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Commit b3b0e6ac authored by Andrey Konovalov's avatar Andrey Konovalov Committed by Linus Torvalds
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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.comSigned-off-by: default avatarAndrey Konovalov <andreyknvl@google.com>
Reviewed-by: default avatarAndrey Ryabinin <aryabinin@virtuozzo.com>
Reviewed-by: default avatarDmitry 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: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 2d4acb90
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Documentation/dev-tools/kasan.rst
View file @ b3b0e6ac
...@@ -4,15 +4,25 @@ The Kernel Address Sanitizer (KASAN) ...@@ -4,15 +4,25 @@ The Kernel Address Sanitizer (KASAN)
Overview Overview
-------- --------
KernelAddressSANitizer (KASAN) is a dynamic memory error detector. It provides KernelAddressSANitizer (KASAN) is a dynamic memory error detector designed to
a fast and comprehensive solution for finding use-after-free and out-of-bounds find out-of-bound and use-after-free bugs. KASAN has two modes: generic KASAN
bugs. (similar to userspace ASan) and software tag-based KASAN (similar to userspace
HWASan).
KASAN uses compile-time instrumentation for checking every memory access, KASAN uses compile-time instrumentation to insert validity checks before every
therefore you will need a GCC version 4.9.2 or later. GCC 5.0 or later is memory access, and therefore requires a compiler version that supports that.
required for detection of out-of-bounds accesses to stack or global variables.
Currently KASAN is supported only for the x86_64 and arm64 architectures. Generic KASAN is supported in both GCC and Clang. With GCC it requires version
4.9.2 or later for basic support and version 5.0 or later for detection of
out-of-bounds accesses for stack and global variables and for inline
instrumentation mode (see the Usage section). With Clang it requires version
7.0.0 or later and it doesn't support detection of out-of-bounds accesses for
global variables yet.
Tag-based KASAN is only supported in Clang and requires version 7.0.0 or later.
Currently generic KASAN is supported for the x86_64, arm64, xtensa and s390
architectures, and tag-based KASAN is supported only for arm64.
Usage Usage
----- -----
...@@ -21,12 +31,14 @@ To enable KASAN configure kernel with:: ...@@ -21,12 +31,14 @@ To enable KASAN configure kernel with::
CONFIG_KASAN = y CONFIG_KASAN = y
and choose between CONFIG_KASAN_OUTLINE and CONFIG_KASAN_INLINE. Outline and and choose between CONFIG_KASAN_GENERIC (to enable generic KASAN) and
inline are compiler instrumentation types. The former produces smaller binary CONFIG_KASAN_SW_TAGS (to enable software tag-based KASAN).
the latter is 1.1 - 2 times faster. Inline instrumentation requires a GCC
version 5.0 or later. You also need to choose between CONFIG_KASAN_OUTLINE and CONFIG_KASAN_INLINE.
Outline and inline are compiler instrumentation types. The former produces
smaller binary while the latter is 1.1 - 2 times faster.
KASAN works with both SLUB and SLAB memory allocators. Both KASAN modes work with both SLUB and SLAB memory allocators.
For better bug detection and nicer reporting, enable CONFIG_STACKTRACE. For better bug detection and nicer reporting, enable CONFIG_STACKTRACE.
To disable instrumentation for specific files or directories, add a line To disable instrumentation for specific files or directories, add a line
...@@ -43,85 +55,85 @@ similar to the following to the respective kernel Makefile: ...@@ -43,85 +55,85 @@ similar to the following to the respective kernel Makefile:
Error reports Error reports
~~~~~~~~~~~~~ ~~~~~~~~~~~~~
A typical out of bounds access report looks like this:: A typical out-of-bounds access generic KASAN report looks like this::
================================================================== ==================================================================
BUG: AddressSanitizer: out of bounds access in kmalloc_oob_right+0x65/0x75 [test_kasan] at addr ffff8800693bc5d3 BUG: KASAN: slab-out-of-bounds in kmalloc_oob_right+0xa8/0xbc [test_kasan]
Write of size 1 by task modprobe/1689 Write of size 1 at addr ffff8801f44ec37b by task insmod/2760
=============================================================================
BUG kmalloc-128 (Not tainted): kasan error CPU: 1 PID: 2760 Comm: insmod Not tainted 4.19.0-rc3+ #698
----------------------------------------------------------------------------- Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
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: Call Trace:
[<ffffffff81cc68ae>] dump_stack+0x46/0x58 dump_stack+0x94/0xd8
[<ffffffff811fd848>] print_trailer+0xf8/0x160 print_address_description+0x73/0x280
[<ffffffffa00026a7>] ? kmem_cache_oob+0xc3/0xc3 [test_kasan] kasan_report+0x144/0x187
[<ffffffff811ff0f5>] object_err+0x35/0x40 __asan_report_store1_noabort+0x17/0x20
[<ffffffffa0002065>] ? kmalloc_oob_right+0x65/0x75 [test_kasan] kmalloc_oob_right+0xa8/0xbc [test_kasan]
[<ffffffff8120b9fa>] kasan_report_error+0x38a/0x3f0 kmalloc_tests_init+0x16/0x700 [test_kasan]
[<ffffffff8120a79f>] ? kasan_poison_shadow+0x2f/0x40 do_one_initcall+0xa5/0x3ae
[<ffffffff8120b344>] ? kasan_unpoison_shadow+0x14/0x40 do_init_module+0x1b6/0x547
[<ffffffff8120a79f>] ? kasan_poison_shadow+0x2f/0x40 load_module+0x75df/0x8070
[<ffffffffa00026a7>] ? kmem_cache_oob+0xc3/0xc3 [test_kasan] __do_sys_init_module+0x1c6/0x200
[<ffffffff8120a995>] __asan_store1+0x75/0xb0 __x64_sys_init_module+0x6e/0xb0
[<ffffffffa0002601>] ? kmem_cache_oob+0x1d/0xc3 [test_kasan] do_syscall_64+0x9f/0x2c0
[<ffffffffa0002065>] ? kmalloc_oob_right+0x65/0x75 [test_kasan] entry_SYSCALL_64_after_hwframe+0x44/0xa9
[<ffffffffa0002065>] kmalloc_oob_right+0x65/0x75 [test_kasan] RIP: 0033:0x7f96443109da
[<ffffffffa00026b0>] init_module+0x9/0x47 [test_kasan] RSP: 002b:00007ffcf0b51b08 EFLAGS: 00000202 ORIG_RAX: 00000000000000af
[<ffffffff810002d9>] do_one_initcall+0x99/0x200 RAX: ffffffffffffffda RBX: 000055dc3ee521a0 RCX: 00007f96443109da
[<ffffffff811e4e5c>] ? __vunmap+0xec/0x160 RDX: 00007f96445cff88 RSI: 0000000000057a50 RDI: 00007f9644992000
[<ffffffff81114f63>] load_module+0x2cb3/0x3b20 RBP: 000055dc3ee510b0 R08: 0000000000000003 R09: 0000000000000000
[<ffffffff8110fd70>] ? m_show+0x240/0x240 R10: 00007f964430cd0a R11: 0000000000000202 R12: 00007f96445cff88
[<ffffffff81115f06>] SyS_finit_module+0x76/0x80 R13: 000055dc3ee51090 R14: 0000000000000000 R15: 0000000000000000
[<ffffffff81cd3129>] system_call_fastpath+0x12/0x17
Allocated by task 2760:
save_stack+0x43/0xd0
kasan_kmalloc+0xa7/0xd0
kmem_cache_alloc_trace+0xe1/0x1b0
kmalloc_oob_right+0x56/0xbc [test_kasan]
kmalloc_tests_init+0x16/0x700 [test_kasan]
do_one_initcall+0xa5/0x3ae
do_init_module+0x1b6/0x547
load_module+0x75df/0x8070
__do_sys_init_module+0x1c6/0x200
__x64_sys_init_module+0x6e/0xb0
do_syscall_64+0x9f/0x2c0
entry_SYSCALL_64_after_hwframe+0x44/0xa9
Freed by task 815:
save_stack+0x43/0xd0
__kasan_slab_free+0x135/0x190
kasan_slab_free+0xe/0x10
kfree+0x93/0x1a0
umh_complete+0x6a/0xa0
call_usermodehelper_exec_async+0x4c3/0x640
ret_from_fork+0x35/0x40
The buggy address belongs to the object at ffff8801f44ec300
which belongs to the cache kmalloc-128 of size 128
The buggy address is located 123 bytes inside of
128-byte region [ffff8801f44ec300, ffff8801f44ec380)
The buggy address belongs to the page:
page:ffffea0007d13b00 count:1 mapcount:0 mapping:ffff8801f7001640 index:0x0
flags: 0x200000000000100(slab)
raw: 0200000000000100 ffffea0007d11dc0 0000001a0000001a ffff8801f7001640
raw: 0000000000000000 0000000080150015 00000001ffffffff 0000000000000000
page dumped because: kasan: bad access detected
Memory state around the buggy address: Memory state around the buggy address:
ffff8800693bc300: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc ffff8801f44ec200: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
ffff8800693bc380: fc fc 00 00 00 00 00 00 00 00 00 00 00 00 00 fc ffff8801f44ec280: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
ffff8800693bc400: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc >ffff8801f44ec300: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 03
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 ffff8801f44ec380: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
ffff8800693bc680: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc ffff8801f44ec400: fb fb fb fb fb fb fb fb 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 The header of the report provides a short summary of what kind of bug happened
access caused it. It's followed by the description of the accessed slub object and what kind of access caused it. It's followed by a stack trace of the bad
(see 'SLUB Debug output' section in Documentation/vm/slub.rst for details) and access, a stack trace of where the accessed memory was allocated (in case bad
the description of the accessed memory page. access happens on a slab object), and a stack trace of where the object was
freed (in case of a use-after-free bug report). Next comes a description of
the accessed slab object and information about the accessed memory page.
In the last section the report shows memory state around the accessed address. In the last section the report shows memory state around the accessed address.
Reading this part requires some understanding of how KASAN works. Reading this part requires some understanding of how KASAN works.
...@@ -138,18 +150,24 @@ inaccessible memory like redzones or freed memory (see mm/kasan/kasan.h). ...@@ -138,18 +150,24 @@ 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 In the report above the arrows point to the shadow byte 03, which means that
the accessed address is partially accessible. the accessed address is partially accessible.
For tag-based KASAN this last report section shows the memory tags around the
accessed address (see Implementation details section).
Implementation details Implementation details
---------------------- ----------------------
Generic KASAN
~~~~~~~~~~~~~
From a high level, our approach to memory error detection is similar to that 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 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 to access, and use compile-time instrumentation to insert checks of shadow
memory access. memory on each memory access.
AddressSanitizer dedicates 1/8 of kernel memory to its shadow memory Generic KASAN dedicates 1/8th of kernel memory to its shadow memory (e.g. 16TB
(e.g. 16TB to cover 128TB on x86_64) and uses direct mapping with a scale and to cover 128TB on x86_64) and uses direct mapping with a scale and offset to
offset to translate a memory address to its corresponding shadow address. translate a memory address to its corresponding shadow address.
Here is the function which translates an address to its corresponding shadow Here is the function which translates an address to its corresponding shadow
address:: address::
...@@ -162,12 +180,38 @@ address:: ...@@ -162,12 +180,38 @@ address::
where ``KASAN_SHADOW_SCALE_SHIFT = 3``. where ``KASAN_SHADOW_SCALE_SHIFT = 3``.
Compile-time instrumentation used for checking memory accesses. Compiler inserts Compile-time instrumentation is used to insert memory access checks. Compiler
function calls (__asan_load*(addr), __asan_store*(addr)) before each memory inserts function calls (__asan_load*(addr), __asan_store*(addr)) before each
access of size 1, 2, 4, 8 or 16. These functions check whether memory access is memory access of size 1, 2, 4, 8 or 16. These functions check whether memory
valid or not by checking corresponding shadow memory. access is valid or not by checking corresponding shadow memory.
GCC 5.0 has possibility to perform inline instrumentation. Instead of making GCC 5.0 has possibility to perform inline instrumentation. Instead of making
function calls GCC directly inserts the code to check the shadow memory. function calls GCC directly inserts the code to check the shadow memory.
This option significantly enlarges kernel but it gives x1.1-x2 performance This option significantly enlarges kernel but it gives x1.1-x2 performance
boost over outline instrumented kernel. boost over outline instrumented kernel.
Software tag-based KASAN
~~~~~~~~~~~~~~~~~~~~~~~~
Tag-based KASAN uses the Top Byte Ignore (TBI) feature of modern arm64 CPUs to
store a pointer tag in the top byte of kernel pointers. Like generic KASAN it
uses shadow memory to store memory tags associated with each 16-byte memory
cell (therefore it dedicates 1/16th of the kernel memory for shadow memory).
On each memory allocation tag-based KASAN generates a random tag, tags the
allocated memory with this tag, and embeds this tag into the returned pointer.
Software tag-based KASAN uses compile-time instrumentation to insert checks
before each memory access. These checks make sure that tag of the memory that
is being accessed is equal to tag of the pointer that is used to access this
memory. In case of a tag mismatch tag-based KASAN prints a bug report.
Software tag-based KASAN also has two instrumentation modes (outline, that
emits callbacks to check memory accesses; and inline, that performs the shadow
memory checks inline). With outline instrumentation mode, a bug report is
simply printed from the function that performs the access check. With inline
instrumentation a brk instruction is emitted by the compiler, and a dedicated
brk handler is used to print bug reports.
A potential expansion of this mode is a hardware tag-based mode, which would
use hardware memory tagging support instead of compiler instrumentation and
manual shadow memory manipulation.
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