1 .. SPDX-License-Identifier: GPL-2.0 1 .. SPDX-License-Identifier: GPL-2.0
2 .. Copyright (C) 2020, Google LLC. 2 .. Copyright (C) 2020, Google LLC.
3 3
4 Kernel Electric-Fence (KFENCE) 4 Kernel Electric-Fence (KFENCE)
5 ============================== 5 ==============================
6 6
7 Kernel Electric-Fence (KFENCE) is a low-overhe 7 Kernel Electric-Fence (KFENCE) is a low-overhead sampling-based memory safety
8 error detector. KFENCE detects heap out-of-bou 8 error detector. KFENCE detects heap out-of-bounds access, use-after-free, and
9 invalid-free errors. 9 invalid-free errors.
10 10
11 KFENCE is designed to be enabled in production 11 KFENCE is designed to be enabled in production kernels, and has near zero
12 performance overhead. Compared to KASAN, KFENC 12 performance overhead. Compared to KASAN, KFENCE trades performance for
13 precision. The main motivation behind KFENCE's 13 precision. The main motivation behind KFENCE's design, is that with enough
14 total uptime KFENCE will detect bugs in code p 14 total uptime KFENCE will detect bugs in code paths not typically exercised by
15 non-production test workloads. One way to quic 15 non-production test workloads. One way to quickly achieve a large enough total
16 uptime is when the tool is deployed across a l 16 uptime is when the tool is deployed across a large fleet of machines.
17 17
18 Usage 18 Usage
19 ----- 19 -----
20 20
21 To enable KFENCE, configure the kernel with:: 21 To enable KFENCE, configure the kernel with::
22 22
23 CONFIG_KFENCE=y 23 CONFIG_KFENCE=y
24 24
25 To build a kernel with KFENCE support, but dis 25 To build a kernel with KFENCE support, but disabled by default (to enable, set
26 ``kfence.sample_interval`` to non-zero value), 26 ``kfence.sample_interval`` to non-zero value), configure the kernel with::
27 27
28 CONFIG_KFENCE=y 28 CONFIG_KFENCE=y
29 CONFIG_KFENCE_SAMPLE_INTERVAL=0 29 CONFIG_KFENCE_SAMPLE_INTERVAL=0
30 30
31 KFENCE provides several other configuration op 31 KFENCE provides several other configuration options to customize behaviour (see
32 the respective help text in ``lib/Kconfig.kfen 32 the respective help text in ``lib/Kconfig.kfence`` for more info).
33 33
34 Tuning performance 34 Tuning performance
35 ~~~~~~~~~~~~~~~~~~ 35 ~~~~~~~~~~~~~~~~~~
36 36
37 The most important parameter is KFENCE's sampl 37 The most important parameter is KFENCE's sample interval, which can be set via
38 the kernel boot parameter ``kfence.sample_inte 38 the kernel boot parameter ``kfence.sample_interval`` in milliseconds. The
39 sample interval determines the frequency with 39 sample interval determines the frequency with which heap allocations will be
40 guarded by KFENCE. The default is configurable 40 guarded by KFENCE. The default is configurable via the Kconfig option
41 ``CONFIG_KFENCE_SAMPLE_INTERVAL``. Setting ``k 41 ``CONFIG_KFENCE_SAMPLE_INTERVAL``. Setting ``kfence.sample_interval=0``
42 disables KFENCE. 42 disables KFENCE.
43 43
44 The sample interval controls a timer that sets <<
45 default, to keep the real sample interval pred <<
46 causes CPU wake-ups when the system is complet <<
47 on power-constrained systems. The boot paramet <<
48 instead switches to a "deferrable" timer which <<
49 idle systems, at the risk of unpredictable sam <<
50 configurable via the Kconfig option ``CONFIG_K <<
51 <<
52 .. warning:: <<
53 The KUnit test suite is very likely to fail <<
54 since it currently causes very unpredictabl <<
55 <<
56 By default KFENCE will only sample 1 heap allo <<
57 interval. *Burst mode* allows to sample succes <<
58 kernel boot parameter ``kfence.burst`` can be <<
59 denotes the *additional* successive allocation <<
60 setting ``kfence.burst=N`` means that ``1 + N` <<
61 attempted through KFENCE for each sample inter <<
62 <<
63 The KFENCE memory pool is of fixed size, and i 44 The KFENCE memory pool is of fixed size, and if the pool is exhausted, no
64 further KFENCE allocations occur. With ``CONFI 45 further KFENCE allocations occur. With ``CONFIG_KFENCE_NUM_OBJECTS`` (default
65 255), the number of available guarded objects 46 255), the number of available guarded objects can be controlled. Each object
66 requires 2 pages, one for the object itself an 47 requires 2 pages, one for the object itself and the other one used as a guard
67 page; object pages are interleaved with guard 48 page; object pages are interleaved with guard pages, and every object page is
68 therefore surrounded by two guard pages. 49 therefore surrounded by two guard pages.
69 50
70 The total memory dedicated to the KFENCE memor 51 The total memory dedicated to the KFENCE memory pool can be computed as::
71 52
72 ( #objects + 1 ) * 2 * PAGE_SIZE 53 ( #objects + 1 ) * 2 * PAGE_SIZE
73 54
74 Using the default config, and assuming a page 55 Using the default config, and assuming a page size of 4 KiB, results in
75 dedicating 2 MiB to the KFENCE memory pool. 56 dedicating 2 MiB to the KFENCE memory pool.
76 57
77 Note: On architectures that support huge pages 58 Note: On architectures that support huge pages, KFENCE will ensure that the
78 pool is using pages of size ``PAGE_SIZE``. Thi 59 pool is using pages of size ``PAGE_SIZE``. This will result in additional page
79 tables being allocated. 60 tables being allocated.
80 61
81 Error reports 62 Error reports
82 ~~~~~~~~~~~~~ 63 ~~~~~~~~~~~~~
83 64
84 A typical out-of-bounds access looks like this 65 A typical out-of-bounds access looks like this::
85 66
86 ========================================== 67 ==================================================================
87 BUG: KFENCE: out-of-bounds read in test_ou !! 68 BUG: KFENCE: out-of-bounds read in test_out_of_bounds_read+0xa3/0x22b
88 69
89 Out-of-bounds read at 0xffff8c3f2e291fff ( !! 70 Out-of-bounds read at 0xffffffffb672efff (1B left of kfence-#17):
90 test_out_of_bounds_read+0xa6/0x234 !! 71 test_out_of_bounds_read+0xa3/0x22b
91 kunit_try_run_case+0x61/0xa0 !! 72 kunit_try_run_case+0x51/0x85
92 kunit_generic_run_threadfn_adapter+0x16/0 73 kunit_generic_run_threadfn_adapter+0x16/0x30
93 kthread+0x176/0x1b0 !! 74 kthread+0x137/0x160
94 ret_from_fork+0x22/0x30 75 ret_from_fork+0x22/0x30
95 76
96 kfence-#72: 0xffff8c3f2e292000-0xffff8c3f2 !! 77 kfence-#17 [0xffffffffb672f000-0xffffffffb672f01f, size=32, cache=kmalloc-32] allocated by task 507:
97 !! 78 test_alloc+0xf3/0x25b
98 allocated by task 484 on cpu 0 at 32.91933 !! 79 test_out_of_bounds_read+0x98/0x22b
99 test_alloc+0xfe/0x738 !! 80 kunit_try_run_case+0x51/0x85
100 test_out_of_bounds_read+0x9b/0x234 <<
101 kunit_try_run_case+0x61/0xa0 <<
102 kunit_generic_run_threadfn_adapter+0x16/0 81 kunit_generic_run_threadfn_adapter+0x16/0x30
103 kthread+0x176/0x1b0 !! 82 kthread+0x137/0x160
104 ret_from_fork+0x22/0x30 83 ret_from_fork+0x22/0x30
105 84
106 CPU: 0 PID: 484 Comm: kunit_try_catch Not !! 85 CPU: 4 PID: 107 Comm: kunit_try_catch Not tainted 5.8.0-rc6+ #7
107 Hardware name: QEMU Standard PC (i440FX + !! 86 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1 04/01/2014
108 ========================================== 87 ==================================================================
109 88
110 The header of the report provides a short summ 89 The header of the report provides a short summary of the function involved in
111 the access. It is followed by more detailed in 90 the access. It is followed by more detailed information about the access and
112 its origin. Note that, real kernel addresses a 91 its origin. Note that, real kernel addresses are only shown when using the
113 kernel command line option ``no_hash_pointers` 92 kernel command line option ``no_hash_pointers``.
114 93
115 Use-after-free accesses are reported as:: 94 Use-after-free accesses are reported as::
116 95
117 ========================================== 96 ==================================================================
118 BUG: KFENCE: use-after-free read in test_u 97 BUG: KFENCE: use-after-free read in test_use_after_free_read+0xb3/0x143
119 98
120 Use-after-free read at 0xffff8c3f2e2a0000 !! 99 Use-after-free read at 0xffffffffb673dfe0 (in kfence-#24):
121 test_use_after_free_read+0xb3/0x143 100 test_use_after_free_read+0xb3/0x143
122 kunit_try_run_case+0x61/0xa0 !! 101 kunit_try_run_case+0x51/0x85
123 kunit_generic_run_threadfn_adapter+0x16/0 102 kunit_generic_run_threadfn_adapter+0x16/0x30
124 kthread+0x176/0x1b0 !! 103 kthread+0x137/0x160
125 ret_from_fork+0x22/0x30 104 ret_from_fork+0x22/0x30
126 105
127 kfence-#79: 0xffff8c3f2e2a0000-0xffff8c3f2 !! 106 kfence-#24 [0xffffffffb673dfe0-0xffffffffb673dfff, size=32, cache=kmalloc-32] allocated by task 507:
128 !! 107 test_alloc+0xf3/0x25b
129 allocated by task 488 on cpu 2 at 33.87132 <<
130 test_alloc+0xfe/0x738 <<
131 test_use_after_free_read+0x76/0x143 108 test_use_after_free_read+0x76/0x143
132 kunit_try_run_case+0x61/0xa0 !! 109 kunit_try_run_case+0x51/0x85
133 kunit_generic_run_threadfn_adapter+0x16/0 110 kunit_generic_run_threadfn_adapter+0x16/0x30
134 kthread+0x176/0x1b0 !! 111 kthread+0x137/0x160
135 ret_from_fork+0x22/0x30 112 ret_from_fork+0x22/0x30
136 113
137 freed by task 488 on cpu 2 at 33.871358s: !! 114 freed by task 507:
138 test_use_after_free_read+0xa8/0x143 115 test_use_after_free_read+0xa8/0x143
139 kunit_try_run_case+0x61/0xa0 !! 116 kunit_try_run_case+0x51/0x85
140 kunit_generic_run_threadfn_adapter+0x16/0 117 kunit_generic_run_threadfn_adapter+0x16/0x30
141 kthread+0x176/0x1b0 !! 118 kthread+0x137/0x160
142 ret_from_fork+0x22/0x30 119 ret_from_fork+0x22/0x30
143 120
144 CPU: 2 PID: 488 Comm: kunit_try_catch Tain !! 121 CPU: 4 PID: 109 Comm: kunit_try_catch Tainted: G W 5.8.0-rc6+ #7
145 Hardware name: QEMU Standard PC (i440FX + !! 122 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1 04/01/2014
146 ========================================== 123 ==================================================================
147 124
148 KFENCE also reports on invalid frees, such as 125 KFENCE also reports on invalid frees, such as double-frees::
149 126
150 ========================================== 127 ==================================================================
151 BUG: KFENCE: invalid free in test_double_f 128 BUG: KFENCE: invalid free in test_double_free+0xdc/0x171
152 129
153 Invalid free of 0xffff8c3f2e2a4000 (in kfe !! 130 Invalid free of 0xffffffffb6741000:
154 test_double_free+0xdc/0x171 131 test_double_free+0xdc/0x171
155 kunit_try_run_case+0x61/0xa0 !! 132 kunit_try_run_case+0x51/0x85
156 kunit_generic_run_threadfn_adapter+0x16/0 133 kunit_generic_run_threadfn_adapter+0x16/0x30
157 kthread+0x176/0x1b0 !! 134 kthread+0x137/0x160
158 ret_from_fork+0x22/0x30 135 ret_from_fork+0x22/0x30
159 136
160 kfence-#81: 0xffff8c3f2e2a4000-0xffff8c3f2 !! 137 kfence-#26 [0xffffffffb6741000-0xffffffffb674101f, size=32, cache=kmalloc-32] allocated by task 507:
161 !! 138 test_alloc+0xf3/0x25b
162 allocated by task 490 on cpu 1 at 34.17532 <<
163 test_alloc+0xfe/0x738 <<
164 test_double_free+0x76/0x171 139 test_double_free+0x76/0x171
165 kunit_try_run_case+0x61/0xa0 !! 140 kunit_try_run_case+0x51/0x85
166 kunit_generic_run_threadfn_adapter+0x16/0 141 kunit_generic_run_threadfn_adapter+0x16/0x30
167 kthread+0x176/0x1b0 !! 142 kthread+0x137/0x160
168 ret_from_fork+0x22/0x30 143 ret_from_fork+0x22/0x30
169 144
170 freed by task 490 on cpu 1 at 34.175348s: !! 145 freed by task 507:
171 test_double_free+0xa8/0x171 146 test_double_free+0xa8/0x171
172 kunit_try_run_case+0x61/0xa0 !! 147 kunit_try_run_case+0x51/0x85
173 kunit_generic_run_threadfn_adapter+0x16/0 148 kunit_generic_run_threadfn_adapter+0x16/0x30
174 kthread+0x176/0x1b0 !! 149 kthread+0x137/0x160
175 ret_from_fork+0x22/0x30 150 ret_from_fork+0x22/0x30
176 151
177 CPU: 1 PID: 490 Comm: kunit_try_catch Tain !! 152 CPU: 4 PID: 111 Comm: kunit_try_catch Tainted: G W 5.8.0-rc6+ #7
178 Hardware name: QEMU Standard PC (i440FX + !! 153 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1 04/01/2014
179 ========================================== 154 ==================================================================
180 155
181 KFENCE also uses pattern-based redzones on the 156 KFENCE also uses pattern-based redzones on the other side of an object's guard
182 page, to detect out-of-bounds writes on the un 157 page, to detect out-of-bounds writes on the unprotected side of the object.
183 These are reported on frees:: 158 These are reported on frees::
184 159
185 ========================================== 160 ==================================================================
186 BUG: KFENCE: memory corruption in test_kma 161 BUG: KFENCE: memory corruption in test_kmalloc_aligned_oob_write+0xef/0x184
187 162
188 Corrupted memory at 0xffff8c3f2e33aff9 [ 0 !! 163 Corrupted memory at 0xffffffffb6797ff9 [ 0xac . . . . . . ] (in kfence-#69):
189 test_kmalloc_aligned_oob_write+0xef/0x184 164 test_kmalloc_aligned_oob_write+0xef/0x184
190 kunit_try_run_case+0x61/0xa0 !! 165 kunit_try_run_case+0x51/0x85
191 kunit_generic_run_threadfn_adapter+0x16/0 166 kunit_generic_run_threadfn_adapter+0x16/0x30
192 kthread+0x176/0x1b0 !! 167 kthread+0x137/0x160
193 ret_from_fork+0x22/0x30 168 ret_from_fork+0x22/0x30
194 169
195 kfence-#156: 0xffff8c3f2e33afb0-0xffff8c3f !! 170 kfence-#69 [0xffffffffb6797fb0-0xffffffffb6797ff8, size=73, cache=kmalloc-96] allocated by task 507:
196 !! 171 test_alloc+0xf3/0x25b
197 allocated by task 502 on cpu 7 at 42.15930 <<
198 test_alloc+0xfe/0x738 <<
199 test_kmalloc_aligned_oob_write+0x57/0x184 172 test_kmalloc_aligned_oob_write+0x57/0x184
200 kunit_try_run_case+0x61/0xa0 !! 173 kunit_try_run_case+0x51/0x85
201 kunit_generic_run_threadfn_adapter+0x16/0 174 kunit_generic_run_threadfn_adapter+0x16/0x30
202 kthread+0x176/0x1b0 !! 175 kthread+0x137/0x160
203 ret_from_fork+0x22/0x30 176 ret_from_fork+0x22/0x30
204 177
205 CPU: 7 PID: 502 Comm: kunit_try_catch Tain !! 178 CPU: 4 PID: 120 Comm: kunit_try_catch Tainted: G W 5.8.0-rc6+ #7
206 Hardware name: QEMU Standard PC (i440FX + !! 179 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1 04/01/2014
207 ========================================== 180 ==================================================================
208 181
209 For such errors, the address where the corrupt 182 For such errors, the address where the corruption occurred as well as the
210 invalidly written bytes (offset from the addre 183 invalidly written bytes (offset from the address) are shown; in this
211 representation, '.' denote untouched bytes. In 184 representation, '.' denote untouched bytes. In the example above ``0xac`` is
212 the value written to the invalid address at of 185 the value written to the invalid address at offset 0, and the remaining '.'
213 denote that no following bytes have been touch 186 denote that no following bytes have been touched. Note that, real values are
214 only shown if the kernel was booted with ``no_ 187 only shown if the kernel was booted with ``no_hash_pointers``; to avoid
215 information disclosure otherwise, '!' is used 188 information disclosure otherwise, '!' is used instead to denote invalidly
216 written bytes. 189 written bytes.
217 190
218 And finally, KFENCE may also report on invalid 191 And finally, KFENCE may also report on invalid accesses to any protected page
219 where it was not possible to determine an asso 192 where it was not possible to determine an associated object, e.g. if adjacent
220 object pages had not yet been allocated:: 193 object pages had not yet been allocated::
221 194
222 ========================================== 195 ==================================================================
223 BUG: KFENCE: invalid read in test_invalid_ 196 BUG: KFENCE: invalid read in test_invalid_access+0x26/0xe0
224 197
225 Invalid read at 0xffffffffb670b00a: 198 Invalid read at 0xffffffffb670b00a:
226 test_invalid_access+0x26/0xe0 199 test_invalid_access+0x26/0xe0
227 kunit_try_run_case+0x51/0x85 200 kunit_try_run_case+0x51/0x85
228 kunit_generic_run_threadfn_adapter+0x16/0 201 kunit_generic_run_threadfn_adapter+0x16/0x30
229 kthread+0x137/0x160 202 kthread+0x137/0x160
230 ret_from_fork+0x22/0x30 203 ret_from_fork+0x22/0x30
231 204
232 CPU: 4 PID: 124 Comm: kunit_try_catch Tain 205 CPU: 4 PID: 124 Comm: kunit_try_catch Tainted: G W 5.8.0-rc6+ #7
233 Hardware name: QEMU Standard PC (i440FX + 206 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1 04/01/2014
234 ========================================== 207 ==================================================================
235 208
236 DebugFS interface 209 DebugFS interface
237 ~~~~~~~~~~~~~~~~~ 210 ~~~~~~~~~~~~~~~~~
238 211
239 Some debugging information is exposed via debu 212 Some debugging information is exposed via debugfs:
240 213
241 * The file ``/sys/kernel/debug/kfence/stats`` 214 * The file ``/sys/kernel/debug/kfence/stats`` provides runtime statistics.
242 215
243 * The file ``/sys/kernel/debug/kfence/objects` 216 * The file ``/sys/kernel/debug/kfence/objects`` provides a list of objects
244 allocated via KFENCE, including those alread 217 allocated via KFENCE, including those already freed but protected.
245 218
246 Implementation Details 219 Implementation Details
247 ---------------------- 220 ----------------------
248 221
249 Guarded allocations are set up based on the sa 222 Guarded allocations are set up based on the sample interval. After expiration
250 of the sample interval, the next allocation th 223 of the sample interval, the next allocation through the main allocator (SLAB or
251 SLUB) returns a guarded allocation from the KF 224 SLUB) returns a guarded allocation from the KFENCE object pool (allocation
252 sizes up to PAGE_SIZE are supported). At this 225 sizes up to PAGE_SIZE are supported). At this point, the timer is reset, and
253 the next allocation is set up after the expira !! 226 the next allocation is set up after the expiration of the interval. To "gate" a
254 !! 227 KFENCE allocation through the main allocator's fast-path without overhead,
255 When using ``CONFIG_KFENCE_STATIC_KEYS=y``, KF !! 228 KFENCE relies on static branches via the static keys infrastructure. The static
256 through the main allocator's fast-path by rely !! 229 branch is toggled to redirect the allocation to KFENCE.
257 static keys infrastructure. The static branch <<
258 allocation to KFENCE. Depending on sample inte <<
259 system architecture, this may perform better t <<
260 Careful benchmarking is recommended. <<
261 230
262 KFENCE objects each reside on a dedicated page 231 KFENCE objects each reside on a dedicated page, at either the left or right
263 page boundaries selected at random. The pages 232 page boundaries selected at random. The pages to the left and right of the
264 object page are "guard pages", whose attribute 233 object page are "guard pages", whose attributes are changed to a protected
265 state, and cause page faults on any attempted 234 state, and cause page faults on any attempted access. Such page faults are then
266 intercepted by KFENCE, which handles the fault 235 intercepted by KFENCE, which handles the fault gracefully by reporting an
267 out-of-bounds access, and marking the page as 236 out-of-bounds access, and marking the page as accessible so that the faulting
268 code can (wrongly) continue executing (set ``p 237 code can (wrongly) continue executing (set ``panic_on_warn`` to panic instead).
269 238
270 To detect out-of-bounds writes to memory withi 239 To detect out-of-bounds writes to memory within the object's page itself,
271 KFENCE also uses pattern-based redzones. For e 240 KFENCE also uses pattern-based redzones. For each object page, a redzone is set
272 up for all non-object memory. For typical alig 241 up for all non-object memory. For typical alignments, the redzone is only
273 required on the unguarded side of an object. B 242 required on the unguarded side of an object. Because KFENCE must honor the
274 cache's requested alignment, special alignment 243 cache's requested alignment, special alignments may result in unprotected gaps
275 on either side of an object, all of which are 244 on either side of an object, all of which are redzoned.
276 245
277 The following figure illustrates the page layo 246 The following figure illustrates the page layout::
278 247
279 ---+-----------+-----------+-----------+-- 248 ---+-----------+-----------+-----------+-----------+-----------+---
280 | xxxxxxxxx | O : | xxxxxxxxx | 249 | xxxxxxxxx | O : | xxxxxxxxx | : O | xxxxxxxxx |
281 | xxxxxxxxx | B : | xxxxxxxxx | 250 | xxxxxxxxx | B : | xxxxxxxxx | : B | xxxxxxxxx |
282 | x GUARD x | J : RED- | x GUARD x | R 251 | x GUARD x | J : RED- | x GUARD x | RED- : J | x GUARD x |
283 | xxxxxxxxx | E : ZONE | xxxxxxxxx | 252 | xxxxxxxxx | E : ZONE | xxxxxxxxx | ZONE : E | xxxxxxxxx |
284 | xxxxxxxxx | C : | xxxxxxxxx | 253 | xxxxxxxxx | C : | xxxxxxxxx | : C | xxxxxxxxx |
285 | xxxxxxxxx | T : | xxxxxxxxx | 254 | xxxxxxxxx | T : | xxxxxxxxx | : T | xxxxxxxxx |
286 ---+-----------+-----------+-----------+-- 255 ---+-----------+-----------+-----------+-----------+-----------+---
287 256
288 Upon deallocation of a KFENCE object, the obje 257 Upon deallocation of a KFENCE object, the object's page is again protected and
289 the object is marked as freed. Any further acc 258 the object is marked as freed. Any further access to the object causes a fault
290 and KFENCE reports a use-after-free access. Fr 259 and KFENCE reports a use-after-free access. Freed objects are inserted at the
291 tail of KFENCE's freelist, so that the least r 260 tail of KFENCE's freelist, so that the least recently freed objects are reused
292 first, and the chances of detecting use-after- 261 first, and the chances of detecting use-after-frees of recently freed objects
293 is increased. 262 is increased.
294 <<
295 If pool utilization reaches 75% (default) or a <<
296 pool eventually being fully occupied by alloca <<
297 coverage of allocations, KFENCE limits current <<
298 same source from further filling up the pool. <<
299 based on its partial allocation stack trace. A <<
300 limits frequent long-lived allocations (e.g. p <<
301 filling up the pool permanently, which is the <<
302 becoming full and the sampled allocation rate <<
303 at which to start limiting currently covered a <<
304 the boot parameter ``kfence.skip_covered_thres <<
305 263
306 Interface 264 Interface
307 --------- 265 ---------
308 266
309 The following describes the functions which ar 267 The following describes the functions which are used by allocators as well as
310 page handling code to set up and deal with KFE 268 page handling code to set up and deal with KFENCE allocations.
311 269
312 .. kernel-doc:: include/linux/kfence.h 270 .. kernel-doc:: include/linux/kfence.h
313 :functions: is_kfence_address 271 :functions: is_kfence_address
314 kfence_shutdown_cache 272 kfence_shutdown_cache
315 kfence_alloc kfence_free __kfen 273 kfence_alloc kfence_free __kfence_free
316 kfence_ksize kfence_object_star 274 kfence_ksize kfence_object_start
317 kfence_handle_page_fault 275 kfence_handle_page_fault
318 276
319 Related Tools 277 Related Tools
320 ------------- 278 -------------
321 279
322 In userspace, a similar approach is taken by ` 280 In userspace, a similar approach is taken by `GWP-ASan
323 <http://llvm.org/docs/GwpAsan.html>`_. GWP-ASa 281 <http://llvm.org/docs/GwpAsan.html>`_. GWP-ASan also relies on guard pages and
324 a sampling strategy to detect memory unsafety 282 a sampling strategy to detect memory unsafety bugs at scale. KFENCE's design is
325 directly influenced by GWP-ASan, and can be se 283 directly influenced by GWP-ASan, and can be seen as its kernel sibling. Another
326 similar but non-sampling approach, that also i 284 similar but non-sampling approach, that also inspired the name "KFENCE", can be
327 found in the userspace `Electric Fence Malloc 285 found in the userspace `Electric Fence Malloc Debugger
328 <https://linux.die.net/man/3/efence>`_. 286 <https://linux.die.net/man/3/efence>`_.
329 287
330 In the kernel, several tools exist to debug me 288 In the kernel, several tools exist to debug memory access errors, and in
331 particular KASAN can detect all bug classes th 289 particular KASAN can detect all bug classes that KFENCE can detect. While KASAN
332 is more precise, relying on compiler instrumen 290 is more precise, relying on compiler instrumentation, this comes at a
333 performance cost. 291 performance cost.
334 292
335 It is worth highlighting that KASAN and KFENCE 293 It is worth highlighting that KASAN and KFENCE are complementary, with
336 different target environments. For instance, K 294 different target environments. For instance, KASAN is the better debugging-aid,
337 where test cases or reproducers exists: due to 295 where test cases or reproducers exists: due to the lower chance to detect the
338 error, it would require more effort using KFEN 296 error, it would require more effort using KFENCE to debug. Deployments at scale
339 that cannot afford to enable KASAN, however, w 297 that cannot afford to enable KASAN, however, would benefit from using KFENCE to
340 discover bugs due to code paths not exercised 298 discover bugs due to code paths not exercised by test cases or fuzzers.
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