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 9
28 Hardware Tag-Based KASAN or HW_TAGS KASAN, ena !! 10 1. generic KASAN (similar to userspace ASan),
29 is the mode intended to be used as an in-field !! 11 2. software tag-based KASAN (similar to userspace HWASan),
30 security mitigation. This mode only works on a !! 12 3. hardware tag-based KASAN (based on hardware memory tagging).
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 13
47 Generic KASAN is supported on x86_64, arm, arm !! 14 Software KASAN modes (1 and 2) use compile-time instrumentation to insert
48 and loongarch, and the tag-based KASAN modes a !! 15 validity checks before every memory access, and therefore require a compiler
>> 16 version that supports that.
49 17
50 Compilers !! 18 Generic KASAN is supported in both GCC and Clang. With GCC it requires version
51 ~~~~~~~~~ !! 19 8.3.0 or later. Any supported Clang version is compatible, but detection of
>> 20 out-of-bounds accesses for global variables is only supported since Clang 11.
52 21
53 Software KASAN modes use compile-time instrume !! 22 Tag-based KASAN is only supported in Clang.
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 23
59 Generic KASAN requires GCC version 8.3.0 or la !! 24 Currently generic KASAN is supported for the x86_64, arm, arm64, xtensa, s390
60 or any Clang version supported by the kernel. !! 25 and riscv architectures, and tag-based KASAN modes are supported only for arm64.
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 <<
73 Software Tag-Based KASAN supports slab, page_a <<
74 <<
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 26
81 Usage 27 Usage
82 ----- 28 -----
83 29
84 To enable KASAN, configure the kernel with:: !! 30 To enable KASAN configure kernel with::
85 <<
86 CONFIG_KASAN=y <<
87 31
88 and choose between ``CONFIG_KASAN_GENERIC`` (t !! 32 CONFIG_KASAN = y
89 ``CONFIG_KASAN_SW_TAGS`` (to enable Software T <<
90 ``CONFIG_KASAN_HW_TAGS`` (to enable Hardware T <<
91 33
92 For the software modes, also choose between `` !! 34 and choose between CONFIG_KASAN_GENERIC (to enable generic KASAN),
93 ``CONFIG_KASAN_INLINE``. Outline and inline ar !! 35 CONFIG_KASAN_SW_TAGS (to enable software tag-based KASAN), and
94 The former produces a smaller binary while the !! 36 CONFIG_KASAN_HW_TAGS (to enable hardware tag-based KASAN).
95 37
96 To include alloc and free stack traces of affe !! 38 For software modes, you also need to choose between CONFIG_KASAN_OUTLINE and
97 enable ``CONFIG_STACKTRACE``. To include alloc !! 39 CONFIG_KASAN_INLINE. Outline and inline are compiler instrumentation types.
98 physical pages, enable ``CONFIG_PAGE_OWNER`` a !! 40 The former produces smaller binary while the latter is 1.1 - 2 times faster.
99 41
100 Boot parameters !! 42 Both software KASAN modes work with both SLUB and SLAB memory allocators,
101 ~~~~~~~~~~~~~~~ !! 43 while the hardware tag-based KASAN currently only support SLUB.
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 44
123 - ``kasan.stacktrace=off`` or ``=on`` disables !! 45 For better error reports that include stack traces, enable CONFIG_STACKTRACE.
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 46
134 - ``kasan.mode=sync``, ``=async`` or ``=asymm` !! 47 To augment reports with last allocation and freeing stack of the physical page,
135 is configured in synchronous, asynchronous o !! 48 it is recommended to enable also CONFIG_PAGE_OWNER and boot with page_owner=on.
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 49
166 Error reports 50 Error reports
167 ~~~~~~~~~~~~~ 51 ~~~~~~~~~~~~~
168 52
169 A typical KASAN report looks like this:: !! 53 A typical out-of-bounds access generic KASAN report looks like this::
170 54
171 ========================================== 55 ==================================================================
172 BUG: KASAN: slab-out-of-bounds in kmalloc_ !! 56 BUG: KASAN: slab-out-of-bounds in kmalloc_oob_right+0xa8/0xbc [test_kasan]
173 Write of size 1 at addr ffff8801f44ec37b b 57 Write of size 1 at addr ffff8801f44ec37b by task insmod/2760
174 58
175 CPU: 1 PID: 2760 Comm: insmod Not tainted 59 CPU: 1 PID: 2760 Comm: insmod Not tainted 4.19.0-rc3+ #698
176 Hardware name: QEMU Standard PC (i440FX + 60 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
177 Call Trace: 61 Call Trace:
178 dump_stack+0x94/0xd8 62 dump_stack+0x94/0xd8
179 print_address_description+0x73/0x280 63 print_address_description+0x73/0x280
180 kasan_report+0x144/0x187 64 kasan_report+0x144/0x187
181 __asan_report_store1_noabort+0x17/0x20 65 __asan_report_store1_noabort+0x17/0x20
182 kmalloc_oob_right+0xa8/0xbc [kasan_test] !! 66 kmalloc_oob_right+0xa8/0xbc [test_kasan]
183 kmalloc_tests_init+0x16/0x700 [kasan_test !! 67 kmalloc_tests_init+0x16/0x700 [test_kasan]
184 do_one_initcall+0xa5/0x3ae 68 do_one_initcall+0xa5/0x3ae
185 do_init_module+0x1b6/0x547 69 do_init_module+0x1b6/0x547
186 load_module+0x75df/0x8070 70 load_module+0x75df/0x8070
187 __do_sys_init_module+0x1c6/0x200 71 __do_sys_init_module+0x1c6/0x200
188 __x64_sys_init_module+0x6e/0xb0 72 __x64_sys_init_module+0x6e/0xb0
189 do_syscall_64+0x9f/0x2c0 73 do_syscall_64+0x9f/0x2c0
190 entry_SYSCALL_64_after_hwframe+0x44/0xa9 74 entry_SYSCALL_64_after_hwframe+0x44/0xa9
191 RIP: 0033:0x7f96443109da 75 RIP: 0033:0x7f96443109da
192 RSP: 002b:00007ffcf0b51b08 EFLAGS: 0000020 76 RSP: 002b:00007ffcf0b51b08 EFLAGS: 00000202 ORIG_RAX: 00000000000000af
193 RAX: ffffffffffffffda RBX: 000055dc3ee521a 77 RAX: ffffffffffffffda RBX: 000055dc3ee521a0 RCX: 00007f96443109da
194 RDX: 00007f96445cff88 RSI: 0000000000057a5 78 RDX: 00007f96445cff88 RSI: 0000000000057a50 RDI: 00007f9644992000
195 RBP: 000055dc3ee510b0 R08: 000000000000000 79 RBP: 000055dc3ee510b0 R08: 0000000000000003 R09: 0000000000000000
196 R10: 00007f964430cd0a R11: 000000000000020 80 R10: 00007f964430cd0a R11: 0000000000000202 R12: 00007f96445cff88
197 R13: 000055dc3ee51090 R14: 000000000000000 81 R13: 000055dc3ee51090 R14: 0000000000000000 R15: 0000000000000000
198 82
199 Allocated by task 2760: 83 Allocated by task 2760:
200 save_stack+0x43/0xd0 84 save_stack+0x43/0xd0
201 kasan_kmalloc+0xa7/0xd0 85 kasan_kmalloc+0xa7/0xd0
202 kmem_cache_alloc_trace+0xe1/0x1b0 86 kmem_cache_alloc_trace+0xe1/0x1b0
203 kmalloc_oob_right+0x56/0xbc [kasan_test] !! 87 kmalloc_oob_right+0x56/0xbc [test_kasan]
204 kmalloc_tests_init+0x16/0x700 [kasan_test !! 88 kmalloc_tests_init+0x16/0x700 [test_kasan]
205 do_one_initcall+0xa5/0x3ae 89 do_one_initcall+0xa5/0x3ae
206 do_init_module+0x1b6/0x547 90 do_init_module+0x1b6/0x547
207 load_module+0x75df/0x8070 91 load_module+0x75df/0x8070
208 __do_sys_init_module+0x1c6/0x200 92 __do_sys_init_module+0x1c6/0x200
209 __x64_sys_init_module+0x6e/0xb0 93 __x64_sys_init_module+0x6e/0xb0
210 do_syscall_64+0x9f/0x2c0 94 do_syscall_64+0x9f/0x2c0
211 entry_SYSCALL_64_after_hwframe+0x44/0xa9 95 entry_SYSCALL_64_after_hwframe+0x44/0xa9
212 96
213 Freed by task 815: 97 Freed by task 815:
214 save_stack+0x43/0xd0 98 save_stack+0x43/0xd0
215 __kasan_slab_free+0x135/0x190 99 __kasan_slab_free+0x135/0x190
216 kasan_slab_free+0xe/0x10 100 kasan_slab_free+0xe/0x10
217 kfree+0x93/0x1a0 101 kfree+0x93/0x1a0
218 umh_complete+0x6a/0xa0 102 umh_complete+0x6a/0xa0
219 call_usermodehelper_exec_async+0x4c3/0x64 103 call_usermodehelper_exec_async+0x4c3/0x640
220 ret_from_fork+0x35/0x40 104 ret_from_fork+0x35/0x40
221 105
222 The buggy address belongs to the object at 106 The buggy address belongs to the object at ffff8801f44ec300
223 which belongs to the cache kmalloc-128 of 107 which belongs to the cache kmalloc-128 of size 128
224 The buggy address is located 123 bytes ins 108 The buggy address is located 123 bytes inside of
225 128-byte region [ffff8801f44ec300, ffff88 109 128-byte region [ffff8801f44ec300, ffff8801f44ec380)
226 The buggy address belongs to the page: 110 The buggy address belongs to the page:
227 page:ffffea0007d13b00 count:1 mapcount:0 m 111 page:ffffea0007d13b00 count:1 mapcount:0 mapping:ffff8801f7001640 index:0x0
228 flags: 0x200000000000100(slab) 112 flags: 0x200000000000100(slab)
229 raw: 0200000000000100 ffffea0007d11dc0 000 113 raw: 0200000000000100 ffffea0007d11dc0 0000001a0000001a ffff8801f7001640
230 raw: 0000000000000000 0000000080150015 000 114 raw: 0000000000000000 0000000080150015 00000001ffffffff 0000000000000000
231 page dumped because: kasan: bad access det 115 page dumped because: kasan: bad access detected
232 116
233 Memory state around the buggy address: 117 Memory state around the buggy address:
234 ffff8801f44ec200: fc fc fc fc fc fc fc fc 118 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 119 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 00 120 >ffff8801f44ec300: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 03
237 121 ^
238 ffff8801f44ec380: fc fc fc fc fc fc fc fc 122 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 123 ffff8801f44ec400: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
240 ========================================== 124 ==================================================================
241 125
242 The report header summarizes what kind of bug !! 126 The header of the report provides a short summary of what kind of bug happened
243 caused it. It is followed by a stack trace of !! 127 and what kind of access caused it. It's followed by a stack trace of the bad
244 where the accessed memory was allocated (in ca !! 128 access, a stack trace of where the accessed memory was allocated (in case bad
245 and a stack trace of where the object was free !! 129 access happens on a slab object), and a stack trace of where the object was
246 bug report). Next comes a description of the a !! 130 freed (in case of a use-after-free bug report). Next comes a description of
247 information about the accessed memory page. !! 131 the accessed slab object and information about the accessed memory page.
248 132
249 In the end, the report shows the memory state !! 133 In the last section the report shows memory state around the accessed address.
250 Internally, KASAN tracks memory state separate !! 134 Internally KASAN tracks memory state separately for each memory granule, which
251 is either 8 or 16 aligned bytes depending on K 135 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 136 memory state section of the report shows the state of one of the memory
253 granules that surround the accessed address. 137 granules that surround the accessed address.
254 138
255 For Generic KASAN, the size of each memory gra !! 139 For generic KASAN the size of each memory granule is 8. The state of each
256 granule is encoded in one shadow byte. Those 8 140 granule is encoded in one shadow byte. Those 8 bytes can be accessible,
257 partially accessible, freed, or be a part of a !! 141 partially accessible, freed or be a part of a redzone. KASAN uses the following
258 encoding for each shadow byte: 00 means that a !! 142 encoding for each shadow byte: 0 means that all 8 bytes of the corresponding
259 memory region are accessible; number N (1 <= N 143 memory region are accessible; number N (1 <= N <= 7) means that the first N
260 bytes are accessible, and other (8 - N) bytes 144 bytes are accessible, and other (8 - N) bytes are not; any negative value
261 indicates that the entire 8-byte word is inacc 145 indicates that the entire 8-byte word is inaccessible. KASAN uses different
262 negative values to distinguish between differe 146 negative values to distinguish between different kinds of inaccessible memory
263 like redzones or freed memory (see mm/kasan/ka 147 like redzones or freed memory (see mm/kasan/kasan.h).
264 148
265 In the report above, the arrow points to the s !! 149 In the report above the arrows point to the shadow byte 03, which means that
266 that the accessed address is partially accessi !! 150 the accessed address is partially accessible.
267 151
268 For tag-based KASAN modes, this last report se !! 152 For tag-based KASAN this last report section shows the memory tags around the
269 the accessed address (see the `Implementation !! 153 accessed address (see `Implementation details`_ section).
270 154
271 Note that KASAN bug titles (like ``slab-out-of !! 155 Boot parameters
272 are best-effort: KASAN prints the most probabl !! 156 ~~~~~~~~~~~~~~~
273 information it has. The actual type of the bug <<
274 <<
275 Generic KASAN also reports up to two auxiliary <<
276 traces point to places in code that interacted <<
277 directly present in the bad access stack trace <<
278 call_rcu() and workqueue queuing. <<
279 157
280 CONFIG_KASAN_EXTRA_INFO !! 158 Hardware tag-based KASAN mode (see the section about different mode below) is
281 ~~~~~~~~~~~~~~~~~~~~~~~ !! 159 intended for use in production as a security mitigation. Therefore it supports
>> 160 boot parameters that allow to disable KASAN competely or otherwise control
>> 161 particular KASAN features.
282 162
283 Enabling CONFIG_KASAN_EXTRA_INFO allows KASAN !! 163 - ``kasan=off`` or ``=on`` controls whether KASAN is enabled (default: ``on``).
284 information. The extra information currently s <<
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). <<
289 164
290 Here is the report with CONFIG_KASAN_EXTRA_INF !! 165 - ``kasan.stacktrace=off`` or ``=on`` disables or enables alloc and free stack
291 different parts are shown):: !! 166 traces collection (default: ``on``).
>> 167
>> 168 - ``kasan.fault=report`` or ``=panic`` controls whether to only print a KASAN
>> 169 report or also panic the kernel (default: ``report``).
>> 170
>> 171 For developers
>> 172 ~~~~~~~~~~~~~~
>> 173
>> 174 Software KASAN modes use compiler instrumentation to insert validity checks.
>> 175 Such instrumentation might be incompatible with some part of the kernel, and
>> 176 therefore needs to be disabled. To disable instrumentation for specific files
>> 177 or directories, add a line similar to the following to the respective kernel
>> 178 Makefile:
>> 179
>> 180 - For a single file (e.g. main.o)::
>> 181
>> 182 KASAN_SANITIZE_main.o := n
>> 183
>> 184 - For all files in one directory::
>> 185
>> 186 KASAN_SANITIZE := n
292 187
293 ========================================== <<
294 ... <<
295 Allocated by task 134 on cpu 5 at 229.1338 <<
296 ... <<
297 Freed by task 136 on cpu 3 at 230.199335s: <<
298 ... <<
299 ========================================== <<
300 188
301 Implementation details 189 Implementation details
302 ---------------------- 190 ----------------------
303 191
304 Generic KASAN 192 Generic KASAN
305 ~~~~~~~~~~~~~ 193 ~~~~~~~~~~~~~
306 194
307 Software KASAN modes use shadow memory to reco !! 195 From a high level perspective, KASAN's approach to memory error detection is
308 safe to access and use compile-time instrument !! 196 similar to that of kmemcheck: use shadow memory to record whether each byte of
309 checks before each memory access. !! 197 memory is safe to access, and use compile-time instrumentation to insert checks
>> 198 of shadow memory on each memory access.
310 199
311 Generic KASAN dedicates 1/8th of kernel memory !! 200 Generic KASAN dedicates 1/8th of kernel memory to its shadow memory (e.g. 16TB
312 to cover 128TB on x86_64) and uses direct mapp 201 to cover 128TB on x86_64) and uses direct mapping with a scale and offset to
313 translate a memory address to its correspondin 202 translate a memory address to its corresponding shadow address.
314 203
315 Here is the function which translates an addre 204 Here is the function which translates an address to its corresponding shadow
316 address:: 205 address::
317 206
318 static inline void *kasan_mem_to_shadow(co 207 static inline void *kasan_mem_to_shadow(const void *addr)
319 { 208 {
320 return (void *)((unsigned long)addr >> !! 209 return ((unsigned long)addr >> KASAN_SHADOW_SCALE_SHIFT)
321 + KASAN_SHADOW_OFFSET; 210 + KASAN_SHADOW_OFFSET;
322 } 211 }
323 212
324 where ``KASAN_SHADOW_SCALE_SHIFT = 3``. 213 where ``KASAN_SHADOW_SCALE_SHIFT = 3``.
325 214
326 Compile-time instrumentation is used to insert 215 Compile-time instrumentation is used to insert memory access checks. Compiler
327 inserts function calls (``__asan_load*(addr)`` !! 216 inserts function calls (__asan_load*(addr), __asan_store*(addr)) before each
328 each memory access of size 1, 2, 4, 8, or 16. !! 217 memory access of size 1, 2, 4, 8 or 16. These functions check whether memory
329 memory accesses are valid or not by checking c !! 218 access is valid or not by checking corresponding shadow memory.
330 !! 219
331 With inline instrumentation, instead of making !! 220 GCC 5.0 has possibility to perform inline instrumentation. Instead of making
332 directly inserts the code to check shadow memo !! 221 function calls GCC directly inserts the code to check the shadow memory.
333 enlarges the kernel, but it gives an x1.1-x2 p !! 222 This option significantly enlarges kernel but it gives x1.1-x2 performance
334 outline-instrumented kernel. !! 223 boost over outline instrumented kernel.
335 224
336 Generic KASAN is the only mode that delays the !! 225 Generic KASAN also reports the last 2 call stacks to creation of work that
>> 226 potentially has access to an object. Call stacks for the following are shown:
>> 227 call_rcu() and workqueue queuing.
>> 228
>> 229 Generic KASAN is the only mode that delays the reuse of freed object via
337 quarantine (see mm/kasan/quarantine.c for impl 230 quarantine (see mm/kasan/quarantine.c for implementation).
338 231
339 Software Tag-Based KASAN !! 232 Software tag-based KASAN
340 ~~~~~~~~~~~~~~~~~~~~~~~~ 233 ~~~~~~~~~~~~~~~~~~~~~~~~
341 234
342 Software Tag-Based KASAN uses a software memor !! 235 Software tag-based KASAN requires software memory tagging support in the form
343 access validity. It is currently only implemen !! 236 of HWASan-like compiler instrumentation (see HWASan documentation for details).
>> 237
>> 238 Software tag-based KASAN is currently only implemented for arm64 architecture.
344 239
345 Software Tag-Based KASAN uses the Top Byte Ign !! 240 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 !! 241 to store a pointer tag in the top byte of kernel pointers. Like generic KASAN
347 to store memory tags associated with each 16-b !! 242 it uses shadow memory to store memory tags associated with each 16-byte memory
348 dedicates 1/16th of the kernel memory for shad !! 243 cell (therefore it dedicates 1/16th of the kernel memory for shadow memory).
349 244
350 On each memory allocation, Software Tag-Based !! 245 On each memory allocation software tag-based KASAN generates a random tag, tags
351 the allocated memory with this tag, and embeds !! 246 the allocated memory with this tag, and embeds this tag into the returned
352 pointer. 247 pointer.
353 248
354 Software Tag-Based KASAN uses compile-time ins !! 249 Software tag-based KASAN uses compile-time instrumentation to insert checks
355 before each memory access. These checks make s !! 250 before each memory access. These checks make sure that tag of the memory that
356 that is being accessed is equal to the tag of !! 251 is being accessed is equal to tag of the pointer that is used to access this
357 this memory. In case of a tag mismatch, Softwa !! 252 memory. In case of a tag mismatch software tag-based KASAN prints a bug report.
358 report. <<
359 253
360 Software Tag-Based KASAN also has two instrume !! 254 Software tag-based KASAN also has two instrumentation modes (outline, that
361 emits callbacks to check memory accesses; and !! 255 emits callbacks to check memory accesses; and inline, that performs the shadow
362 memory checks inline). With outline instrument 256 memory checks inline). With outline instrumentation mode, a bug report is
363 printed from the function that performs the ac !! 257 simply printed from the function that performs the access check. With inline
364 instrumentation, a ``brk`` instruction is emit !! 258 instrumentation a brk instruction is emitted by the compiler, and a dedicated
365 dedicated ``brk`` handler is used to print bug !! 259 brk handler is used to print bug reports.
366 260
367 Software Tag-Based KASAN uses 0xFF as a match- !! 261 Software tag-based KASAN uses 0xFF as a match-all pointer tag (accesses through
368 pointers with the 0xFF pointer tag are not che !! 262 pointers with 0xFF pointer tag aren't checked). The value 0xFE is currently
369 reserved to tag freed memory regions. 263 reserved to tag freed memory regions.
370 264
371 Hardware Tag-Based KASAN !! 265 Software tag-based KASAN currently only supports tagging of
>> 266 kmem_cache_alloc/kmalloc and page_alloc memory.
>> 267
>> 268 Hardware tag-based KASAN
372 ~~~~~~~~~~~~~~~~~~~~~~~~ 269 ~~~~~~~~~~~~~~~~~~~~~~~~
373 270
374 Hardware Tag-Based KASAN is similar to the sof !! 271 Hardware tag-based KASAN is similar to the software mode in concept, but uses
375 hardware memory tagging support instead of com 272 hardware memory tagging support instead of compiler instrumentation and
376 shadow memory. 273 shadow memory.
377 274
378 Hardware Tag-Based KASAN is currently only imp !! 275 Hardware tag-based KASAN is currently only implemented for arm64 architecture
379 and based on both arm64 Memory Tagging Extensi 276 and based on both arm64 Memory Tagging Extension (MTE) introduced in ARMv8.5
380 Instruction Set Architecture and Top Byte Igno !! 277 Instruction Set Architecture, and Top Byte Ignore (TBI).
381 278
382 Special arm64 instructions are used to assign 279 Special arm64 instructions are used to assign memory tags for each allocation.
383 Same tags are assigned to pointers to those al 280 Same tags are assigned to pointers to those allocations. On every memory
384 access, hardware makes sure that the tag of th !! 281 access, hardware makes sure that tag of the memory that is being accessed is
385 equal to the tag of the pointer that is used t !! 282 equal to 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 !! 283 tag mismatch a fault is generated and a report is printed.
387 284
388 Hardware Tag-Based KASAN uses 0xFF as a match- !! 285 Hardware tag-based KASAN uses 0xFF as a match-all pointer tag (accesses through
389 pointers with the 0xFF pointer tag are not che !! 286 pointers with 0xFF pointer tag aren't checked). The value 0xFE is currently
390 reserved to tag freed memory regions. 287 reserved to tag freed memory regions.
391 288
392 If the hardware does not support MTE (pre ARMv !! 289 Hardware tag-based KASAN currently only supports tagging of
393 will not be enabled. In this case, all KASAN b !! 290 kmem_cache_alloc/kmalloc and page_alloc memory.
394 <<
395 Note that enabling CONFIG_KASAN_HW_TAGS always <<
396 enabled. Even when ``kasan.mode=off`` is provi <<
397 support MTE (but supports TBI). <<
398 <<
399 Hardware Tag-Based KASAN only reports the firs <<
400 checking gets disabled. <<
401 <<
402 Shadow memory <<
403 ------------- <<
404 291
405 The contents of this section are only applicab !! 292 What memory accesses are sanitised by KASAN?
>> 293 --------------------------------------------
406 294
407 The kernel maps memory in several different pa !! 295 The kernel maps memory in a number of different parts of the address
408 The range of kernel virtual addresses is large !! 296 space. This poses something of a problem for KASAN, which requires
409 memory to support a real shadow region for eve !! 297 that all addresses accessed by instrumented code have a valid shadow
410 accessed by the kernel. Therefore, KASAN only !! 298 region.
411 parts of the address space. !! 299
>> 300 The range of kernel virtual addresses is large: there is not enough
>> 301 real memory to support a real shadow region for every address that
>> 302 could be accessed by the kernel.
412 303
413 Default behaviour !! 304 By default
414 ~~~~~~~~~~~~~~~~~ !! 305 ~~~~~~~~~~
415 306
416 By default, architectures only map real memory 307 By default, architectures only map real memory over the shadow region
417 for the linear mapping (and potentially other 308 for the linear mapping (and potentially other small areas). For all
418 other areas - such as vmalloc and vmemmap spac 309 other areas - such as vmalloc and vmemmap space - a single read-only
419 page is mapped over the shadow area. This read 310 page is mapped over the shadow area. This read-only shadow page
420 declares all memory accesses as permitted. 311 declares all memory accesses as permitted.
421 312
422 This presents a problem for modules: they do n 313 This presents a problem for modules: they do not live in the linear
423 mapping but in a dedicated module space. By ho !! 314 mapping, but in a dedicated module space. By hooking in to the module
424 allocator, KASAN temporarily maps real shadow !! 315 allocator, KASAN can temporarily map real shadow memory to cover
425 This allows detection of invalid accesses to m !! 316 them. This allows detection of invalid accesses to module globals, for
>> 317 example.
426 318
427 This also creates an incompatibility with ``VM 319 This also creates an incompatibility with ``VMAP_STACK``: if the stack
428 lives in vmalloc space, it will be shadowed by 320 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 321 the kernel will fault when trying to set up the shadow data for stack
430 variables. 322 variables.
431 323
432 CONFIG_KASAN_VMALLOC 324 CONFIG_KASAN_VMALLOC
433 ~~~~~~~~~~~~~~~~~~~~ 325 ~~~~~~~~~~~~~~~~~~~~
434 326
435 With ``CONFIG_KASAN_VMALLOC``, KASAN can cover 327 With ``CONFIG_KASAN_VMALLOC``, KASAN can cover vmalloc space at the
436 cost of greater memory usage. Currently, this !! 328 cost of greater memory usage. Currently this is only supported on x86.
437 arm64, riscv, s390, and powerpc. <<
438 329
439 This works by hooking into vmalloc and vmap an !! 330 This works by hooking into vmalloc and vmap, and dynamically
440 allocating real shadow memory to back the mapp 331 allocating real shadow memory to back the mappings.
441 332
442 Most mappings in vmalloc space are small, requ 333 Most mappings in vmalloc space are small, requiring less than a full
443 page of shadow space. Allocating a full shadow 334 page of shadow space. Allocating a full shadow page per mapping would
444 therefore be wasteful. Furthermore, to ensure 335 therefore be wasteful. Furthermore, to ensure that different mappings
445 use different shadow pages, mappings would hav 336 use different shadow pages, mappings would have to be aligned to
446 ``KASAN_GRANULE_SIZE * PAGE_SIZE``. 337 ``KASAN_GRANULE_SIZE * PAGE_SIZE``.
447 338
448 Instead, KASAN shares backing space across mul 339 Instead, KASAN shares backing space across multiple mappings. It allocates
449 a backing page when a mapping in vmalloc space 340 a backing page when a mapping in vmalloc space uses a particular page
450 of the shadow region. This page can be shared 341 of the shadow region. This page can be shared by other vmalloc
451 mappings later on. 342 mappings later on.
452 343
453 KASAN hooks into the vmap infrastructure to la 344 KASAN hooks into the vmap infrastructure to lazily clean up unused shadow
454 memory. 345 memory.
455 346
456 To avoid the difficulties around swapping mapp 347 To avoid the difficulties around swapping mappings around, KASAN expects
457 that the part of the shadow region that covers 348 that the part of the shadow region that covers the vmalloc space will
458 not be covered by the early shadow page but wi !! 349 not be covered by the early shadow page, but will be left
459 This will require changes in arch-specific cod !! 350 unmapped. This will require changes in arch-specific code.
460 351
461 This allows ``VMAP_STACK`` support on x86 and !! 352 This allows ``VMAP_STACK`` support on x86, and can simplify support of
462 architectures that do not have a fixed module 353 architectures that do not have a fixed module region.
463 354
464 For developers !! 355 CONFIG_KASAN_KUNIT_TEST & CONFIG_TEST_KASAN_MODULE
465 -------------- !! 356 --------------------------------------------------
466 <<
467 Ignoring accesses <<
468 ~~~~~~~~~~~~~~~~~ <<
469 <<
470 Software KASAN modes use compiler instrumentat <<
471 Such instrumentation might be incompatible wit <<
472 therefore needs to be disabled. <<
473 <<
474 Other parts of the kernel might access metadat <<
475 Normally, KASAN detects and reports such acces <<
476 in memory allocators), these accesses are vali <<
477 <<
478 For software KASAN modes, to disable instrumen <<
479 directory, add a ``KASAN_SANITIZE`` annotation <<
480 Makefile: <<
481 <<
482 - For a single file (e.g., main.o):: <<
483 <<
484 KASAN_SANITIZE_main.o := n <<
485 <<
486 - For all files in one directory:: <<
487 <<
488 KASAN_SANITIZE := n <<
489 357
490 For software KASAN modes, to disable instrumen !! 358 KASAN tests consist on two parts:
491 use the KASAN-specific ``__no_sanitize_address <<
492 generic ``noinstr`` one. <<
493 <<
494 Note that disabling compiler instrumentation ( <<
495 per-function basis) makes KASAN ignore the acc <<
496 that code for software KASAN modes. It does no <<
497 indirectly (through calls to instrumented func <<
498 Tag-Based KASAN, which does not use compiler i <<
499 <<
500 For software KASAN modes, to disable KASAN rep <<
501 for the current task, annotate this part of th <<
502 ``kasan_disable_current()``/``kasan_enable_cur <<
503 disables the reports for indirect accesses tha <<
504 <<
505 For tag-based KASAN modes, to disable access c <<
506 ``kasan_reset_tag()`` or ``page_kasan_tag_rese <<
507 disabling access checking via ``page_kasan_tag <<
508 restoring the per-page KASAN tag via ``page_ka <<
509 <<
510 Tests <<
511 ~~~~~ <<
512 <<
513 There are KASAN tests that allow verifying tha <<
514 certain types of memory corruptions. The tests <<
515 359
516 1. Tests that are integrated with the KUnit Te 360 1. Tests that are integrated with the KUnit Test Framework. Enabled with
517 ``CONFIG_KASAN_KUNIT_TEST``. These tests can b 361 ``CONFIG_KASAN_KUNIT_TEST``. These tests can be run and partially verified
518 automatically in a few different ways; see the !! 362 automatically in a few different ways, see the instructions below.
519 363
520 2. Tests that are currently incompatible with 364 2. Tests that are currently incompatible with KUnit. Enabled with
521 ``CONFIG_KASAN_MODULE_TEST`` and can only be r !! 365 ``CONFIG_TEST_KASAN_MODULE`` and can only be run as a module. These tests can
522 only be verified manually by loading the kerne !! 366 only be verified manually, by loading the kernel module and inspecting the
523 kernel log for KASAN reports. 367 kernel log for KASAN reports.
524 368
525 Each KUnit-compatible KASAN test prints one of !! 369 Each KUnit-compatible KASAN test prints a KASAN report if an error is detected.
526 error is detected. Then the test prints its nu !! 370 Then the test prints its number and status.
527 371
528 When a test passes:: 372 When a test passes::
529 373
530 ok 28 - kmalloc_double_kzfree 374 ok 28 - kmalloc_double_kzfree
531 375
532 When a test fails due to a failed ``kmalloc``: 376 When a test fails due to a failed ``kmalloc``::
533 377
534 # kmalloc_large_oob_right: ASSERTION F !! 378 # kmalloc_large_oob_right: ASSERTION FAILED at lib/test_kasan.c:163
535 Expected ptr is not null, but is 379 Expected ptr is not null, but is
536 not ok 5 - kmalloc_large_oob_right !! 380 not ok 4 - kmalloc_large_oob_right
537 381
538 When a test fails due to a missing KASAN repor 382 When a test fails due to a missing KASAN report::
539 383
540 # kmalloc_double_kzfree: EXPECTATION F !! 384 # kmalloc_double_kzfree: EXPECTATION FAILED at lib/test_kasan.c:629
541 KASAN failure expected in "kfree_sensi !! 385 Expected kasan_data->report_expected == kasan_data->report_found, but
>> 386 kasan_data->report_expected == 1
>> 387 kasan_data->report_found == 0
542 not ok 28 - kmalloc_double_kzfree 388 not ok 28 - kmalloc_double_kzfree
543 389
544 <<
545 At the end the cumulative status of all KASAN 390 At the end the cumulative status of all KASAN tests is printed. On success::
546 391
547 ok 1 - kasan 392 ok 1 - kasan
548 393
549 Or, if one of the tests failed:: 394 Or, if one of the tests failed::
550 395
551 not ok 1 - kasan 396 not ok 1 - kasan
552 397
>> 398
553 There are a few ways to run KUnit-compatible K 399 There are a few ways to run KUnit-compatible KASAN tests.
554 400
555 1. Loadable module 401 1. Loadable module
>> 402 ~~~~~~~~~~~~~~~~~~
556 403
557 With ``CONFIG_KUNIT`` enabled, KASAN-KUnit !! 404 With ``CONFIG_KUNIT`` enabled, ``CONFIG_KASAN_KUNIT_TEST`` can be built as
558 module and run by loading ``kasan_test.ko`` !! 405 a loadable module and run on any architecture that supports KASAN by loading
>> 406 the module with insmod or modprobe. The module is called ``test_kasan``.
559 407
560 2. Built-In 408 2. Built-In
>> 409 ~~~~~~~~~~~
561 410
562 With ``CONFIG_KUNIT`` built-in, KASAN-KUnit !! 411 With ``CONFIG_KUNIT`` built-in, ``CONFIG_KASAN_KUNIT_TEST`` can be built-in
563 In this case, the tests will run at boot as !! 412 on any architecure that supports KASAN. These and any other KUnit tests enabled
>> 413 will run and print the results at boot as a late-init call.
564 414
565 3. Using kunit_tool 415 3. Using kunit_tool
>> 416 ~~~~~~~~~~~~~~~~~~~
566 417
567 With ``CONFIG_KUNIT`` and ``CONFIG_KASAN_KU !! 418 With ``CONFIG_KUNIT`` and ``CONFIG_KASAN_KUNIT_TEST`` built-in, it's also
568 possible to use ``kunit_tool`` to see the r !! 419 possible use ``kunit_tool`` to see the results of these and other KUnit tests
569 readable way. This will not print the KASAN !! 420 in a more readable way. This will not print the KASAN reports of the tests that
570 See `KUnit documentation <https://www.kerne !! 421 passed. Use `KUnit documentation <https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html>`_
571 for more up-to-date information on ``kunit_ !! 422 for more up-to-date information on ``kunit_tool``.
572 423
573 .. _KUnit: https://www.kernel.org/doc/html/lat 424 .. _KUnit: https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html
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