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 <<
67 Memory types <<
68 ~~~~~~~~~~~~ <<
69 <<
70 Generic KASAN supports finding bugs in all of <<
71 stack, and global memory. <<
72 9
73 Software Tag-Based KASAN supports slab, page_a !! 10 1. generic KASAN (similar to userspace ASan),
>> 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+.
74 34
75 Hardware Tag-Based KASAN supports slab, page_a !! 35 Both software KASAN modes work with SLUB and SLAB memory allocators,
76 memory. !! 36 while the hardware tag-based KASAN currently only supports SLUB.
77 37
78 For slab, both software KASAN modes support SL !! 38 Currently, generic KASAN is supported for the x86_64, arm, arm64, xtensa, s390,
79 Hardware Tag-Based KASAN only supports SLUB. !! 39 and riscv architectures, and tag-based KASAN modes are supported only for arm64.
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 ~~~~~~~~~~~~~~~
282 176
283 Enabling CONFIG_KASAN_EXTRA_INFO allows KASAN !! 177 KASAN is affected by the generic ``panic_on_warn`` command line parameter.
284 information. The extra information currently s !! 178 When it is enabled, KASAN panics the kernel after printing a bug report.
285 timestamp at allocation and free. More informa <<
286 the bug and correlate the error with other sys <<
287 extra memory to record more information (more <<
288 CONFIG_KASAN_EXTRA_INFO). <<
289 179
290 Here is the report with CONFIG_KASAN_EXTRA_INF !! 180 By default, KASAN prints a bug report only for the first invalid memory access.
291 different parts are shown):: !! 181 With ``kasan_multi_shot``, KASAN prints a report on every invalid access. This
>> 182 effectively disables ``panic_on_warn`` for KASAN reports.
292 183
293 ========================================== !! 184 Hardware tag-based KASAN mode (see the section about various modes below) is
294 ... !! 185 intended for use in production as a security mitigation. Therefore, it supports
295 Allocated by task 134 on cpu 5 at 229.1338 !! 186 boot parameters that allow disabling KASAN or controlling its features.
296 ... !! 187
297 Freed by task 136 on cpu 3 at 230.199335s: !! 188 - ``kasan=off`` or ``=on`` controls whether KASAN is enabled (default: ``on``).
298 ... !! 189
299 ========================================== !! 190 - ``kasan.mode=sync`` or ``=async`` controls whether KASAN is configured in
>> 191 synchronous or asynchronous mode of execution (default: ``sync``).
>> 192 Synchronous mode: a bad access is detected immediately when a tag
>> 193 check fault occurs.
>> 194 Asynchronous mode: a bad access detection is delayed. When a tag check
>> 195 fault occurs, the information is stored in hardware (in the TFSR_EL1
>> 196 register for arm64). The kernel periodically checks the hardware and
>> 197 only reports tag faults during these checks.
>> 198
>> 199 - ``kasan.stacktrace=off`` or ``=on`` disables or enables alloc and free stack
>> 200 traces collection (default: ``on``).
>> 201
>> 202 - ``kasan.fault=report`` or ``=panic`` controls whether to only print a KASAN
>> 203 report or also panic the kernel (default: ``report``). The panic happens even
>> 204 if ``kasan_multi_shot`` is enabled.
300 205
301 Implementation details 206 Implementation details
302 ---------------------- 207 ----------------------
303 208
304 Generic KASAN 209 Generic KASAN
305 ~~~~~~~~~~~~~ 210 ~~~~~~~~~~~~~
306 211
307 Software KASAN modes use shadow memory to reco 212 Software KASAN modes use shadow memory to record whether each byte of memory is
308 safe to access and use compile-time instrument 213 safe to access and use compile-time instrumentation to insert shadow memory
309 checks before each memory access. 214 checks before each memory access.
310 215
311 Generic KASAN dedicates 1/8th of kernel memory 216 Generic KASAN dedicates 1/8th of kernel memory to its shadow memory (16TB
312 to cover 128TB on x86_64) and uses direct mapp 217 to cover 128TB on x86_64) and uses direct mapping with a scale and offset to
313 translate a memory address to its correspondin 218 translate a memory address to its corresponding shadow address.
314 219
315 Here is the function which translates an addre 220 Here is the function which translates an address to its corresponding shadow
316 address:: 221 address::
317 222
318 static inline void *kasan_mem_to_shadow(co 223 static inline void *kasan_mem_to_shadow(const void *addr)
319 { 224 {
320 return (void *)((unsigned long)addr >> 225 return (void *)((unsigned long)addr >> KASAN_SHADOW_SCALE_SHIFT)
321 + KASAN_SHADOW_OFFSET; 226 + KASAN_SHADOW_OFFSET;
322 } 227 }
323 228
324 where ``KASAN_SHADOW_SCALE_SHIFT = 3``. 229 where ``KASAN_SHADOW_SCALE_SHIFT = 3``.
325 230
326 Compile-time instrumentation is used to insert 231 Compile-time instrumentation is used to insert memory access checks. Compiler
327 inserts function calls (``__asan_load*(addr)`` 232 inserts function calls (``__asan_load*(addr)``, ``__asan_store*(addr)``) before
328 each memory access of size 1, 2, 4, 8, or 16. 233 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 234 memory accesses are valid or not by checking corresponding shadow memory.
330 235
331 With inline instrumentation, instead of making 236 With inline instrumentation, instead of making function calls, the compiler
332 directly inserts the code to check shadow memo 237 directly inserts the code to check shadow memory. This option significantly
333 enlarges the kernel, but it gives an x1.1-x2 p 238 enlarges the kernel, but it gives an x1.1-x2 performance boost over the
334 outline-instrumented kernel. 239 outline-instrumented kernel.
335 240
336 Generic KASAN is the only mode that delays the 241 Generic KASAN is the only mode that delays the reuse of freed objects via
337 quarantine (see mm/kasan/quarantine.c for impl 242 quarantine (see mm/kasan/quarantine.c for implementation).
338 243
339 Software Tag-Based KASAN !! 244 Software tag-based KASAN
340 ~~~~~~~~~~~~~~~~~~~~~~~~ 245 ~~~~~~~~~~~~~~~~~~~~~~~~
341 246
342 Software Tag-Based KASAN uses a software memor !! 247 Software tag-based KASAN uses a software memory tagging approach to checking
343 access validity. It is currently only implemen 248 access validity. It is currently only implemented for the arm64 architecture.
344 249
345 Software Tag-Based KASAN uses the Top Byte Ign !! 250 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 251 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 252 to store memory tags associated with each 16-byte memory cell (therefore, it
348 dedicates 1/16th of the kernel memory for shad 253 dedicates 1/16th of the kernel memory for shadow memory).
349 254
350 On each memory allocation, Software Tag-Based !! 255 On each memory allocation, software tag-based KASAN generates a random tag, tags
351 the allocated memory with this tag, and embeds 256 the allocated memory with this tag, and embeds the same tag into the returned
352 pointer. 257 pointer.
353 258
354 Software Tag-Based KASAN uses compile-time ins !! 259 Software tag-based KASAN uses compile-time instrumentation to insert checks
355 before each memory access. These checks make s 260 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 261 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 !! 262 this memory. In case of a tag mismatch, software tag-based KASAN prints a bug
358 report. 263 report.
359 264
360 Software Tag-Based KASAN also has two instrume !! 265 Software tag-based KASAN also has two instrumentation modes (outline, which
361 emits callbacks to check memory accesses; and 266 emits callbacks to check memory accesses; and inline, which performs the shadow
362 memory checks inline). With outline instrument 267 memory checks inline). With outline instrumentation mode, a bug report is
363 printed from the function that performs the ac 268 printed from the function that performs the access check. With inline
364 instrumentation, a ``brk`` instruction is emit 269 instrumentation, a ``brk`` instruction is emitted by the compiler, and a
365 dedicated ``brk`` handler is used to print bug 270 dedicated ``brk`` handler is used to print bug reports.
366 271
367 Software Tag-Based KASAN uses 0xFF as a match- !! 272 Software tag-based KASAN uses 0xFF as a match-all pointer tag (accesses through
368 pointers with the 0xFF pointer tag are not che 273 pointers with the 0xFF pointer tag are not checked). The value 0xFE is currently
369 reserved to tag freed memory regions. 274 reserved to tag freed memory regions.
370 275
371 Hardware Tag-Based KASAN !! 276 Software tag-based KASAN currently only supports tagging of slab and page_alloc
>> 277 memory.
>> 278
>> 279 Hardware tag-based KASAN
372 ~~~~~~~~~~~~~~~~~~~~~~~~ 280 ~~~~~~~~~~~~~~~~~~~~~~~~
373 281
374 Hardware Tag-Based KASAN is similar to the sof !! 282 Hardware tag-based KASAN is similar to the software mode in concept but uses
375 hardware memory tagging support instead of com 283 hardware memory tagging support instead of compiler instrumentation and
376 shadow memory. 284 shadow memory.
377 285
378 Hardware Tag-Based KASAN is currently only imp !! 286 Hardware tag-based KASAN is currently only implemented for arm64 architecture
379 and based on both arm64 Memory Tagging Extensi 287 and based on both arm64 Memory Tagging Extension (MTE) introduced in ARMv8.5
380 Instruction Set Architecture and Top Byte Igno 288 Instruction Set Architecture and Top Byte Ignore (TBI).
381 289
382 Special arm64 instructions are used to assign 290 Special arm64 instructions are used to assign memory tags for each allocation.
383 Same tags are assigned to pointers to those al 291 Same tags are assigned to pointers to those allocations. On every memory
384 access, hardware makes sure that the tag of th 292 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 293 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 294 tag mismatch, a fault is generated, and a report is printed.
387 295
388 Hardware Tag-Based KASAN uses 0xFF as a match- !! 296 Hardware tag-based KASAN uses 0xFF as a match-all pointer tag (accesses through
389 pointers with the 0xFF pointer tag are not che 297 pointers with the 0xFF pointer tag are not checked). The value 0xFE is currently
390 reserved to tag freed memory regions. 298 reserved to tag freed memory regions.
391 299
392 If the hardware does not support MTE (pre ARMv !! 300 Hardware tag-based KASAN currently only supports tagging of slab and page_alloc
>> 301 memory.
>> 302
>> 303 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 304 will not be enabled. In this case, all KASAN boot parameters are ignored.
394 305
395 Note that enabling CONFIG_KASAN_HW_TAGS always 306 Note that enabling CONFIG_KASAN_HW_TAGS always results in in-kernel TBI being
396 enabled. Even when ``kasan.mode=off`` is provi 307 enabled. Even when ``kasan.mode=off`` is provided or when the hardware does not
397 support MTE (but supports TBI). 308 support MTE (but supports TBI).
398 309
399 Hardware Tag-Based KASAN only reports the firs !! 310 Hardware tag-based KASAN only reports the first found bug. After that, MTE tag
400 checking gets disabled. 311 checking gets disabled.
401 312
402 Shadow memory 313 Shadow memory
403 ------------- 314 -------------
404 315
405 The contents of this section are only applicab <<
406 <<
407 The kernel maps memory in several different pa 316 The kernel maps memory in several different parts of the address space.
408 The range of kernel virtual addresses is large 317 The range of kernel virtual addresses is large: there is not enough real
409 memory to support a real shadow region for eve 318 memory to support a real shadow region for every address that could be
410 accessed by the kernel. Therefore, KASAN only 319 accessed by the kernel. Therefore, KASAN only maps real shadow for certain
411 parts of the address space. 320 parts of the address space.
412 321
413 Default behaviour 322 Default behaviour
414 ~~~~~~~~~~~~~~~~~ 323 ~~~~~~~~~~~~~~~~~
415 324
416 By default, architectures only map real memory 325 By default, architectures only map real memory over the shadow region
417 for the linear mapping (and potentially other 326 for the linear mapping (and potentially other small areas). For all
418 other areas - such as vmalloc and vmemmap spac 327 other areas - such as vmalloc and vmemmap space - a single read-only
419 page is mapped over the shadow area. This read 328 page is mapped over the shadow area. This read-only shadow page
420 declares all memory accesses as permitted. 329 declares all memory accesses as permitted.
421 330
422 This presents a problem for modules: they do n 331 This presents a problem for modules: they do not live in the linear
423 mapping but in a dedicated module space. By ho 332 mapping but in a dedicated module space. By hooking into the module
424 allocator, KASAN temporarily maps real shadow 333 allocator, KASAN temporarily maps real shadow memory to cover them.
425 This allows detection of invalid accesses to m 334 This allows detection of invalid accesses to module globals, for example.
426 335
427 This also creates an incompatibility with ``VM 336 This also creates an incompatibility with ``VMAP_STACK``: if the stack
428 lives in vmalloc space, it will be shadowed by 337 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 338 the kernel will fault when trying to set up the shadow data for stack
430 variables. 339 variables.
431 340
432 CONFIG_KASAN_VMALLOC 341 CONFIG_KASAN_VMALLOC
433 ~~~~~~~~~~~~~~~~~~~~ 342 ~~~~~~~~~~~~~~~~~~~~
434 343
435 With ``CONFIG_KASAN_VMALLOC``, KASAN can cover 344 With ``CONFIG_KASAN_VMALLOC``, KASAN can cover vmalloc space at the
436 cost of greater memory usage. Currently, this 345 cost of greater memory usage. Currently, this is supported on x86,
437 arm64, riscv, s390, and powerpc. !! 346 riscv, s390, and powerpc.
438 347
439 This works by hooking into vmalloc and vmap an 348 This works by hooking into vmalloc and vmap and dynamically
440 allocating real shadow memory to back the mapp 349 allocating real shadow memory to back the mappings.
441 350
442 Most mappings in vmalloc space are small, requ 351 Most mappings in vmalloc space are small, requiring less than a full
443 page of shadow space. Allocating a full shadow 352 page of shadow space. Allocating a full shadow page per mapping would
444 therefore be wasteful. Furthermore, to ensure 353 therefore be wasteful. Furthermore, to ensure that different mappings
445 use different shadow pages, mappings would hav 354 use different shadow pages, mappings would have to be aligned to
446 ``KASAN_GRANULE_SIZE * PAGE_SIZE``. 355 ``KASAN_GRANULE_SIZE * PAGE_SIZE``.
447 356
448 Instead, KASAN shares backing space across mul 357 Instead, KASAN shares backing space across multiple mappings. It allocates
449 a backing page when a mapping in vmalloc space 358 a backing page when a mapping in vmalloc space uses a particular page
450 of the shadow region. This page can be shared 359 of the shadow region. This page can be shared by other vmalloc
451 mappings later on. 360 mappings later on.
452 361
453 KASAN hooks into the vmap infrastructure to la 362 KASAN hooks into the vmap infrastructure to lazily clean up unused shadow
454 memory. 363 memory.
455 364
456 To avoid the difficulties around swapping mapp 365 To avoid the difficulties around swapping mappings around, KASAN expects
457 that the part of the shadow region that covers 366 that the part of the shadow region that covers the vmalloc space will
458 not be covered by the early shadow page but wi 367 not be covered by the early shadow page but will be left unmapped.
459 This will require changes in arch-specific cod 368 This will require changes in arch-specific code.
460 369
461 This allows ``VMAP_STACK`` support on x86 and 370 This allows ``VMAP_STACK`` support on x86 and can simplify support of
462 architectures that do not have a fixed module 371 architectures that do not have a fixed module region.
463 372
464 For developers 373 For developers
465 -------------- 374 --------------
466 375
467 Ignoring accesses 376 Ignoring accesses
468 ~~~~~~~~~~~~~~~~~ 377 ~~~~~~~~~~~~~~~~~
469 378
470 Software KASAN modes use compiler instrumentat 379 Software KASAN modes use compiler instrumentation to insert validity checks.
471 Such instrumentation might be incompatible wit 380 Such instrumentation might be incompatible with some parts of the kernel, and
472 therefore needs to be disabled. 381 therefore needs to be disabled.
473 382
474 Other parts of the kernel might access metadat 383 Other parts of the kernel might access metadata for allocated objects.
475 Normally, KASAN detects and reports such acces 384 Normally, KASAN detects and reports such accesses, but in some cases (e.g.,
476 in memory allocators), these accesses are vali 385 in memory allocators), these accesses are valid.
477 386
478 For software KASAN modes, to disable instrumen 387 For software KASAN modes, to disable instrumentation for a specific file or
479 directory, add a ``KASAN_SANITIZE`` annotation 388 directory, add a ``KASAN_SANITIZE`` annotation to the respective kernel
480 Makefile: 389 Makefile:
481 390
482 - For a single file (e.g., main.o):: 391 - For a single file (e.g., main.o)::
483 392
484 KASAN_SANITIZE_main.o := n 393 KASAN_SANITIZE_main.o := n
485 394
486 - For all files in one directory:: 395 - For all files in one directory::
487 396
488 KASAN_SANITIZE := n 397 KASAN_SANITIZE := n
489 398
490 For software KASAN modes, to disable instrumen 399 For software KASAN modes, to disable instrumentation on a per-function basis,
491 use the KASAN-specific ``__no_sanitize_address 400 use the KASAN-specific ``__no_sanitize_address`` function attribute or the
492 generic ``noinstr`` one. 401 generic ``noinstr`` one.
493 402
494 Note that disabling compiler instrumentation ( 403 Note that disabling compiler instrumentation (either on a per-file or a
495 per-function basis) makes KASAN ignore the acc 404 per-function basis) makes KASAN ignore the accesses that happen directly in
496 that code for software KASAN modes. It does no 405 that code for software KASAN modes. It does not help when the accesses happen
497 indirectly (through calls to instrumented func !! 406 indirectly (through calls to instrumented functions) or with the hardware
498 Tag-Based KASAN, which does not use compiler i !! 407 tag-based mode that does not use compiler instrumentation.
499 408
500 For software KASAN modes, to disable KASAN rep 409 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 410 for the current task, annotate this part of the code with a
502 ``kasan_disable_current()``/``kasan_enable_cur 411 ``kasan_disable_current()``/``kasan_enable_current()`` section. This also
503 disables the reports for indirect accesses tha 412 disables the reports for indirect accesses that happen through function calls.
504 413
505 For tag-based KASAN modes, to disable access c !! 414 For tag-based KASAN modes (include the hardware one), to disable access
506 ``kasan_reset_tag()`` or ``page_kasan_tag_rese !! 415 checking, use ``kasan_reset_tag()`` or ``page_kasan_tag_reset()``. Note that
507 disabling access checking via ``page_kasan_tag !! 416 temporarily disabling access checking via ``page_kasan_tag_reset()`` requires
508 restoring the per-page KASAN tag via ``page_ka !! 417 saving and restoring the per-page KASAN tag via
>> 418 ``page_kasan_tag``/``page_kasan_tag_set``.
509 419
510 Tests 420 Tests
511 ~~~~~ 421 ~~~~~
512 422
513 There are KASAN tests that allow verifying tha 423 There are KASAN tests that allow verifying that KASAN works and can detect
514 certain types of memory corruptions. The tests 424 certain types of memory corruptions. The tests consist of two parts:
515 425
516 1. Tests that are integrated with the KUnit Te 426 1. Tests that are integrated with the KUnit Test Framework. Enabled with
517 ``CONFIG_KASAN_KUNIT_TEST``. These tests can b 427 ``CONFIG_KASAN_KUNIT_TEST``. These tests can be run and partially verified
518 automatically in a few different ways; see the 428 automatically in a few different ways; see the instructions below.
519 429
520 2. Tests that are currently incompatible with 430 2. Tests that are currently incompatible with KUnit. Enabled with
521 ``CONFIG_KASAN_MODULE_TEST`` and can only be r 431 ``CONFIG_KASAN_MODULE_TEST`` and can only be run as a module. These tests can
522 only be verified manually by loading the kerne 432 only be verified manually by loading the kernel module and inspecting the
523 kernel log for KASAN reports. 433 kernel log for KASAN reports.
524 434
525 Each KUnit-compatible KASAN test prints one of 435 Each KUnit-compatible KASAN test prints one of multiple KASAN reports if an
526 error is detected. Then the test prints its nu 436 error is detected. Then the test prints its number and status.
527 437
528 When a test passes:: 438 When a test passes::
529 439
530 ok 28 - kmalloc_double_kzfree 440 ok 28 - kmalloc_double_kzfree
531 441
532 When a test fails due to a failed ``kmalloc``: 442 When a test fails due to a failed ``kmalloc``::
533 443
534 # kmalloc_large_oob_right: ASSERTION F !! 444 # kmalloc_large_oob_right: ASSERTION FAILED at lib/test_kasan.c:163
535 Expected ptr is not null, but is 445 Expected ptr is not null, but is
536 not ok 5 - kmalloc_large_oob_right !! 446 not ok 4 - kmalloc_large_oob_right
537 447
538 When a test fails due to a missing KASAN repor 448 When a test fails due to a missing KASAN report::
539 449
540 # kmalloc_double_kzfree: EXPECTATION F !! 450 # kmalloc_double_kzfree: EXPECTATION FAILED at lib/test_kasan.c:629
541 KASAN failure expected in "kfree_sensi !! 451 Expected kasan_data->report_expected == kasan_data->report_found, but
>> 452 kasan_data->report_expected == 1
>> 453 kasan_data->report_found == 0
542 not ok 28 - kmalloc_double_kzfree 454 not ok 28 - kmalloc_double_kzfree
543 455
544 <<
545 At the end the cumulative status of all KASAN 456 At the end the cumulative status of all KASAN tests is printed. On success::
546 457
547 ok 1 - kasan 458 ok 1 - kasan
548 459
549 Or, if one of the tests failed:: 460 Or, if one of the tests failed::
550 461
551 not ok 1 - kasan 462 not ok 1 - kasan
552 463
553 There are a few ways to run KUnit-compatible K 464 There are a few ways to run KUnit-compatible KASAN tests.
554 465
555 1. Loadable module 466 1. Loadable module
556 467
557 With ``CONFIG_KUNIT`` enabled, KASAN-KUnit 468 With ``CONFIG_KUNIT`` enabled, KASAN-KUnit tests can be built as a loadable
558 module and run by loading ``kasan_test.ko`` !! 469 module and run by loading ``test_kasan.ko`` with ``insmod`` or ``modprobe``.
559 470
560 2. Built-In 471 2. Built-In
561 472
562 With ``CONFIG_KUNIT`` built-in, KASAN-KUnit 473 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 474 In this case, the tests will run at boot as a late-init call.
564 475
565 3. Using kunit_tool 476 3. Using kunit_tool
566 477
567 With ``CONFIG_KUNIT`` and ``CONFIG_KASAN_KU 478 With ``CONFIG_KUNIT`` and ``CONFIG_KASAN_KUNIT_TEST`` built-in, it is also
568 possible to use ``kunit_tool`` to see the r 479 possible to use ``kunit_tool`` to see the results of KUnit tests in a more
569 readable way. This will not print the KASAN 480 readable way. This will not print the KASAN reports of the tests that passed.
570 See `KUnit documentation <https://www.kerne 481 See `KUnit documentation <https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html>`_
571 for more up-to-date information on ``kunit_ 482 for more up-to-date information on ``kunit_tool``.
572 483
573 .. _KUnit: https://www.kernel.org/doc/html/lat 484 .. _KUnit: https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html
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