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kasan: update documentation
This patch updates KASAN documentation to reflect the addition of the new tag-based mode. Link: http://lkml.kernel.org/r/aabef9de317c54b8a3919a4946ce534c6576726a.1544099024.git.andreyknvl@google.com Signed-off-by: Andrey Konovalov <andreyknvl@google.com> Reviewed-by: Andrey Ryabinin <aryabinin@virtuozzo.com> Reviewed-by: Dmitry Vyukov <dvyukov@google.com> Cc: Christoph Lameter <cl@linux.com> Cc: Mark Rutland <mark.rutland@arm.com> Cc: Will Deacon <will.deacon@arm.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Andrey Konovalov
Linus Torvalds
parent
2d4acb9087
commit
b3b0e6accb
@ -4,15 +4,25 @@ The Kernel Address Sanitizer (KASAN)
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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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KernelAddressSANitizer (KASAN) is a dynamic memory error detector designed to
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find out-of-bound and use-after-free bugs. KASAN has two modes: generic KASAN
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(similar to userspace ASan) and software tag-based KASAN (similar to userspace
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HWASan).
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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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KASAN uses compile-time instrumentation to insert validity checks before every
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memory access, and therefore requires a compiler version that supports that.
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Currently KASAN is supported only for the x86_64 and arm64 architectures.
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Generic KASAN is supported in both GCC and Clang. With GCC it requires version
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4.9.2 or later for basic support and version 5.0 or later for detection of
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out-of-bounds accesses for stack and global variables and for inline
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instrumentation mode (see the Usage section). With Clang it requires version
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7.0.0 or later and it doesn't support detection of out-of-bounds accesses for
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global variables yet.
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Tag-based KASAN is only supported in Clang and requires version 7.0.0 or later.
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Currently generic KASAN is supported for the x86_64, arm64, xtensa and s390
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architectures, and tag-based KASAN is supported only for arm64.
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Usage
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-----
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@ -21,12 +31,14 @@ 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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and choose between CONFIG_KASAN_GENERIC (to enable generic KASAN) and
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CONFIG_KASAN_SW_TAGS (to enable software tag-based KASAN).
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KASAN works with both SLUB and SLAB memory allocators.
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You also need to choose between CONFIG_KASAN_OUTLINE and CONFIG_KASAN_INLINE.
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Outline and inline are compiler instrumentation types. The former produces
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smaller binary while the latter is 1.1 - 2 times faster.
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Both KASAN modes work with both SLUB and SLAB memory allocators.
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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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@ -43,85 +55,85 @@ similar to the following to the respective kernel Makefile:
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Error reports
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~~~~~~~~~~~~~
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A typical out of bounds access report looks like this::
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A typical out-of-bounds access generic KASAN 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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BUG: KASAN: slab-out-of-bounds in kmalloc_oob_right+0xa8/0xbc [test_kasan]
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Write of size 1 at addr ffff8801f44ec37b by task insmod/2760
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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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CPU: 1 PID: 2760 Comm: insmod Not tainted 4.19.0-rc3+ #698
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Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
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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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dump_stack+0x94/0xd8
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print_address_description+0x73/0x280
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kasan_report+0x144/0x187
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__asan_report_store1_noabort+0x17/0x20
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kmalloc_oob_right+0xa8/0xbc [test_kasan]
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kmalloc_tests_init+0x16/0x700 [test_kasan]
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do_one_initcall+0xa5/0x3ae
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do_init_module+0x1b6/0x547
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load_module+0x75df/0x8070
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__do_sys_init_module+0x1c6/0x200
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__x64_sys_init_module+0x6e/0xb0
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do_syscall_64+0x9f/0x2c0
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entry_SYSCALL_64_after_hwframe+0x44/0xa9
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RIP: 0033:0x7f96443109da
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RSP: 002b:00007ffcf0b51b08 EFLAGS: 00000202 ORIG_RAX: 00000000000000af
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RAX: ffffffffffffffda RBX: 000055dc3ee521a0 RCX: 00007f96443109da
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RDX: 00007f96445cff88 RSI: 0000000000057a50 RDI: 00007f9644992000
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RBP: 000055dc3ee510b0 R08: 0000000000000003 R09: 0000000000000000
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R10: 00007f964430cd0a R11: 0000000000000202 R12: 00007f96445cff88
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R13: 000055dc3ee51090 R14: 0000000000000000 R15: 0000000000000000
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Allocated by task 2760:
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save_stack+0x43/0xd0
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kasan_kmalloc+0xa7/0xd0
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kmem_cache_alloc_trace+0xe1/0x1b0
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kmalloc_oob_right+0x56/0xbc [test_kasan]
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kmalloc_tests_init+0x16/0x700 [test_kasan]
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do_one_initcall+0xa5/0x3ae
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do_init_module+0x1b6/0x547
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load_module+0x75df/0x8070
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__do_sys_init_module+0x1c6/0x200
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__x64_sys_init_module+0x6e/0xb0
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do_syscall_64+0x9f/0x2c0
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entry_SYSCALL_64_after_hwframe+0x44/0xa9
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Freed by task 815:
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save_stack+0x43/0xd0
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__kasan_slab_free+0x135/0x190
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kasan_slab_free+0xe/0x10
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kfree+0x93/0x1a0
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umh_complete+0x6a/0xa0
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call_usermodehelper_exec_async+0x4c3/0x640
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ret_from_fork+0x35/0x40
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The buggy address belongs to the object at ffff8801f44ec300
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which belongs to the cache kmalloc-128 of size 128
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The buggy address is located 123 bytes inside of
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128-byte region [ffff8801f44ec300, ffff8801f44ec380)
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The buggy address belongs to the page:
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page:ffffea0007d13b00 count:1 mapcount:0 mapping:ffff8801f7001640 index:0x0
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flags: 0x200000000000100(slab)
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raw: 0200000000000100 ffffea0007d11dc0 0000001a0000001a ffff8801f7001640
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raw: 0000000000000000 0000000080150015 00000001ffffffff 0000000000000000
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page dumped because: kasan: bad access detected
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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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ffff8801f44ec200: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
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ffff8801f44ec280: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
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>ffff8801f44ec300: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 03
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^
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ffff8801f44ec380: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
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ffff8801f44ec400: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
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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.rst for details) and
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the description of the accessed memory page.
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The header of the report provides a short summary of what kind of bug happened
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and what kind of access caused it. It's followed by a stack trace of the bad
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access, a stack trace of where the accessed memory was allocated (in case bad
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access happens on a slab object), and a stack trace of where the object was
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freed (in case of a use-after-free bug report). Next comes a description of
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the accessed slab object and information about 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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@ -138,18 +150,24 @@ 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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For tag-based KASAN this last report section shows the memory tags around the
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accessed address (see Implementation details section).
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Implementation details
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----------------------
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Generic KASAN
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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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to access, and use compile-time instrumentation to insert checks of shadow
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memory on each 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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Generic KASAN dedicates 1/8th of kernel memory to its shadow memory (e.g. 16TB
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to cover 128TB on x86_64) and uses direct mapping with a scale and offset to
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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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@ -162,12 +180,38 @@ address::
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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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Compile-time instrumentation is used to insert memory access checks. Compiler
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inserts function calls (__asan_load*(addr), __asan_store*(addr)) before each
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memory access of size 1, 2, 4, 8 or 16. These functions check whether memory
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access is 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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Software tag-based KASAN
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~~~~~~~~~~~~~~~~~~~~~~~~
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Tag-based KASAN uses the Top Byte Ignore (TBI) feature of modern arm64 CPUs to
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store a pointer tag in the top byte of kernel pointers. Like generic KASAN it
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uses shadow memory to store memory tags associated with each 16-byte memory
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cell (therefore it dedicates 1/16th of the kernel memory for shadow memory).
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On each memory allocation tag-based KASAN generates a random tag, tags the
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allocated memory with this tag, and embeds this tag into the returned pointer.
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Software tag-based KASAN uses compile-time instrumentation to insert checks
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before each memory access. These checks make sure that tag of the memory that
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is being accessed is equal to tag of the pointer that is used to access this
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memory. In case of a tag mismatch tag-based KASAN prints a bug report.
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Software tag-based KASAN also has two instrumentation modes (outline, that
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emits callbacks to check memory accesses; and inline, that performs the shadow
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memory checks inline). With outline instrumentation mode, a bug report is
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simply printed from the function that performs the access check. With inline
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instrumentation a brk instruction is emitted by the compiler, and a dedicated
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brk handler is used to print bug reports.
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A potential expansion of this mode is a hardware tag-based mode, which would
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use hardware memory tagging support instead of compiler instrumentation and
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manual shadow memory manipulation.
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