KernelAddressSanitizer (KASAN) ============================== 0. Overview =========== 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 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 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 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. To disable instrumentation for specific files or directories, add a line similar to the following to the respective kernel Makefile: For a single file (e.g. main.o): KASAN_SANITIZE_main.o := n For all files in one directory: KASAN_SANITIZE := n 1.1 Error reports ================= A typical out of bounds access report looks like this: ================================================================== BUG: AddressSanitizer: out of bounds access in kmalloc_oob_right+0x65/0x75 [test_kasan] at addr ffff8800693bc5d3 Write of size 1 by task modprobe/1689 ============================================================================= BUG kmalloc-128 (Not tainted): kasan error ----------------------------------------------------------------------------- Disabling lock debugging due to kernel taint INFO: Allocated in kmalloc_oob_right+0x3d/0x75 [test_kasan] age=0 cpu=0 pid=1689 __slab_alloc+0x4b4/0x4f0 kmem_cache_alloc_trace+0x10b/0x190 kmalloc_oob_right+0x3d/0x75 [test_kasan] init_module+0x9/0x47 [test_kasan] do_one_initcall+0x99/0x200 load_module+0x2cb3/0x3b20 SyS_finit_module+0x76/0x80 system_call_fastpath+0x12/0x17 INFO: Slab 0xffffea0001a4ef00 objects=17 used=7 fp=0xffff8800693bd728 flags=0x100000000004080 INFO: Object 0xffff8800693bc558 @offset=1368 fp=0xffff8800693bc720 Bytes b4 ffff8800693bc548: 00 00 00 00 00 00 00 00 5a 5a 5a 5a 5a 5a 5a 5a ........ZZZZZZZZ Object ffff8800693bc558: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk Object ffff8800693bc568: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk Object ffff8800693bc578: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk Object ffff8800693bc588: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk Object ffff8800693bc598: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk Object ffff8800693bc5a8: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk Object ffff8800693bc5b8: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk Object ffff8800693bc5c8: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b a5 kkkkkkkkkkkkkkk. Redzone ffff8800693bc5d8: cc cc cc cc cc cc cc cc ........ Padding ffff8800693bc718: 5a 5a 5a 5a 5a 5a 5a 5a ZZZZZZZZ CPU: 0 PID: 1689 Comm: modprobe Tainted: G B 3.18.0-rc1-mm1+ #98 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.7.5-0-ge51488c-20140602_164612-nilsson.home.kraxel.org 04/01/2014 ffff8800693bc000 0000000000000000 ffff8800693bc558 ffff88006923bb78 ffffffff81cc68ae 00000000000000f3 ffff88006d407600 ffff88006923bba8 ffffffff811fd848 ffff88006d407600 ffffea0001a4ef00 ffff8800693bc558 Call Trace: [] dump_stack+0x46/0x58 [] print_trailer+0xf8/0x160 [] ? kmem_cache_oob+0xc3/0xc3 [test_kasan] [] object_err+0x35/0x40 [] ? kmalloc_oob_right+0x65/0x75 [test_kasan] [] kasan_report_error+0x38a/0x3f0 [] ? kasan_poison_shadow+0x2f/0x40 [] ? kasan_unpoison_shadow+0x14/0x40 [] ? kasan_poison_shadow+0x2f/0x40 [] ? kmem_cache_oob+0xc3/0xc3 [test_kasan] [] __asan_store1+0x75/0xb0 [] ? kmem_cache_oob+0x1d/0xc3 [test_kasan] [] ? kmalloc_oob_right+0x65/0x75 [test_kasan] [] kmalloc_oob_right+0x65/0x75 [test_kasan] [] init_module+0x9/0x47 [test_kasan] [] do_one_initcall+0x99/0x200 [] ? __vunmap+0xec/0x160 [] load_module+0x2cb3/0x3b20 [] ? m_show+0x240/0x240 [] SyS_finit_module+0x76/0x80 [] system_call_fastpath+0x12/0x17 Memory state around the buggy address: ffff8800693bc300: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc ffff8800693bc380: fc fc 00 00 00 00 00 00 00 00 00 00 00 00 00 fc ffff8800693bc400: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc ffff8800693bc480: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc ffff8800693bc500: fc fc fc fc fc fc fc fc fc fc fc 00 00 00 00 00 >ffff8800693bc580: 00 00 00 00 00 00 00 00 00 00 03 fc fc fc fc fc ^ ffff8800693bc600: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc ffff8800693bc680: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc ffff8800693bc700: fc fc fc fc fb fb fb fb fb fb fb fb fb fb fb fb ffff8800693bc780: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb ffff8800693bc800: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb ================================================================== 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 understanding of how KASAN works. 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; any negative value indicates that the entire 8-byte word is inaccessible. We use different negative values to distinguish between different kinds of inaccessible memory like redzones or freed memory (see mm/kasan/kasan.h). In the report above the arrows point to the shadow byte 03, which means that 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 to access, and use compile-time instrumentation to check shadow memory on each memory access. AddressSanitizer dedicates 1/8 of kernel memory to its shadow memory (e.g. 16TB to cover 128TB on x86_64) and uses direct mapping with a scale and offset to translate a memory address to its corresponding shadow address. Here is the function which translates an address to its corresponding shadow address: static inline void *kasan_mem_to_shadow(const void *addr) { return ((unsigned long)addr >> KASAN_SHADOW_SCALE_SHIFT) + KASAN_SHADOW_OFFSET; } where KASAN_SHADOW_SCALE_SHIFT = 3. Compile-time instrumentation used for checking memory accesses. Compiler inserts function calls (__asan_load*(addr), __asan_store*(addr)) before each memory access of size 1, 2, 4, 8 or 16. These functions check whether memory access is valid or not by checking corresponding shadow memory. GCC 5.0 has possibility to perform inline instrumentation. Instead of making function calls GCC directly inserts the code to check the shadow memory. This option significantly enlarges kernel but it gives x1.1-x2 performance boost over outline instrumented kernel.