1 .. SPDX-License-Identifier: GPL-2.0 !! 1 The Kernel Address Sanitizer (KASAN)
2 .. Copyright (C) 2023, Google LLC. !! 2 ====================================
3 <<
4 Kernel Address Sanitizer (KASAN) <<
5 ================================ <<
6 3
7 Overview 4 Overview
8 -------- 5 --------
9 6
10 Kernel Address Sanitizer (KASAN) is a dynamic !! 7 KernelAddressSANitizer (KASAN) is a dynamic memory safety error detector
11 designed to find out-of-bounds and use-after-f !! 8 designed to find out-of-bound and use-after-free bugs. KASAN has three modes:
12 <<
13 KASAN has three modes: <<
14 <<
15 1. Generic KASAN <<
16 2. Software Tag-Based KASAN <<
17 3. Hardware Tag-Based KASAN <<
18 <<
19 Generic KASAN, enabled with CONFIG_KASAN_GENER <<
20 debugging, similar to userspace ASan. This mod <<
21 architectures, but it has significant performa <<
22 <<
23 Software Tag-Based KASAN or SW_TAGS KASAN, ena <<
24 can be used for both debugging and dogfood tes <<
25 This mode is only supported for arm64, but its <<
26 using it for testing on memory-restricted devi <<
27 <<
28 Hardware Tag-Based KASAN or HW_TAGS KASAN, ena <<
29 is the mode intended to be used as an in-field <<
30 security mitigation. This mode only works on a <<
31 (Memory Tagging Extension), but it has low mem <<
32 thus can be used in production. <<
33 <<
34 For details about the memory and performance i <<
35 descriptions of the corresponding Kconfig opti <<
36 <<
37 The Generic and the Software Tag-Based modes a <<
38 software modes. The Software Tag-Based and the <<
39 referred to as the tag-based modes. <<
40 <<
41 Support <<
42 ------- <<
43 <<
44 Architectures <<
45 ~~~~~~~~~~~~~ <<
46 <<
47 Generic KASAN is supported on x86_64, arm, arm <<
48 and loongarch, and the tag-based KASAN modes a <<
49 <<
50 Compilers <<
51 ~~~~~~~~~ <<
52 <<
53 Software KASAN modes use compile-time instrume <<
54 before every memory access and thus require a <<
55 support for that. The Hardware Tag-Based mode <<
56 these checks but still requires a compiler ver <<
57 tagging instructions. <<
58 <<
59 Generic KASAN requires GCC version 8.3.0 or la <<
60 or any Clang version supported by the kernel. <<
61 <<
62 Software Tag-Based KASAN requires GCC 11+ <<
63 or any Clang version supported by the kernel. <<
64 <<
65 Hardware Tag-Based KASAN requires GCC 10+ or C <<
66 9
67 Memory types !! 10 1. generic KASAN (similar to userspace ASan),
68 ~~~~~~~~~~~~ !! 11 2. software tag-based KASAN (similar to userspace HWASan),
>> 12 3. hardware tag-based KASAN (based on hardware memory tagging).
>> 13
>> 14 Generic KASAN is mainly used for debugging due to a large memory overhead.
>> 15 Software tag-based KASAN can be used for dogfood testing as it has a lower
>> 16 memory overhead that allows using it with real workloads. Hardware tag-based
>> 17 KASAN comes with low memory and performance overheads and, therefore, can be
>> 18 used in production. Either as an in-field memory bug detector or as a security
>> 19 mitigation.
>> 20
>> 21 Software KASAN modes (#1 and #2) use compile-time instrumentation to insert
>> 22 validity checks before every memory access and, therefore, require a compiler
>> 23 version that supports that.
>> 24
>> 25 Generic KASAN is supported in GCC and Clang. With GCC, it requires version
>> 26 8.3.0 or later. Any supported Clang version is compatible, but detection of
>> 27 out-of-bounds accesses for global variables is only supported since Clang 11.
>> 28
>> 29 Software tag-based KASAN mode is only supported in Clang.
>> 30
>> 31 The hardware KASAN mode (#3) relies on hardware to perform the checks but
>> 32 still requires a compiler version that supports memory tagging instructions.
>> 33 This mode is supported in GCC 10+ and Clang 11+.
69 34
70 Generic KASAN supports finding bugs in all of !! 35 Both software KASAN modes work with SLUB and SLAB memory allocators,
71 stack, and global memory. !! 36 while the hardware tag-based KASAN currently only supports SLUB.
72 37
73 Software Tag-Based KASAN supports slab, page_a !! 38 Currently, generic KASAN is supported for the x86_64, arm, arm64, xtensa, s390,
74 !! 39 and riscv architectures, and tag-based KASAN modes are supported only for arm64.
75 Hardware Tag-Based KASAN supports slab, page_a <<
76 memory. <<
77 <<
78 For slab, both software KASAN modes support SL <<
79 Hardware Tag-Based KASAN only supports SLUB. <<
80 40
81 Usage 41 Usage
82 ----- 42 -----
83 43
84 To enable KASAN, configure the kernel with:: 44 To enable KASAN, configure the kernel with::
85 45
86 CONFIG_KASAN=y 46 CONFIG_KASAN=y
87 47
88 and choose between ``CONFIG_KASAN_GENERIC`` (t !! 48 and choose between ``CONFIG_KASAN_GENERIC`` (to enable generic KASAN),
89 ``CONFIG_KASAN_SW_TAGS`` (to enable Software T !! 49 ``CONFIG_KASAN_SW_TAGS`` (to enable software tag-based KASAN), and
90 ``CONFIG_KASAN_HW_TAGS`` (to enable Hardware T !! 50 ``CONFIG_KASAN_HW_TAGS`` (to enable hardware tag-based KASAN).
91 51
92 For the software modes, also choose between `` !! 52 For software modes, also choose between ``CONFIG_KASAN_OUTLINE`` and
93 ``CONFIG_KASAN_INLINE``. Outline and inline ar 53 ``CONFIG_KASAN_INLINE``. Outline and inline are compiler instrumentation types.
94 The former produces a smaller binary while the !! 54 The former produces a smaller binary while the latter is 1.1-2 times faster.
95 55
96 To include alloc and free stack traces of affe 56 To include alloc and free stack traces of affected slab objects into reports,
97 enable ``CONFIG_STACKTRACE``. To include alloc 57 enable ``CONFIG_STACKTRACE``. To include alloc and free stack traces of affected
98 physical pages, enable ``CONFIG_PAGE_OWNER`` a 58 physical pages, enable ``CONFIG_PAGE_OWNER`` and boot with ``page_owner=on``.
99 59
100 Boot parameters <<
101 ~~~~~~~~~~~~~~~ <<
102 <<
103 KASAN is affected by the generic ``panic_on_wa <<
104 When it is enabled, KASAN panics the kernel af <<
105 <<
106 By default, KASAN prints a bug report only for <<
107 With ``kasan_multi_shot``, KASAN prints a repo <<
108 effectively disables ``panic_on_warn`` for KAS <<
109 <<
110 Alternatively, independent of ``panic_on_warn` <<
111 parameter can be used to control panic and rep <<
112 <<
113 - ``kasan.fault=report``, ``=panic``, or ``=pa <<
114 to only print a KASAN report, panic the kern <<
115 invalid writes only (default: ``report``). T <<
116 ``kasan_multi_shot`` is enabled. Note that w <<
117 Hardware Tag-Based KASAN, ``kasan.fault=pani <<
118 asynchronously checked accesses (including r <<
119 <<
120 Software and Hardware Tag-Based KASAN modes (s <<
121 modes below) support altering stack trace coll <<
122 <<
123 - ``kasan.stacktrace=off`` or ``=on`` disables <<
124 traces collection (default: ``on``). <<
125 - ``kasan.stack_ring_size=<number of entries>` <<
126 in the stack ring (default: ``32768``). <<
127 <<
128 Hardware Tag-Based KASAN mode is intended for <<
129 mitigation. Therefore, it supports additional <<
130 disabling KASAN altogether or controlling its <<
131 <<
132 - ``kasan=off`` or ``=on`` controls whether KA <<
133 <<
134 - ``kasan.mode=sync``, ``=async`` or ``=asymm` <<
135 is configured in synchronous, asynchronous o <<
136 execution (default: ``sync``). <<
137 Synchronous mode: a bad access is detected i <<
138 check fault occurs. <<
139 Asynchronous mode: a bad access detection is <<
140 fault occurs, the information is stored in h <<
141 register for arm64). The kernel periodically <<
142 only reports tag faults during these checks. <<
143 Asymmetric mode: a bad access is detected sy <<
144 asynchronously on writes. <<
145 <<
146 - ``kasan.vmalloc=off`` or ``=on`` disables or <<
147 allocations (default: ``on``). <<
148 <<
149 - ``kasan.page_alloc.sample=<sampling interval <<
150 Nth page_alloc allocation with the order equ <<
151 ``kasan.page_alloc.sample.order``, where N i <<
152 parameter (default: ``1``, or tag every such <<
153 This parameter is intended to mitigate the p <<
154 by KASAN. <<
155 Note that enabling this parameter makes Hard <<
156 of allocations chosen by sampling and thus m <<
157 allocations. Use the default value for accur <<
158 <<
159 - ``kasan.page_alloc.sample.order=<minimum pag <<
160 order of allocations that are affected by sa <<
161 Only applies when ``kasan.page_alloc.sample` <<
162 than ``1``. <<
163 This parameter is intended to allow sampling <<
164 allocations, which is the biggest source of <<
165 <<
166 Error reports 60 Error reports
167 ~~~~~~~~~~~~~ 61 ~~~~~~~~~~~~~
168 62
169 A typical KASAN report looks like this:: 63 A typical KASAN report looks like this::
170 64
171 ========================================== 65 ==================================================================
172 BUG: KASAN: slab-out-of-bounds in kmalloc_ !! 66 BUG: KASAN: slab-out-of-bounds in kmalloc_oob_right+0xa8/0xbc [test_kasan]
173 Write of size 1 at addr ffff8801f44ec37b b 67 Write of size 1 at addr ffff8801f44ec37b by task insmod/2760
174 68
175 CPU: 1 PID: 2760 Comm: insmod Not tainted 69 CPU: 1 PID: 2760 Comm: insmod Not tainted 4.19.0-rc3+ #698
176 Hardware name: QEMU Standard PC (i440FX + 70 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
177 Call Trace: 71 Call Trace:
178 dump_stack+0x94/0xd8 72 dump_stack+0x94/0xd8
179 print_address_description+0x73/0x280 73 print_address_description+0x73/0x280
180 kasan_report+0x144/0x187 74 kasan_report+0x144/0x187
181 __asan_report_store1_noabort+0x17/0x20 75 __asan_report_store1_noabort+0x17/0x20
182 kmalloc_oob_right+0xa8/0xbc [kasan_test] !! 76 kmalloc_oob_right+0xa8/0xbc [test_kasan]
183 kmalloc_tests_init+0x16/0x700 [kasan_test !! 77 kmalloc_tests_init+0x16/0x700 [test_kasan]
184 do_one_initcall+0xa5/0x3ae 78 do_one_initcall+0xa5/0x3ae
185 do_init_module+0x1b6/0x547 79 do_init_module+0x1b6/0x547
186 load_module+0x75df/0x8070 80 load_module+0x75df/0x8070
187 __do_sys_init_module+0x1c6/0x200 81 __do_sys_init_module+0x1c6/0x200
188 __x64_sys_init_module+0x6e/0xb0 82 __x64_sys_init_module+0x6e/0xb0
189 do_syscall_64+0x9f/0x2c0 83 do_syscall_64+0x9f/0x2c0
190 entry_SYSCALL_64_after_hwframe+0x44/0xa9 84 entry_SYSCALL_64_after_hwframe+0x44/0xa9
191 RIP: 0033:0x7f96443109da 85 RIP: 0033:0x7f96443109da
192 RSP: 002b:00007ffcf0b51b08 EFLAGS: 0000020 86 RSP: 002b:00007ffcf0b51b08 EFLAGS: 00000202 ORIG_RAX: 00000000000000af
193 RAX: ffffffffffffffda RBX: 000055dc3ee521a 87 RAX: ffffffffffffffda RBX: 000055dc3ee521a0 RCX: 00007f96443109da
194 RDX: 00007f96445cff88 RSI: 0000000000057a5 88 RDX: 00007f96445cff88 RSI: 0000000000057a50 RDI: 00007f9644992000
195 RBP: 000055dc3ee510b0 R08: 000000000000000 89 RBP: 000055dc3ee510b0 R08: 0000000000000003 R09: 0000000000000000
196 R10: 00007f964430cd0a R11: 000000000000020 90 R10: 00007f964430cd0a R11: 0000000000000202 R12: 00007f96445cff88
197 R13: 000055dc3ee51090 R14: 000000000000000 91 R13: 000055dc3ee51090 R14: 0000000000000000 R15: 0000000000000000
198 92
199 Allocated by task 2760: 93 Allocated by task 2760:
200 save_stack+0x43/0xd0 94 save_stack+0x43/0xd0
201 kasan_kmalloc+0xa7/0xd0 95 kasan_kmalloc+0xa7/0xd0
202 kmem_cache_alloc_trace+0xe1/0x1b0 96 kmem_cache_alloc_trace+0xe1/0x1b0
203 kmalloc_oob_right+0x56/0xbc [kasan_test] !! 97 kmalloc_oob_right+0x56/0xbc [test_kasan]
204 kmalloc_tests_init+0x16/0x700 [kasan_test !! 98 kmalloc_tests_init+0x16/0x700 [test_kasan]
205 do_one_initcall+0xa5/0x3ae 99 do_one_initcall+0xa5/0x3ae
206 do_init_module+0x1b6/0x547 100 do_init_module+0x1b6/0x547
207 load_module+0x75df/0x8070 101 load_module+0x75df/0x8070
208 __do_sys_init_module+0x1c6/0x200 102 __do_sys_init_module+0x1c6/0x200
209 __x64_sys_init_module+0x6e/0xb0 103 __x64_sys_init_module+0x6e/0xb0
210 do_syscall_64+0x9f/0x2c0 104 do_syscall_64+0x9f/0x2c0
211 entry_SYSCALL_64_after_hwframe+0x44/0xa9 105 entry_SYSCALL_64_after_hwframe+0x44/0xa9
212 106
213 Freed by task 815: 107 Freed by task 815:
214 save_stack+0x43/0xd0 108 save_stack+0x43/0xd0
215 __kasan_slab_free+0x135/0x190 109 __kasan_slab_free+0x135/0x190
216 kasan_slab_free+0xe/0x10 110 kasan_slab_free+0xe/0x10
217 kfree+0x93/0x1a0 111 kfree+0x93/0x1a0
218 umh_complete+0x6a/0xa0 112 umh_complete+0x6a/0xa0
219 call_usermodehelper_exec_async+0x4c3/0x64 113 call_usermodehelper_exec_async+0x4c3/0x640
220 ret_from_fork+0x35/0x40 114 ret_from_fork+0x35/0x40
221 115
222 The buggy address belongs to the object at 116 The buggy address belongs to the object at ffff8801f44ec300
223 which belongs to the cache kmalloc-128 of 117 which belongs to the cache kmalloc-128 of size 128
224 The buggy address is located 123 bytes ins 118 The buggy address is located 123 bytes inside of
225 128-byte region [ffff8801f44ec300, ffff88 119 128-byte region [ffff8801f44ec300, ffff8801f44ec380)
226 The buggy address belongs to the page: 120 The buggy address belongs to the page:
227 page:ffffea0007d13b00 count:1 mapcount:0 m 121 page:ffffea0007d13b00 count:1 mapcount:0 mapping:ffff8801f7001640 index:0x0
228 flags: 0x200000000000100(slab) 122 flags: 0x200000000000100(slab)
229 raw: 0200000000000100 ffffea0007d11dc0 000 123 raw: 0200000000000100 ffffea0007d11dc0 0000001a0000001a ffff8801f7001640
230 raw: 0000000000000000 0000000080150015 000 124 raw: 0000000000000000 0000000080150015 00000001ffffffff 0000000000000000
231 page dumped because: kasan: bad access det 125 page dumped because: kasan: bad access detected
232 126
233 Memory state around the buggy address: 127 Memory state around the buggy address:
234 ffff8801f44ec200: fc fc fc fc fc fc fc fc 128 ffff8801f44ec200: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
235 ffff8801f44ec280: fb fb fb fb fb fb fb fb 129 ffff8801f44ec280: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
236 >ffff8801f44ec300: 00 00 00 00 00 00 00 00 130 >ffff8801f44ec300: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 03
237 131 ^
238 ffff8801f44ec380: fc fc fc fc fc fc fc fc 132 ffff8801f44ec380: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
239 ffff8801f44ec400: fb fb fb fb fb fb fb fb 133 ffff8801f44ec400: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
240 ========================================== 134 ==================================================================
241 135
242 The report header summarizes what kind of bug 136 The report header summarizes what kind of bug happened and what kind of access
243 caused it. It is followed by a stack trace of 137 caused it. It is followed by a stack trace of the bad access, a stack trace of
244 where the accessed memory was allocated (in ca 138 where the accessed memory was allocated (in case a slab object was accessed),
245 and a stack trace of where the object was free 139 and a stack trace of where the object was freed (in case of a use-after-free
246 bug report). Next comes a description of the a 140 bug report). Next comes a description of the accessed slab object and the
247 information about the accessed memory page. 141 information about the accessed memory page.
248 142
249 In the end, the report shows the memory state 143 In the end, the report shows the memory state around the accessed address.
250 Internally, KASAN tracks memory state separate 144 Internally, KASAN tracks memory state separately for each memory granule, which
251 is either 8 or 16 aligned bytes depending on K 145 is either 8 or 16 aligned bytes depending on KASAN mode. Each number in the
252 memory state section of the report shows the s 146 memory state section of the report shows the state of one of the memory
253 granules that surround the accessed address. 147 granules that surround the accessed address.
254 148
255 For Generic KASAN, the size of each memory gra !! 149 For generic KASAN, the size of each memory granule is 8. The state of each
256 granule is encoded in one shadow byte. Those 8 150 granule is encoded in one shadow byte. Those 8 bytes can be accessible,
257 partially accessible, freed, or be a part of a 151 partially accessible, freed, or be a part of a redzone. KASAN uses the following
258 encoding for each shadow byte: 00 means that a 152 encoding for each shadow byte: 00 means that all 8 bytes of the corresponding
259 memory region are accessible; number N (1 <= N 153 memory region are accessible; number N (1 <= N <= 7) means that the first N
260 bytes are accessible, and other (8 - N) bytes 154 bytes are accessible, and other (8 - N) bytes are not; any negative value
261 indicates that the entire 8-byte word is inacc 155 indicates that the entire 8-byte word is inaccessible. KASAN uses different
262 negative values to distinguish between differe 156 negative values to distinguish between different kinds of inaccessible memory
263 like redzones or freed memory (see mm/kasan/ka 157 like redzones or freed memory (see mm/kasan/kasan.h).
264 158
265 In the report above, the arrow points to the s 159 In the report above, the arrow points to the shadow byte ``03``, which means
266 that the accessed address is partially accessi 160 that the accessed address is partially accessible.
267 161
268 For tag-based KASAN modes, this last report se 162 For tag-based KASAN modes, this last report section shows the memory tags around
269 the accessed address (see the `Implementation 163 the accessed address (see the `Implementation details`_ section).
270 164
271 Note that KASAN bug titles (like ``slab-out-of 165 Note that KASAN bug titles (like ``slab-out-of-bounds`` or ``use-after-free``)
272 are best-effort: KASAN prints the most probabl 166 are best-effort: KASAN prints the most probable bug type based on the limited
273 information it has. The actual type of the bug 167 information it has. The actual type of the bug might be different.
274 168
275 Generic KASAN also reports up to two auxiliary 169 Generic KASAN also reports up to two auxiliary call stack traces. These stack
276 traces point to places in code that interacted 170 traces point to places in code that interacted with the object but that are not
277 directly present in the bad access stack trace 171 directly present in the bad access stack trace. Currently, this includes
278 call_rcu() and workqueue queuing. 172 call_rcu() and workqueue queuing.
279 173
280 CONFIG_KASAN_EXTRA_INFO !! 174 Boot parameters
281 ~~~~~~~~~~~~~~~~~~~~~~~ !! 175 ~~~~~~~~~~~~~~~
>> 176
>> 177 KASAN is affected by the generic ``panic_on_warn`` command line parameter.
>> 178 When it is enabled, KASAN panics the kernel after printing a bug report.
282 179
283 Enabling CONFIG_KASAN_EXTRA_INFO allows KASAN !! 180 By default, KASAN prints a bug report only for the first invalid memory access.
284 information. The extra information currently s !! 181 With ``kasan_multi_shot``, KASAN prints a report on every invalid access. This
285 timestamp at allocation and free. More informa !! 182 effectively disables ``panic_on_warn`` for KASAN reports.
286 the bug and correlate the error with other sys <<
287 extra memory to record more information (more <<
288 CONFIG_KASAN_EXTRA_INFO). <<
289 183
290 Here is the report with CONFIG_KASAN_EXTRA_INF !! 184 Alternatively, independent of ``panic_on_warn`` the ``kasan.fault=`` boot
291 different parts are shown):: !! 185 parameter can be used to control panic and reporting behaviour:
292 186
293 ========================================== !! 187 - ``kasan.fault=report`` or ``=panic`` controls whether to only print a KASAN
294 ... !! 188 report or also panic the kernel (default: ``report``). The panic happens even
295 Allocated by task 134 on cpu 5 at 229.1338 !! 189 if ``kasan_multi_shot`` is enabled.
296 ... !! 190
297 Freed by task 136 on cpu 3 at 230.199335s: !! 191 Hardware tag-based KASAN mode (see the section about various modes below) is
298 ... !! 192 intended for use in production as a security mitigation. Therefore, it supports
299 ========================================== !! 193 additional boot parameters that allow disabling KASAN or controlling features:
>> 194
>> 195 - ``kasan=off`` or ``=on`` controls whether KASAN is enabled (default: ``on``).
>> 196
>> 197 - ``kasan.mode=sync``, ``=async`` or ``=asymm`` controls whether KASAN
>> 198 is configured in synchronous, asynchronous or asymmetric mode of
>> 199 execution (default: ``sync``).
>> 200 Synchronous mode: a bad access is detected immediately when a tag
>> 201 check fault occurs.
>> 202 Asynchronous mode: a bad access detection is delayed. When a tag check
>> 203 fault occurs, the information is stored in hardware (in the TFSR_EL1
>> 204 register for arm64). The kernel periodically checks the hardware and
>> 205 only reports tag faults during these checks.
>> 206 Asymmetric mode: a bad access is detected synchronously on reads and
>> 207 asynchronously on writes.
>> 208
>> 209 - ``kasan.stacktrace=off`` or ``=on`` disables or enables alloc and free stack
>> 210 traces collection (default: ``on``).
300 211
301 Implementation details 212 Implementation details
302 ---------------------- 213 ----------------------
303 214
304 Generic KASAN 215 Generic KASAN
305 ~~~~~~~~~~~~~ 216 ~~~~~~~~~~~~~
306 217
307 Software KASAN modes use shadow memory to reco 218 Software KASAN modes use shadow memory to record whether each byte of memory is
308 safe to access and use compile-time instrument 219 safe to access and use compile-time instrumentation to insert shadow memory
309 checks before each memory access. 220 checks before each memory access.
310 221
311 Generic KASAN dedicates 1/8th of kernel memory 222 Generic KASAN dedicates 1/8th of kernel memory to its shadow memory (16TB
312 to cover 128TB on x86_64) and uses direct mapp 223 to cover 128TB on x86_64) and uses direct mapping with a scale and offset to
313 translate a memory address to its correspondin 224 translate a memory address to its corresponding shadow address.
314 225
315 Here is the function which translates an addre 226 Here is the function which translates an address to its corresponding shadow
316 address:: 227 address::
317 228
318 static inline void *kasan_mem_to_shadow(co 229 static inline void *kasan_mem_to_shadow(const void *addr)
319 { 230 {
320 return (void *)((unsigned long)addr >> 231 return (void *)((unsigned long)addr >> KASAN_SHADOW_SCALE_SHIFT)
321 + KASAN_SHADOW_OFFSET; 232 + KASAN_SHADOW_OFFSET;
322 } 233 }
323 234
324 where ``KASAN_SHADOW_SCALE_SHIFT = 3``. 235 where ``KASAN_SHADOW_SCALE_SHIFT = 3``.
325 236
326 Compile-time instrumentation is used to insert 237 Compile-time instrumentation is used to insert memory access checks. Compiler
327 inserts function calls (``__asan_load*(addr)`` 238 inserts function calls (``__asan_load*(addr)``, ``__asan_store*(addr)``) before
328 each memory access of size 1, 2, 4, 8, or 16. 239 each memory access of size 1, 2, 4, 8, or 16. These functions check whether
329 memory accesses are valid or not by checking c 240 memory accesses are valid or not by checking corresponding shadow memory.
330 241
331 With inline instrumentation, instead of making 242 With inline instrumentation, instead of making function calls, the compiler
332 directly inserts the code to check shadow memo 243 directly inserts the code to check shadow memory. This option significantly
333 enlarges the kernel, but it gives an x1.1-x2 p 244 enlarges the kernel, but it gives an x1.1-x2 performance boost over the
334 outline-instrumented kernel. 245 outline-instrumented kernel.
335 246
336 Generic KASAN is the only mode that delays the 247 Generic KASAN is the only mode that delays the reuse of freed objects via
337 quarantine (see mm/kasan/quarantine.c for impl 248 quarantine (see mm/kasan/quarantine.c for implementation).
338 249
339 Software Tag-Based KASAN !! 250 Software tag-based KASAN
340 ~~~~~~~~~~~~~~~~~~~~~~~~ 251 ~~~~~~~~~~~~~~~~~~~~~~~~
341 252
342 Software Tag-Based KASAN uses a software memor !! 253 Software tag-based KASAN uses a software memory tagging approach to checking
343 access validity. It is currently only implemen 254 access validity. It is currently only implemented for the arm64 architecture.
344 255
345 Software Tag-Based KASAN uses the Top Byte Ign !! 256 Software tag-based KASAN uses the Top Byte Ignore (TBI) feature of arm64 CPUs
346 to store a pointer tag in the top byte of kern 257 to store a pointer tag in the top byte of kernel pointers. It uses shadow memory
347 to store memory tags associated with each 16-b 258 to store memory tags associated with each 16-byte memory cell (therefore, it
348 dedicates 1/16th of the kernel memory for shad 259 dedicates 1/16th of the kernel memory for shadow memory).
349 260
350 On each memory allocation, Software Tag-Based !! 261 On each memory allocation, software tag-based KASAN generates a random tag, tags
351 the allocated memory with this tag, and embeds 262 the allocated memory with this tag, and embeds the same tag into the returned
352 pointer. 263 pointer.
353 264
354 Software Tag-Based KASAN uses compile-time ins !! 265 Software tag-based KASAN uses compile-time instrumentation to insert checks
355 before each memory access. These checks make s 266 before each memory access. These checks make sure that the tag of the memory
356 that is being accessed is equal to the tag of 267 that is being accessed is equal to the tag of the pointer that is used to access
357 this memory. In case of a tag mismatch, Softwa !! 268 this memory. In case of a tag mismatch, software tag-based KASAN prints a bug
358 report. 269 report.
359 270
360 Software Tag-Based KASAN also has two instrume !! 271 Software tag-based KASAN also has two instrumentation modes (outline, which
361 emits callbacks to check memory accesses; and 272 emits callbacks to check memory accesses; and inline, which performs the shadow
362 memory checks inline). With outline instrument 273 memory checks inline). With outline instrumentation mode, a bug report is
363 printed from the function that performs the ac 274 printed from the function that performs the access check. With inline
364 instrumentation, a ``brk`` instruction is emit 275 instrumentation, a ``brk`` instruction is emitted by the compiler, and a
365 dedicated ``brk`` handler is used to print bug 276 dedicated ``brk`` handler is used to print bug reports.
366 277
367 Software Tag-Based KASAN uses 0xFF as a match- !! 278 Software tag-based KASAN uses 0xFF as a match-all pointer tag (accesses through
368 pointers with the 0xFF pointer tag are not che 279 pointers with the 0xFF pointer tag are not checked). The value 0xFE is currently
369 reserved to tag freed memory regions. 280 reserved to tag freed memory regions.
370 281
371 Hardware Tag-Based KASAN !! 282 Software tag-based KASAN currently only supports tagging of slab and page_alloc
>> 283 memory.
>> 284
>> 285 Hardware tag-based KASAN
372 ~~~~~~~~~~~~~~~~~~~~~~~~ 286 ~~~~~~~~~~~~~~~~~~~~~~~~
373 287
374 Hardware Tag-Based KASAN is similar to the sof !! 288 Hardware tag-based KASAN is similar to the software mode in concept but uses
375 hardware memory tagging support instead of com 289 hardware memory tagging support instead of compiler instrumentation and
376 shadow memory. 290 shadow memory.
377 291
378 Hardware Tag-Based KASAN is currently only imp !! 292 Hardware tag-based KASAN is currently only implemented for arm64 architecture
379 and based on both arm64 Memory Tagging Extensi 293 and based on both arm64 Memory Tagging Extension (MTE) introduced in ARMv8.5
380 Instruction Set Architecture and Top Byte Igno 294 Instruction Set Architecture and Top Byte Ignore (TBI).
381 295
382 Special arm64 instructions are used to assign 296 Special arm64 instructions are used to assign memory tags for each allocation.
383 Same tags are assigned to pointers to those al 297 Same tags are assigned to pointers to those allocations. On every memory
384 access, hardware makes sure that the tag of th 298 access, hardware makes sure that the tag of the memory that is being accessed is
385 equal to the tag of the pointer that is used t 299 equal to the tag of the pointer that is used to access this memory. In case of a
386 tag mismatch, a fault is generated, and a repo 300 tag mismatch, a fault is generated, and a report is printed.
387 301
388 Hardware Tag-Based KASAN uses 0xFF as a match- !! 302 Hardware tag-based KASAN uses 0xFF as a match-all pointer tag (accesses through
389 pointers with the 0xFF pointer tag are not che 303 pointers with the 0xFF pointer tag are not checked). The value 0xFE is currently
390 reserved to tag freed memory regions. 304 reserved to tag freed memory regions.
391 305
392 If the hardware does not support MTE (pre ARMv !! 306 Hardware tag-based KASAN currently only supports tagging of slab and page_alloc
>> 307 memory.
>> 308
>> 309 If the hardware does not support MTE (pre ARMv8.5), hardware tag-based KASAN
393 will not be enabled. In this case, all KASAN b 310 will not be enabled. In this case, all KASAN boot parameters are ignored.
394 311
395 Note that enabling CONFIG_KASAN_HW_TAGS always 312 Note that enabling CONFIG_KASAN_HW_TAGS always results in in-kernel TBI being
396 enabled. Even when ``kasan.mode=off`` is provi 313 enabled. Even when ``kasan.mode=off`` is provided or when the hardware does not
397 support MTE (but supports TBI). 314 support MTE (but supports TBI).
398 315
399 Hardware Tag-Based KASAN only reports the firs !! 316 Hardware tag-based KASAN only reports the first found bug. After that, MTE tag
400 checking gets disabled. 317 checking gets disabled.
401 318
402 Shadow memory 319 Shadow memory
403 ------------- 320 -------------
404 321
405 The contents of this section are only applicab <<
406 <<
407 The kernel maps memory in several different pa 322 The kernel maps memory in several different parts of the address space.
408 The range of kernel virtual addresses is large 323 The range of kernel virtual addresses is large: there is not enough real
409 memory to support a real shadow region for eve 324 memory to support a real shadow region for every address that could be
410 accessed by the kernel. Therefore, KASAN only 325 accessed by the kernel. Therefore, KASAN only maps real shadow for certain
411 parts of the address space. 326 parts of the address space.
412 327
413 Default behaviour 328 Default behaviour
414 ~~~~~~~~~~~~~~~~~ 329 ~~~~~~~~~~~~~~~~~
415 330
416 By default, architectures only map real memory 331 By default, architectures only map real memory over the shadow region
417 for the linear mapping (and potentially other 332 for the linear mapping (and potentially other small areas). For all
418 other areas - such as vmalloc and vmemmap spac 333 other areas - such as vmalloc and vmemmap space - a single read-only
419 page is mapped over the shadow area. This read 334 page is mapped over the shadow area. This read-only shadow page
420 declares all memory accesses as permitted. 335 declares all memory accesses as permitted.
421 336
422 This presents a problem for modules: they do n 337 This presents a problem for modules: they do not live in the linear
423 mapping but in a dedicated module space. By ho 338 mapping but in a dedicated module space. By hooking into the module
424 allocator, KASAN temporarily maps real shadow 339 allocator, KASAN temporarily maps real shadow memory to cover them.
425 This allows detection of invalid accesses to m 340 This allows detection of invalid accesses to module globals, for example.
426 341
427 This also creates an incompatibility with ``VM 342 This also creates an incompatibility with ``VMAP_STACK``: if the stack
428 lives in vmalloc space, it will be shadowed by 343 lives in vmalloc space, it will be shadowed by the read-only page, and
429 the kernel will fault when trying to set up th 344 the kernel will fault when trying to set up the shadow data for stack
430 variables. 345 variables.
431 346
432 CONFIG_KASAN_VMALLOC 347 CONFIG_KASAN_VMALLOC
433 ~~~~~~~~~~~~~~~~~~~~ 348 ~~~~~~~~~~~~~~~~~~~~
434 349
435 With ``CONFIG_KASAN_VMALLOC``, KASAN can cover 350 With ``CONFIG_KASAN_VMALLOC``, KASAN can cover vmalloc space at the
436 cost of greater memory usage. Currently, this 351 cost of greater memory usage. Currently, this is supported on x86,
437 arm64, riscv, s390, and powerpc. !! 352 riscv, s390, and powerpc.
438 353
439 This works by hooking into vmalloc and vmap an 354 This works by hooking into vmalloc and vmap and dynamically
440 allocating real shadow memory to back the mapp 355 allocating real shadow memory to back the mappings.
441 356
442 Most mappings in vmalloc space are small, requ 357 Most mappings in vmalloc space are small, requiring less than a full
443 page of shadow space. Allocating a full shadow 358 page of shadow space. Allocating a full shadow page per mapping would
444 therefore be wasteful. Furthermore, to ensure 359 therefore be wasteful. Furthermore, to ensure that different mappings
445 use different shadow pages, mappings would hav 360 use different shadow pages, mappings would have to be aligned to
446 ``KASAN_GRANULE_SIZE * PAGE_SIZE``. 361 ``KASAN_GRANULE_SIZE * PAGE_SIZE``.
447 362
448 Instead, KASAN shares backing space across mul 363 Instead, KASAN shares backing space across multiple mappings. It allocates
449 a backing page when a mapping in vmalloc space 364 a backing page when a mapping in vmalloc space uses a particular page
450 of the shadow region. This page can be shared 365 of the shadow region. This page can be shared by other vmalloc
451 mappings later on. 366 mappings later on.
452 367
453 KASAN hooks into the vmap infrastructure to la 368 KASAN hooks into the vmap infrastructure to lazily clean up unused shadow
454 memory. 369 memory.
455 370
456 To avoid the difficulties around swapping mapp 371 To avoid the difficulties around swapping mappings around, KASAN expects
457 that the part of the shadow region that covers 372 that the part of the shadow region that covers the vmalloc space will
458 not be covered by the early shadow page but wi 373 not be covered by the early shadow page but will be left unmapped.
459 This will require changes in arch-specific cod 374 This will require changes in arch-specific code.
460 375
461 This allows ``VMAP_STACK`` support on x86 and 376 This allows ``VMAP_STACK`` support on x86 and can simplify support of
462 architectures that do not have a fixed module 377 architectures that do not have a fixed module region.
463 378
464 For developers 379 For developers
465 -------------- 380 --------------
466 381
467 Ignoring accesses 382 Ignoring accesses
468 ~~~~~~~~~~~~~~~~~ 383 ~~~~~~~~~~~~~~~~~
469 384
470 Software KASAN modes use compiler instrumentat 385 Software KASAN modes use compiler instrumentation to insert validity checks.
471 Such instrumentation might be incompatible wit 386 Such instrumentation might be incompatible with some parts of the kernel, and
472 therefore needs to be disabled. 387 therefore needs to be disabled.
473 388
474 Other parts of the kernel might access metadat 389 Other parts of the kernel might access metadata for allocated objects.
475 Normally, KASAN detects and reports such acces 390 Normally, KASAN detects and reports such accesses, but in some cases (e.g.,
476 in memory allocators), these accesses are vali 391 in memory allocators), these accesses are valid.
477 392
478 For software KASAN modes, to disable instrumen 393 For software KASAN modes, to disable instrumentation for a specific file or
479 directory, add a ``KASAN_SANITIZE`` annotation 394 directory, add a ``KASAN_SANITIZE`` annotation to the respective kernel
480 Makefile: 395 Makefile:
481 396
482 - For a single file (e.g., main.o):: 397 - For a single file (e.g., main.o)::
483 398
484 KASAN_SANITIZE_main.o := n 399 KASAN_SANITIZE_main.o := n
485 400
486 - For all files in one directory:: 401 - For all files in one directory::
487 402
488 KASAN_SANITIZE := n 403 KASAN_SANITIZE := n
489 404
490 For software KASAN modes, to disable instrumen 405 For software KASAN modes, to disable instrumentation on a per-function basis,
491 use the KASAN-specific ``__no_sanitize_address 406 use the KASAN-specific ``__no_sanitize_address`` function attribute or the
492 generic ``noinstr`` one. 407 generic ``noinstr`` one.
493 408
494 Note that disabling compiler instrumentation ( 409 Note that disabling compiler instrumentation (either on a per-file or a
495 per-function basis) makes KASAN ignore the acc 410 per-function basis) makes KASAN ignore the accesses that happen directly in
496 that code for software KASAN modes. It does no 411 that code for software KASAN modes. It does not help when the accesses happen
497 indirectly (through calls to instrumented func !! 412 indirectly (through calls to instrumented functions) or with the hardware
498 Tag-Based KASAN, which does not use compiler i !! 413 tag-based mode that does not use compiler instrumentation.
499 414
500 For software KASAN modes, to disable KASAN rep 415 For software KASAN modes, to disable KASAN reports in a part of the kernel code
501 for the current task, annotate this part of th 416 for the current task, annotate this part of the code with a
502 ``kasan_disable_current()``/``kasan_enable_cur 417 ``kasan_disable_current()``/``kasan_enable_current()`` section. This also
503 disables the reports for indirect accesses tha 418 disables the reports for indirect accesses that happen through function calls.
504 419
505 For tag-based KASAN modes, to disable access c !! 420 For tag-based KASAN modes (include the hardware one), to disable access
506 ``kasan_reset_tag()`` or ``page_kasan_tag_rese !! 421 checking, use ``kasan_reset_tag()`` or ``page_kasan_tag_reset()``. Note that
507 disabling access checking via ``page_kasan_tag !! 422 temporarily disabling access checking via ``page_kasan_tag_reset()`` requires
508 restoring the per-page KASAN tag via ``page_ka !! 423 saving and restoring the per-page KASAN tag via
>> 424 ``page_kasan_tag``/``page_kasan_tag_set``.
509 425
510 Tests 426 Tests
511 ~~~~~ 427 ~~~~~
512 428
513 There are KASAN tests that allow verifying tha 429 There are KASAN tests that allow verifying that KASAN works and can detect
514 certain types of memory corruptions. The tests 430 certain types of memory corruptions. The tests consist of two parts:
515 431
516 1. Tests that are integrated with the KUnit Te 432 1. Tests that are integrated with the KUnit Test Framework. Enabled with
517 ``CONFIG_KASAN_KUNIT_TEST``. These tests can b 433 ``CONFIG_KASAN_KUNIT_TEST``. These tests can be run and partially verified
518 automatically in a few different ways; see the 434 automatically in a few different ways; see the instructions below.
519 435
520 2. Tests that are currently incompatible with 436 2. Tests that are currently incompatible with KUnit. Enabled with
521 ``CONFIG_KASAN_MODULE_TEST`` and can only be r 437 ``CONFIG_KASAN_MODULE_TEST`` and can only be run as a module. These tests can
522 only be verified manually by loading the kerne 438 only be verified manually by loading the kernel module and inspecting the
523 kernel log for KASAN reports. 439 kernel log for KASAN reports.
524 440
525 Each KUnit-compatible KASAN test prints one of 441 Each KUnit-compatible KASAN test prints one of multiple KASAN reports if an
526 error is detected. Then the test prints its nu 442 error is detected. Then the test prints its number and status.
527 443
528 When a test passes:: 444 When a test passes::
529 445
530 ok 28 - kmalloc_double_kzfree 446 ok 28 - kmalloc_double_kzfree
531 447
532 When a test fails due to a failed ``kmalloc``: 448 When a test fails due to a failed ``kmalloc``::
533 449
534 # kmalloc_large_oob_right: ASSERTION F !! 450 # kmalloc_large_oob_right: ASSERTION FAILED at lib/test_kasan.c:163
535 Expected ptr is not null, but is 451 Expected ptr is not null, but is
536 not ok 5 - kmalloc_large_oob_right !! 452 not ok 4 - kmalloc_large_oob_right
537 453
538 When a test fails due to a missing KASAN repor 454 When a test fails due to a missing KASAN report::
539 455
540 # kmalloc_double_kzfree: EXPECTATION F !! 456 # kmalloc_double_kzfree: EXPECTATION FAILED at lib/test_kasan.c:974
541 KASAN failure expected in "kfree_sensi 457 KASAN failure expected in "kfree_sensitive(ptr)", but none occurred
542 not ok 28 - kmalloc_double_kzfree !! 458 not ok 44 - kmalloc_double_kzfree
543 459
544 460
545 At the end the cumulative status of all KASAN 461 At the end the cumulative status of all KASAN tests is printed. On success::
546 462
547 ok 1 - kasan 463 ok 1 - kasan
548 464
549 Or, if one of the tests failed:: 465 Or, if one of the tests failed::
550 466
551 not ok 1 - kasan 467 not ok 1 - kasan
552 468
553 There are a few ways to run KUnit-compatible K 469 There are a few ways to run KUnit-compatible KASAN tests.
554 470
555 1. Loadable module 471 1. Loadable module
556 472
557 With ``CONFIG_KUNIT`` enabled, KASAN-KUnit 473 With ``CONFIG_KUNIT`` enabled, KASAN-KUnit tests can be built as a loadable
558 module and run by loading ``kasan_test.ko`` !! 474 module and run by loading ``test_kasan.ko`` with ``insmod`` or ``modprobe``.
559 475
560 2. Built-In 476 2. Built-In
561 477
562 With ``CONFIG_KUNIT`` built-in, KASAN-KUnit 478 With ``CONFIG_KUNIT`` built-in, KASAN-KUnit tests can be built-in as well.
563 In this case, the tests will run at boot as 479 In this case, the tests will run at boot as a late-init call.
564 480
565 3. Using kunit_tool 481 3. Using kunit_tool
566 482
567 With ``CONFIG_KUNIT`` and ``CONFIG_KASAN_KU 483 With ``CONFIG_KUNIT`` and ``CONFIG_KASAN_KUNIT_TEST`` built-in, it is also
568 possible to use ``kunit_tool`` to see the r 484 possible to use ``kunit_tool`` to see the results of KUnit tests in a more
569 readable way. This will not print the KASAN 485 readable way. This will not print the KASAN reports of the tests that passed.
570 See `KUnit documentation <https://www.kerne 486 See `KUnit documentation <https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html>`_
571 for more up-to-date information on ``kunit_ 487 for more up-to-date information on ``kunit_tool``.
572 488
573 .. _KUnit: https://www.kernel.org/doc/html/lat 489 .. _KUnit: https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html
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