172 lines
8.1 KiB
Plaintext
172 lines
8.1 KiB
Plaintext
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.
|
|
|
|
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.
|
|
|
|
KASAN works with both SLUB and SLAB memory allocators.
|
|
For better bug detection and nicer reporting, enable CONFIG_STACKTRACE.
|
|
|
|
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:
|
|
[<ffffffff81cc68ae>] dump_stack+0x46/0x58
|
|
[<ffffffff811fd848>] print_trailer+0xf8/0x160
|
|
[<ffffffffa00026a7>] ? kmem_cache_oob+0xc3/0xc3 [test_kasan]
|
|
[<ffffffff811ff0f5>] object_err+0x35/0x40
|
|
[<ffffffffa0002065>] ? kmalloc_oob_right+0x65/0x75 [test_kasan]
|
|
[<ffffffff8120b9fa>] kasan_report_error+0x38a/0x3f0
|
|
[<ffffffff8120a79f>] ? kasan_poison_shadow+0x2f/0x40
|
|
[<ffffffff8120b344>] ? kasan_unpoison_shadow+0x14/0x40
|
|
[<ffffffff8120a79f>] ? kasan_poison_shadow+0x2f/0x40
|
|
[<ffffffffa00026a7>] ? kmem_cache_oob+0xc3/0xc3 [test_kasan]
|
|
[<ffffffff8120a995>] __asan_store1+0x75/0xb0
|
|
[<ffffffffa0002601>] ? kmem_cache_oob+0x1d/0xc3 [test_kasan]
|
|
[<ffffffffa0002065>] ? kmalloc_oob_right+0x65/0x75 [test_kasan]
|
|
[<ffffffffa0002065>] kmalloc_oob_right+0x65/0x75 [test_kasan]
|
|
[<ffffffffa00026b0>] init_module+0x9/0x47 [test_kasan]
|
|
[<ffffffff810002d9>] do_one_initcall+0x99/0x200
|
|
[<ffffffff811e4e5c>] ? __vunmap+0xec/0x160
|
|
[<ffffffff81114f63>] load_module+0x2cb3/0x3b20
|
|
[<ffffffff8110fd70>] ? m_show+0x240/0x240
|
|
[<ffffffff81115f06>] SyS_finit_module+0x76/0x80
|
|
[<ffffffff81cd3129>] 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.
|