [<ffffffff81114f63>] load_module+0x2cb3/0x3b20
101 [<ffffffff8110fd70>] ? m_show+0x240/0x240
102 [<ffffffff81115f06>] SyS_finit_module+0x76/0x80
103 [<ffffffff81cd3129>] system_call_fastpath+0x12/0x17
104Memory state around the buggy address:
105 ffff8800693bc300: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
106 ffff8800693bc380: fc fc 00 00 00 00 00 00 00 00 00 00 00 00 00 fc
107 ffff8800693bc400: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
108 ffff8800693bc480: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
109 ffff8800693bc500: fc fc fc fc fc fc fc fc fc fc fc 00 00 00 00 00
110>ffff8800693bc580: 00 00 00 00 00 00 00 00 00 00 03 fc fc fc fc fc
111 ^
112 ffff8800693bc600: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
113 ffff8800693bc680: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
114 ffff8800693bc700: fc fc fc fc fb fb fb fb fb fb fb fb fb fb fb fb
115 ffff8800693bc780: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
116 ffff8800693bc800: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
117==================================================================
118
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119The header of the report discribe what kind of bug happened and what kind of
120access caused it. It's followed by the description of the accessed slub object
121(see 'SLUB Debug output' section in Documentation/vm/slub.txt for details) and
122the description of the accessed memory page.
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123
124In the last section the report shows memory state around the accessed address.
0295fd5d 125Reading this part requires some understanding of how KASAN works.
0b24becc 126
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127The state of each 8 aligned bytes of memory is encoded in one shadow byte.
128Those 8 bytes can be accessible, partially accessible, freed or be a redzone.
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129We use the following encoding for each shadow byte: 0 means that all 8 bytes
130of the corresponding memory region are accessible; number N (1 <= N <= 7) means
131that the first N bytes are accessible, and other (8 - N) bytes are not;
132any negative value indicates that the entire 8-byte word is inaccessible.
133We use different negative values to distinguish between different kinds of
134inaccessible memory like redzones or freed memory (see mm/kasan/kasan.h).
135
136In the report above the arrows point to the shadow byte 03, which means that
137the accessed address is partially accessible.
138
139
1402. Implementation details
0295fd5d 141=========================
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142
143From a high level, our approach to memory error detection is similar to that
144of kmemcheck: use shadow memory to record whether each byte of memory is safe
145to access, and use compile-time instrumentation to check shadow memory on each
146memory access.
147
148AddressSanitizer dedicates 1/8 of kernel memory to its shadow memory
149(e.g. 16TB to cover 128TB on x86_64) and uses direct mapping with a scale and
150offset to translate a memory address to its corresponding shadow address.
151
f66fa08b 152Here is the function which translates an address to its corresponding shadow
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153address:
154
155static inline void *kasan_mem_to_shadow(const void *addr)
156{
157 return ((unsigned long)addr >> KASAN_SHADOW_SCALE_SHIFT)
158 + KASAN_SHADOW_OFFSET;
159}
160
161where KASAN_SHADOW_SCALE_SHIFT = 3.
162
163Compile-time instrumentation used for checking memory accesses. Compiler inserts
164function calls (__asan_load*(addr), __asan_store*(addr)) before each memory
165access of size 1, 2, 4, 8 or 16. These functions check whether memory access is
166valid or not by checking corresponding shadow memory.
167
168GCC 5.0 has possibility to perform inline instrumentation. Instead of making
169function calls GCC directly inserts the code to check the shadow memory.
170This option significantly enlarges kernel but it gives x1.1-x2 performance
171boost over outline instrumented kernel.