Documentation/dev-tools/kasan.rst

The Kernel Address Sanitizer (KASAN)
====================================

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 the x86_64 and arm64 architectures.

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

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.rst 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.


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.