1KernelAddressSanitizer (KASAN) 2============================== 3 40. Overview 5=========== 6 7KernelAddressSANitizer (KASAN) is a dynamic memory error detector. It provides 8a fast and comprehensive solution for finding use-after-free and out-of-bounds 9bugs. 10 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. 14 15Currently KASAN is supported only for x86_64 architecture. 16 171. Usage 18======== 19 20To enable KASAN configure kernel with: 21 22 CONFIG_KASAN = y 23 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. 28 29KASAN works with both SLUB and SLAB memory allocators. 30For better bug detection and nicer reporting, enable CONFIG_STACKTRACE. 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 42================= 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 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. 123 124In the last section the report shows memory state around the accessed address. 125Reading this part requires some understanding of how KASAN works. 126 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. 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 141========================= 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 152Here is the function which translates an address to its corresponding shadow 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. 172