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1.. SPDX-License-Identifier: GPL-2.0
2.. Copyright (C) 2023, Google LLC.
3
4Kernel Address Sanitizer (KASAN)
5================================
6
7Overview
8--------
9
10Kernel Address Sanitizer (KASAN) is a dynamic memory safety error detector
11designed to find out-of-bounds and use-after-free bugs.
12
13KASAN has three modes:
14
151. Generic KASAN
162. Software Tag-Based KASAN
173. Hardware Tag-Based KASAN
18
19Generic KASAN, enabled with CONFIG_KASAN_GENERIC, is the mode intended for
20debugging, similar to userspace ASan. This mode is supported on many CPU
21architectures, but it has significant performance and memory overheads.
22
23Software Tag-Based KASAN or SW_TAGS KASAN, enabled with CONFIG_KASAN_SW_TAGS,
24can be used for both debugging and dogfood testing, similar to userspace HWASan.
25This mode is only supported for arm64, but its moderate memory overhead allows
26using it for testing on memory-restricted devices with real workloads.
27
28Hardware Tag-Based KASAN or HW_TAGS KASAN, enabled with CONFIG_KASAN_HW_TAGS,
29is the mode intended to be used as an in-field memory bug detector or as a
30security mitigation. This mode only works on arm64 CPUs that support MTE
31(Memory Tagging Extension), but it has low memory and performance overheads and
32thus can be used in production.
33
34For details about the memory and performance impact of each KASAN mode, see the
35descriptions of the corresponding Kconfig options.
36
37The Generic and the Software Tag-Based modes are commonly referred to as the
38software modes. The Software Tag-Based and the Hardware Tag-Based modes are
39referred to as the tag-based modes.
40
41Support
42-------
43
44Architectures
45~~~~~~~~~~~~~
46
47Generic KASAN is supported on x86_64, arm, arm64, powerpc, riscv, s390, xtensa,
48and loongarch, and the tag-based KASAN modes are supported only on arm64.
49
50Compilers
51~~~~~~~~~
52
53Software KASAN modes use compile-time instrumentation to insert validity checks
54before every memory access and thus require a compiler version that provides
55support for that. The Hardware Tag-Based mode relies on hardware to perform
56these checks but still requires a compiler version that supports the memory
57tagging instructions.
58
59Generic KASAN requires GCC version 8.3.0 or later
60or any Clang version supported by the kernel.
61
62Software Tag-Based KASAN requires GCC 11+
63or any Clang version supported by the kernel.
64
65Hardware Tag-Based KASAN requires GCC 10+ or Clang 12+.
66
67Memory types
68~~~~~~~~~~~~
69
70Generic KASAN supports finding bugs in all of slab, page_alloc, vmap, vmalloc,
71stack, and global memory.
72
73Software Tag-Based KASAN supports slab, page_alloc, vmalloc, and stack memory.
74
75Hardware Tag-Based KASAN supports slab, page_alloc, and non-executable vmalloc
76memory.
77
78For slab, both software KASAN modes support SLUB and SLAB allocators, while
79Hardware Tag-Based KASAN only supports SLUB.
80
81Usage
82-----
83
84To enable KASAN, configure the kernel with::
85
86 CONFIG_KASAN=y
87
88and choose between ``CONFIG_KASAN_GENERIC`` (to enable Generic KASAN),
89``CONFIG_KASAN_SW_TAGS`` (to enable Software Tag-Based KASAN), and
90``CONFIG_KASAN_HW_TAGS`` (to enable Hardware Tag-Based KASAN).
91
92For the software modes, also choose between ``CONFIG_KASAN_OUTLINE`` and
93``CONFIG_KASAN_INLINE``. Outline and inline are compiler instrumentation types.
94The former produces a smaller binary while the latter is up to 2 times faster.
95
96To include alloc and free stack traces of affected slab objects into reports,
97enable ``CONFIG_STACKTRACE``. To include alloc and free stack traces of affected
98physical pages, enable ``CONFIG_PAGE_OWNER`` and boot with ``page_owner=on``.
99
100Boot parameters
101~~~~~~~~~~~~~~~
102
103KASAN is affected by the generic ``panic_on_warn`` command line parameter.
104When it is enabled, KASAN panics the kernel after printing a bug report.
105
106By default, KASAN prints a bug report only for the first invalid memory access.
107With ``kasan_multi_shot``, KASAN prints a report on every invalid access. This
108effectively disables ``panic_on_warn`` for KASAN reports.
109
110Alternatively, independent of ``panic_on_warn``, the ``kasan.fault=`` boot
111parameter can be used to control panic and reporting behaviour:
112
113- ``kasan.fault=report``, ``=panic``, or ``=panic_on_write`` controls whether
114 to only print a KASAN report, panic the kernel, or panic the kernel on
115 invalid writes only (default: ``report``). The panic happens even if
116 ``kasan_multi_shot`` is enabled. Note that when using asynchronous mode of
117 Hardware Tag-Based KASAN, ``kasan.fault=panic_on_write`` always panics on
118 asynchronously checked accesses (including reads).
119
120Software and Hardware Tag-Based KASAN modes (see the section about various
121modes below) support altering stack trace collection behavior:
122
123- ``kasan.stacktrace=off`` or ``=on`` disables or enables alloc and free stack
124 traces collection (default: ``on``).
125- ``kasan.stack_ring_size=<number of entries>`` specifies the number of entries
126 in the stack ring (default: ``32768``).
127
128Hardware Tag-Based KASAN mode is intended for use in production as a security
129mitigation. Therefore, it supports additional boot parameters that allow
130disabling KASAN altogether or controlling its features:
131
132- ``kasan=off`` or ``=on`` controls whether KASAN is enabled (default: ``on``).
133
134- ``kasan.mode=sync``, ``=async`` or ``=asymm`` controls whether KASAN
135 is configured in synchronous, asynchronous or asymmetric mode of
136 execution (default: ``sync``).
137 Synchronous mode: a bad access is detected immediately when a tag
138 check fault occurs.
139 Asynchronous mode: a bad access detection is delayed. When a tag check
140 fault occurs, the information is stored in hardware (in the TFSR_EL1
141 register for arm64). The kernel periodically checks the hardware and
142 only reports tag faults during these checks.
143 Asymmetric mode: a bad access is detected synchronously on reads and
144 asynchronously on writes.
145
146- ``kasan.vmalloc=off`` or ``=on`` disables or enables tagging of vmalloc
147 allocations (default: ``on``).
148
149- ``kasan.page_alloc.sample=<sampling interval>`` makes KASAN tag only every
150 Nth page_alloc allocation with the order equal or greater than
151 ``kasan.page_alloc.sample.order``, where N is the value of the ``sample``
152 parameter (default: ``1``, or tag every such allocation).
153 This parameter is intended to mitigate the performance overhead introduced
154 by KASAN.
155 Note that enabling this parameter makes Hardware Tag-Based KASAN skip checks
156 of allocations chosen by sampling and thus miss bad accesses to these
157 allocations. Use the default value for accurate bug detection.
158
159- ``kasan.page_alloc.sample.order=<minimum page order>`` specifies the minimum
160 order of allocations that are affected by sampling (default: ``3``).
161 Only applies when ``kasan.page_alloc.sample`` is set to a value greater
162 than ``1``.
163 This parameter is intended to allow sampling only large page_alloc
164 allocations, which is the biggest source of the performance overhead.
165
166Error reports
167~~~~~~~~~~~~~
168
169A typical KASAN report looks like this::
170
171 ==================================================================
172 BUG: KASAN: slab-out-of-bounds in kmalloc_oob_right+0xa8/0xbc [kasan_test]
173 Write of size 1 at addr ffff8801f44ec37b by task insmod/2760
174
175 CPU: 1 PID: 2760 Comm: insmod Not tainted 4.19.0-rc3+ #698
176 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
177 Call Trace:
178 dump_stack+0x94/0xd8
179 print_address_description+0x73/0x280
180 kasan_report+0x144/0x187
181 __asan_report_store1_noabort+0x17/0x20
182 kmalloc_oob_right+0xa8/0xbc [kasan_test]
183 kmalloc_tests_init+0x16/0x700 [kasan_test]
184 do_one_initcall+0xa5/0x3ae
185 do_init_module+0x1b6/0x547
186 load_module+0x75df/0x8070
187 __do_sys_init_module+0x1c6/0x200
188 __x64_sys_init_module+0x6e/0xb0
189 do_syscall_64+0x9f/0x2c0
190 entry_SYSCALL_64_after_hwframe+0x44/0xa9
191 RIP: 0033:0x7f96443109da
192 RSP: 002b:00007ffcf0b51b08 EFLAGS: 00000202 ORIG_RAX: 00000000000000af
193 RAX: ffffffffffffffda RBX: 000055dc3ee521a0 RCX: 00007f96443109da
194 RDX: 00007f96445cff88 RSI: 0000000000057a50 RDI: 00007f9644992000
195 RBP: 000055dc3ee510b0 R08: 0000000000000003 R09: 0000000000000000
196 R10: 00007f964430cd0a R11: 0000000000000202 R12: 00007f96445cff88
197 R13: 000055dc3ee51090 R14: 0000000000000000 R15: 0000000000000000
198
199 Allocated by task 2760:
200 save_stack+0x43/0xd0
201 kasan_kmalloc+0xa7/0xd0
202 kmem_cache_alloc_trace+0xe1/0x1b0
203 kmalloc_oob_right+0x56/0xbc [kasan_test]
204 kmalloc_tests_init+0x16/0x700 [kasan_test]
205 do_one_initcall+0xa5/0x3ae
206 do_init_module+0x1b6/0x547
207 load_module+0x75df/0x8070
208 __do_sys_init_module+0x1c6/0x200
209 __x64_sys_init_module+0x6e/0xb0
210 do_syscall_64+0x9f/0x2c0
211 entry_SYSCALL_64_after_hwframe+0x44/0xa9
212
213 Freed by task 815:
214 save_stack+0x43/0xd0
215 __kasan_slab_free+0x135/0x190
216 kasan_slab_free+0xe/0x10
217 kfree+0x93/0x1a0
218 umh_complete+0x6a/0xa0
219 call_usermodehelper_exec_async+0x4c3/0x640
220 ret_from_fork+0x35/0x40
221
222 The buggy address belongs to the object at ffff8801f44ec300
223 which belongs to the cache kmalloc-128 of size 128
224 The buggy address is located 123 bytes inside of
225 128-byte region [ffff8801f44ec300, ffff8801f44ec380)
226 The buggy address belongs to the page:
227 page:ffffea0007d13b00 count:1 mapcount:0 mapping:ffff8801f7001640 index:0x0
228 flags: 0x200000000000100(slab)
229 raw: 0200000000000100 ffffea0007d11dc0 0000001a0000001a ffff8801f7001640
230 raw: 0000000000000000 0000000080150015 00000001ffffffff 0000000000000000
231 page dumped because: kasan: bad access detected
232
233 Memory state around the buggy address:
234 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 fc fc fc fc fc fc fc fc
236 >ffff8801f44ec300: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 03
237 ^
238 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 fc fc fc fc fc fc fc fc
240 ==================================================================
241
242The report header summarizes what kind of bug happened and what kind of access
243caused it. It is followed by a stack trace of the bad access, a stack trace of
244where the accessed memory was allocated (in case a slab object was accessed),
245and a stack trace of where the object was freed (in case of a use-after-free
246bug report). Next comes a description of the accessed slab object and the
247information about the accessed memory page.
248
249In the end, the report shows the memory state around the accessed address.
250Internally, KASAN tracks memory state separately for each memory granule, which
251is either 8 or 16 aligned bytes depending on KASAN mode. Each number in the
252memory state section of the report shows the state of one of the memory
253granules that surround the accessed address.
254
255For Generic KASAN, the size of each memory granule is 8. The state of each
256granule is encoded in one shadow byte. Those 8 bytes can be accessible,
257partially accessible, freed, or be a part of a redzone. KASAN uses the following
258encoding for each shadow byte: 00 means that all 8 bytes of the corresponding
259memory region are accessible; number N (1 <= N <= 7) means that the first N
260bytes are accessible, and other (8 - N) bytes are not; any negative value
261indicates that the entire 8-byte word is inaccessible. KASAN uses different
262negative values to distinguish between different kinds of inaccessible memory
263like redzones or freed memory (see mm/kasan/kasan.h).
264
265In the report above, the arrow points to the shadow byte ``03``, which means
266that the accessed address is partially accessible.
267
268For tag-based KASAN modes, this last report section shows the memory tags around
269the accessed address (see the `Implementation details`_ section).
270
271Note that KASAN bug titles (like ``slab-out-of-bounds`` or ``use-after-free``)
272are best-effort: KASAN prints the most probable bug type based on the limited
273information it has. The actual type of the bug might be different.
274
275Generic KASAN also reports up to two auxiliary call stack traces. These stack
276traces point to places in code that interacted with the object but that are not
277directly present in the bad access stack trace. Currently, this includes
278call_rcu() and workqueue queuing.
279
280CONFIG_KASAN_EXTRA_INFO
281~~~~~~~~~~~~~~~~~~~~~~~
282
283Enabling CONFIG_KASAN_EXTRA_INFO allows KASAN to record and report more
284information. The extra information currently supported is the CPU number and
285timestamp at allocation and free. More information can help find the cause of
286the bug and correlate the error with other system events, at the cost of using
287extra memory to record more information (more cost details in the help text of
288CONFIG_KASAN_EXTRA_INFO).
289
290Here is the report with CONFIG_KASAN_EXTRA_INFO enabled (only the
291different parts are shown)::
292
293 ==================================================================
294 ...
295 Allocated by task 134 on cpu 5 at 229.133855s:
296 ...
297 Freed by task 136 on cpu 3 at 230.199335s:
298 ...
299 ==================================================================
300
301Implementation details
302----------------------
303
304Generic KASAN
305~~~~~~~~~~~~~
306
307Software KASAN modes use shadow memory to record whether each byte of memory is
308safe to access and use compile-time instrumentation to insert shadow memory
309checks before each memory access.
310
311Generic KASAN dedicates 1/8th of kernel memory to its shadow memory (16TB
312to cover 128TB on x86_64) and uses direct mapping with a scale and offset to
313translate a memory address to its corresponding shadow address.
314
315Here is the function which translates an address to its corresponding shadow
316address::
317
318 static inline void *kasan_mem_to_shadow(const void *addr)
319 {
320 return (void *)((unsigned long)addr >> KASAN_SHADOW_SCALE_SHIFT)
321 + KASAN_SHADOW_OFFSET;
322 }
323
324where ``KASAN_SHADOW_SCALE_SHIFT = 3``.
325
326Compile-time instrumentation is used to insert memory access checks. Compiler
327inserts function calls (``__asan_load*(addr)``, ``__asan_store*(addr)``) before
328each memory access of size 1, 2, 4, 8, or 16. These functions check whether
329memory accesses are valid or not by checking corresponding shadow memory.
330
331With inline instrumentation, instead of making function calls, the compiler
332directly inserts the code to check shadow memory. This option significantly
333enlarges the kernel, but it gives an x1.1-x2 performance boost over the
334outline-instrumented kernel.
335
336Generic KASAN is the only mode that delays the reuse of freed objects via
337quarantine (see mm/kasan/quarantine.c for implementation).
338
339Software Tag-Based KASAN
340~~~~~~~~~~~~~~~~~~~~~~~~
341
342Software Tag-Based KASAN uses a software memory tagging approach to checking
343access validity. It is currently only implemented for the arm64 architecture.
344
345Software Tag-Based KASAN uses the Top Byte Ignore (TBI) feature of arm64 CPUs
346to store a pointer tag in the top byte of kernel pointers. It uses shadow memory
347to store memory tags associated with each 16-byte memory cell (therefore, it
348dedicates 1/16th of the kernel memory for shadow memory).
349
350On each memory allocation, Software Tag-Based KASAN generates a random tag, tags
351the allocated memory with this tag, and embeds the same tag into the returned
352pointer.
353
354Software Tag-Based KASAN uses compile-time instrumentation to insert checks
355before each memory access. These checks make sure that the tag of the memory
356that is being accessed is equal to the tag of the pointer that is used to access
357this memory. In case of a tag mismatch, Software Tag-Based KASAN prints a bug
358report.
359
360Software Tag-Based KASAN also has two instrumentation modes (outline, which
361emits callbacks to check memory accesses; and inline, which performs the shadow
362memory checks inline). With outline instrumentation mode, a bug report is
363printed from the function that performs the access check. With inline
364instrumentation, a ``brk`` instruction is emitted by the compiler, and a
365dedicated ``brk`` handler is used to print bug reports.
366
367Software Tag-Based KASAN uses 0xFF as a match-all pointer tag (accesses through
368pointers with the 0xFF pointer tag are not checked). The value 0xFE is currently
369reserved to tag freed memory regions.
370
371Hardware Tag-Based KASAN
372~~~~~~~~~~~~~~~~~~~~~~~~
373
374Hardware Tag-Based KASAN is similar to the software mode in concept but uses
375hardware memory tagging support instead of compiler instrumentation and
376shadow memory.
377
378Hardware Tag-Based KASAN is currently only implemented for arm64 architecture
379and based on both arm64 Memory Tagging Extension (MTE) introduced in ARMv8.5
380Instruction Set Architecture and Top Byte Ignore (TBI).
381
382Special arm64 instructions are used to assign memory tags for each allocation.
383Same tags are assigned to pointers to those allocations. On every memory
384access, hardware makes sure that the tag of the memory that is being accessed is
385equal to the tag of the pointer that is used to access this memory. In case of a
386tag mismatch, a fault is generated, and a report is printed.
387
388Hardware Tag-Based KASAN uses 0xFF as a match-all pointer tag (accesses through
389pointers with the 0xFF pointer tag are not checked). The value 0xFE is currently
390reserved to tag freed memory regions.
391
392If the hardware does not support MTE (pre ARMv8.5), Hardware Tag-Based KASAN
393will not be enabled. In this case, all KASAN boot parameters are ignored.
394
395Note that enabling CONFIG_KASAN_HW_TAGS always results in in-kernel TBI being
396enabled. Even when ``kasan.mode=off`` is provided or when the hardware does not
397support MTE (but supports TBI).
398
399Hardware Tag-Based KASAN only reports the first found bug. After that, MTE tag
400checking gets disabled.
401
402Shadow memory
403-------------
404
405The contents of this section are only applicable to software KASAN modes.
406
407The kernel maps memory in several different parts of the address space.
408The range of kernel virtual addresses is large: there is not enough real
409memory to support a real shadow region for every address that could be
410accessed by the kernel. Therefore, KASAN only maps real shadow for certain
411parts of the address space.
412
413Default behaviour
414~~~~~~~~~~~~~~~~~
415
416By default, architectures only map real memory over the shadow region
417for the linear mapping (and potentially other small areas). For all
418other areas - such as vmalloc and vmemmap space - a single read-only
419page is mapped over the shadow area. This read-only shadow page
420declares all memory accesses as permitted.
421
422This presents a problem for modules: they do not live in the linear
423mapping but in a dedicated module space. By hooking into the module
424allocator, KASAN temporarily maps real shadow memory to cover them.
425This allows detection of invalid accesses to module globals, for example.
426
427This also creates an incompatibility with ``VMAP_STACK``: if the stack
428lives in vmalloc space, it will be shadowed by the read-only page, and
429the kernel will fault when trying to set up the shadow data for stack
430variables.
431
432CONFIG_KASAN_VMALLOC
433~~~~~~~~~~~~~~~~~~~~
434
435With ``CONFIG_KASAN_VMALLOC``, KASAN can cover vmalloc space at the
436cost of greater memory usage. Currently, this is supported on x86,
437arm64, riscv, s390, and powerpc.
438
439This works by hooking into vmalloc and vmap and dynamically
440allocating real shadow memory to back the mappings.
441
442Most mappings in vmalloc space are small, requiring less than a full
443page of shadow space. Allocating a full shadow page per mapping would
444therefore be wasteful. Furthermore, to ensure that different mappings
445use different shadow pages, mappings would have to be aligned to
446``KASAN_GRANULE_SIZE * PAGE_SIZE``.
447
448Instead, KASAN shares backing space across multiple mappings. It allocates
449a backing page when a mapping in vmalloc space uses a particular page
450of the shadow region. This page can be shared by other vmalloc
451mappings later on.
452
453KASAN hooks into the vmap infrastructure to lazily clean up unused shadow
454memory.
455
456To avoid the difficulties around swapping mappings around, KASAN expects
457that the part of the shadow region that covers the vmalloc space will
458not be covered by the early shadow page but will be left unmapped.
459This will require changes in arch-specific code.
460
461This allows ``VMAP_STACK`` support on x86 and can simplify support of
462architectures that do not have a fixed module region.
463
464For developers
465--------------
466
467Ignoring accesses
468~~~~~~~~~~~~~~~~~
469
470Software KASAN modes use compiler instrumentation to insert validity checks.
471Such instrumentation might be incompatible with some parts of the kernel, and
472therefore needs to be disabled.
473
474Other parts of the kernel might access metadata for allocated objects.
475Normally, KASAN detects and reports such accesses, but in some cases (e.g.,
476in memory allocators), these accesses are valid.
477
478For software KASAN modes, to disable instrumentation for a specific file or
479directory, add a ``KASAN_SANITIZE`` annotation to the respective kernel
480Makefile:
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
490For software KASAN modes, to disable instrumentation on a per-function basis,
491use the KASAN-specific ``__no_sanitize_address`` function attribute or the
492generic ``noinstr`` one.
493
494Note that disabling compiler instrumentation (either on a per-file or a
495per-function basis) makes KASAN ignore the accesses that happen directly in
496that code for software KASAN modes. It does not help when the accesses happen
497indirectly (through calls to instrumented functions) or with Hardware
498Tag-Based KASAN, which does not use compiler instrumentation.
499
500For software KASAN modes, to disable KASAN reports in a part of the kernel code
501for the current task, annotate this part of the code with a
502``kasan_disable_current()``/``kasan_enable_current()`` section. This also
503disables the reports for indirect accesses that happen through function calls.
504
505For tag-based KASAN modes, to disable access checking, use
506``kasan_reset_tag()`` or ``page_kasan_tag_reset()``. Note that temporarily
507disabling access checking via ``page_kasan_tag_reset()`` requires saving and
508restoring the per-page KASAN tag via ``page_kasan_tag``/``page_kasan_tag_set``.
509
510Tests
511~~~~~
512
513There are KASAN tests that allow verifying that KASAN works and can detect
514certain types of memory corruptions.
515
516All KASAN tests are integrated with the KUnit Test Framework and can be enabled
517via ``CONFIG_KASAN_KUNIT_TEST``. The tests can be run and partially verified
518automatically in a few different ways; see the instructions below.
519
520Each KASAN test prints one of multiple KASAN reports if an error is detected.
521Then the test prints its number and status.
522
523When a test passes::
524
525 ok 28 - kmalloc_double_kzfree
526
527When a test fails due to a failed ``kmalloc``::
528
529 # kmalloc_large_oob_right: ASSERTION FAILED at mm/kasan/kasan_test.c:245
530 Expected ptr is not null, but is
531 not ok 5 - kmalloc_large_oob_right
532
533When a test fails due to a missing KASAN report::
534
535 # kmalloc_double_kzfree: EXPECTATION FAILED at mm/kasan/kasan_test.c:709
536 KASAN failure expected in "kfree_sensitive(ptr)", but none occurred
537 not ok 28 - kmalloc_double_kzfree
538
539
540At the end the cumulative status of all KASAN tests is printed. On success::
541
542 ok 1 - kasan
543
544Or, if one of the tests failed::
545
546 not ok 1 - kasan
547
548There are a few ways to run the KASAN tests.
549
5501. Loadable module
551
552 With ``CONFIG_KUNIT`` enabled, the tests can be built as a loadable module
553 and run by loading ``kasan_test.ko`` with ``insmod`` or ``modprobe``.
554
5552. Built-In
556
557 With ``CONFIG_KUNIT`` built-in, the tests can be built-in as well.
558 In this case, the tests will run at boot as a late-init call.
559
5603. Using kunit_tool
561
562 With ``CONFIG_KUNIT`` and ``CONFIG_KASAN_KUNIT_TEST`` built-in, it is also
563 possible to use ``kunit_tool`` to see the results of KUnit tests in a more
564 readable way. This will not print the KASAN reports of the tests that passed.
565 See `KUnit documentation <https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html>`_
566 for more up-to-date information on ``kunit_tool``.
567
568.. _KUnit: https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html
1The Kernel Address Sanitizer (KASAN)
2====================================
3
4Overview
5--------
6
7Kernel Address Sanitizer (KASAN) is a dynamic memory safety error detector
8designed to find out-of-bounds and use-after-free bugs.
9
10KASAN has three modes:
11
121. Generic KASAN
132. Software Tag-Based KASAN
143. Hardware Tag-Based KASAN
15
16Generic KASAN, enabled with CONFIG_KASAN_GENERIC, is the mode intended for
17debugging, similar to userspace ASan. This mode is supported on many CPU
18architectures, but it has significant performance and memory overheads.
19
20Software Tag-Based KASAN or SW_TAGS KASAN, enabled with CONFIG_KASAN_SW_TAGS,
21can be used for both debugging and dogfood testing, similar to userspace HWASan.
22This mode is only supported for arm64, but its moderate memory overhead allows
23using it for testing on memory-restricted devices with real workloads.
24
25Hardware Tag-Based KASAN or HW_TAGS KASAN, enabled with CONFIG_KASAN_HW_TAGS,
26is the mode intended to be used as an in-field memory bug detector or as a
27security mitigation. This mode only works on arm64 CPUs that support MTE
28(Memory Tagging Extension), but it has low memory and performance overheads and
29thus can be used in production.
30
31For details about the memory and performance impact of each KASAN mode, see the
32descriptions of the corresponding Kconfig options.
33
34The Generic and the Software Tag-Based modes are commonly referred to as the
35software modes. The Software Tag-Based and the Hardware Tag-Based modes are
36referred to as the tag-based modes.
37
38Support
39-------
40
41Architectures
42~~~~~~~~~~~~~
43
44Generic KASAN is supported on x86_64, arm, arm64, powerpc, riscv, s390, and
45xtensa, and the tag-based KASAN modes are supported only on arm64.
46
47Compilers
48~~~~~~~~~
49
50Software KASAN modes use compile-time instrumentation to insert validity checks
51before every memory access and thus require a compiler version that provides
52support for that. The Hardware Tag-Based mode relies on hardware to perform
53these checks but still requires a compiler version that supports the memory
54tagging instructions.
55
56Generic KASAN requires GCC version 8.3.0 or later
57or any Clang version supported by the kernel.
58
59Software Tag-Based KASAN requires GCC 11+
60or any Clang version supported by the kernel.
61
62Hardware Tag-Based KASAN requires GCC 10+ or Clang 12+.
63
64Memory types
65~~~~~~~~~~~~
66
67Generic KASAN supports finding bugs in all of slab, page_alloc, vmap, vmalloc,
68stack, and global memory.
69
70Software Tag-Based KASAN supports slab, page_alloc, vmalloc, and stack memory.
71
72Hardware Tag-Based KASAN supports slab, page_alloc, and non-executable vmalloc
73memory.
74
75For slab, both software KASAN modes support SLUB and SLAB allocators, while
76Hardware Tag-Based KASAN only supports SLUB.
77
78Usage
79-----
80
81To enable KASAN, configure the kernel with::
82
83 CONFIG_KASAN=y
84
85and choose between ``CONFIG_KASAN_GENERIC`` (to enable Generic KASAN),
86``CONFIG_KASAN_SW_TAGS`` (to enable Software Tag-Based KASAN), and
87``CONFIG_KASAN_HW_TAGS`` (to enable Hardware Tag-Based KASAN).
88
89For the software modes, also choose between ``CONFIG_KASAN_OUTLINE`` and
90``CONFIG_KASAN_INLINE``. Outline and inline are compiler instrumentation types.
91The former produces a smaller binary while the latter is up to 2 times faster.
92
93To include alloc and free stack traces of affected slab objects into reports,
94enable ``CONFIG_STACKTRACE``. To include alloc and free stack traces of affected
95physical pages, enable ``CONFIG_PAGE_OWNER`` and boot with ``page_owner=on``.
96
97Boot parameters
98~~~~~~~~~~~~~~~
99
100KASAN is affected by the generic ``panic_on_warn`` command line parameter.
101When it is enabled, KASAN panics the kernel after printing a bug report.
102
103By default, KASAN prints a bug report only for the first invalid memory access.
104With ``kasan_multi_shot``, KASAN prints a report on every invalid access. This
105effectively disables ``panic_on_warn`` for KASAN reports.
106
107Alternatively, independent of ``panic_on_warn``, the ``kasan.fault=`` boot
108parameter can be used to control panic and reporting behaviour:
109
110- ``kasan.fault=report`` or ``=panic`` controls whether to only print a KASAN
111 report or also panic the kernel (default: ``report``). The panic happens even
112 if ``kasan_multi_shot`` is enabled.
113
114Software and Hardware Tag-Based KASAN modes (see the section about various
115modes below) support altering stack trace collection behavior:
116
117- ``kasan.stacktrace=off`` or ``=on`` disables or enables alloc and free stack
118 traces collection (default: ``on``).
119- ``kasan.stack_ring_size=<number of entries>`` specifies the number of entries
120 in the stack ring (default: ``32768``).
121
122Hardware Tag-Based KASAN mode is intended for use in production as a security
123mitigation. Therefore, it supports additional boot parameters that allow
124disabling KASAN altogether or controlling its features:
125
126- ``kasan=off`` or ``=on`` controls whether KASAN is enabled (default: ``on``).
127
128- ``kasan.mode=sync``, ``=async`` or ``=asymm`` controls whether KASAN
129 is configured in synchronous, asynchronous or asymmetric mode of
130 execution (default: ``sync``).
131 Synchronous mode: a bad access is detected immediately when a tag
132 check fault occurs.
133 Asynchronous mode: a bad access detection is delayed. When a tag check
134 fault occurs, the information is stored in hardware (in the TFSR_EL1
135 register for arm64). The kernel periodically checks the hardware and
136 only reports tag faults during these checks.
137 Asymmetric mode: a bad access is detected synchronously on reads and
138 asynchronously on writes.
139
140- ``kasan.vmalloc=off`` or ``=on`` disables or enables tagging of vmalloc
141 allocations (default: ``on``).
142
143Error reports
144~~~~~~~~~~~~~
145
146A typical KASAN report looks like this::
147
148 ==================================================================
149 BUG: KASAN: slab-out-of-bounds in kmalloc_oob_right+0xa8/0xbc [test_kasan]
150 Write of size 1 at addr ffff8801f44ec37b by task insmod/2760
151
152 CPU: 1 PID: 2760 Comm: insmod Not tainted 4.19.0-rc3+ #698
153 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
154 Call Trace:
155 dump_stack+0x94/0xd8
156 print_address_description+0x73/0x280
157 kasan_report+0x144/0x187
158 __asan_report_store1_noabort+0x17/0x20
159 kmalloc_oob_right+0xa8/0xbc [test_kasan]
160 kmalloc_tests_init+0x16/0x700 [test_kasan]
161 do_one_initcall+0xa5/0x3ae
162 do_init_module+0x1b6/0x547
163 load_module+0x75df/0x8070
164 __do_sys_init_module+0x1c6/0x200
165 __x64_sys_init_module+0x6e/0xb0
166 do_syscall_64+0x9f/0x2c0
167 entry_SYSCALL_64_after_hwframe+0x44/0xa9
168 RIP: 0033:0x7f96443109da
169 RSP: 002b:00007ffcf0b51b08 EFLAGS: 00000202 ORIG_RAX: 00000000000000af
170 RAX: ffffffffffffffda RBX: 000055dc3ee521a0 RCX: 00007f96443109da
171 RDX: 00007f96445cff88 RSI: 0000000000057a50 RDI: 00007f9644992000
172 RBP: 000055dc3ee510b0 R08: 0000000000000003 R09: 0000000000000000
173 R10: 00007f964430cd0a R11: 0000000000000202 R12: 00007f96445cff88
174 R13: 000055dc3ee51090 R14: 0000000000000000 R15: 0000000000000000
175
176 Allocated by task 2760:
177 save_stack+0x43/0xd0
178 kasan_kmalloc+0xa7/0xd0
179 kmem_cache_alloc_trace+0xe1/0x1b0
180 kmalloc_oob_right+0x56/0xbc [test_kasan]
181 kmalloc_tests_init+0x16/0x700 [test_kasan]
182 do_one_initcall+0xa5/0x3ae
183 do_init_module+0x1b6/0x547
184 load_module+0x75df/0x8070
185 __do_sys_init_module+0x1c6/0x200
186 __x64_sys_init_module+0x6e/0xb0
187 do_syscall_64+0x9f/0x2c0
188 entry_SYSCALL_64_after_hwframe+0x44/0xa9
189
190 Freed by task 815:
191 save_stack+0x43/0xd0
192 __kasan_slab_free+0x135/0x190
193 kasan_slab_free+0xe/0x10
194 kfree+0x93/0x1a0
195 umh_complete+0x6a/0xa0
196 call_usermodehelper_exec_async+0x4c3/0x640
197 ret_from_fork+0x35/0x40
198
199 The buggy address belongs to the object at ffff8801f44ec300
200 which belongs to the cache kmalloc-128 of size 128
201 The buggy address is located 123 bytes inside of
202 128-byte region [ffff8801f44ec300, ffff8801f44ec380)
203 The buggy address belongs to the page:
204 page:ffffea0007d13b00 count:1 mapcount:0 mapping:ffff8801f7001640 index:0x0
205 flags: 0x200000000000100(slab)
206 raw: 0200000000000100 ffffea0007d11dc0 0000001a0000001a ffff8801f7001640
207 raw: 0000000000000000 0000000080150015 00000001ffffffff 0000000000000000
208 page dumped because: kasan: bad access detected
209
210 Memory state around the buggy address:
211 ffff8801f44ec200: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
212 ffff8801f44ec280: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
213 >ffff8801f44ec300: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 03
214 ^
215 ffff8801f44ec380: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
216 ffff8801f44ec400: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
217 ==================================================================
218
219The report header summarizes what kind of bug happened and what kind of access
220caused it. It is followed by a stack trace of the bad access, a stack trace of
221where the accessed memory was allocated (in case a slab object was accessed),
222and a stack trace of where the object was freed (in case of a use-after-free
223bug report). Next comes a description of the accessed slab object and the
224information about the accessed memory page.
225
226In the end, the report shows the memory state around the accessed address.
227Internally, KASAN tracks memory state separately for each memory granule, which
228is either 8 or 16 aligned bytes depending on KASAN mode. Each number in the
229memory state section of the report shows the state of one of the memory
230granules that surround the accessed address.
231
232For Generic KASAN, the size of each memory granule is 8. The state of each
233granule is encoded in one shadow byte. Those 8 bytes can be accessible,
234partially accessible, freed, or be a part of a redzone. KASAN uses the following
235encoding for each shadow byte: 00 means that all 8 bytes of the corresponding
236memory region are accessible; number N (1 <= N <= 7) means that the first N
237bytes are accessible, and other (8 - N) bytes are not; any negative value
238indicates that the entire 8-byte word is inaccessible. KASAN uses different
239negative values to distinguish between different kinds of inaccessible memory
240like redzones or freed memory (see mm/kasan/kasan.h).
241
242In the report above, the arrow points to the shadow byte ``03``, which means
243that the accessed address is partially accessible.
244
245For tag-based KASAN modes, this last report section shows the memory tags around
246the accessed address (see the `Implementation details`_ section).
247
248Note that KASAN bug titles (like ``slab-out-of-bounds`` or ``use-after-free``)
249are best-effort: KASAN prints the most probable bug type based on the limited
250information it has. The actual type of the bug might be different.
251
252Generic KASAN also reports up to two auxiliary call stack traces. These stack
253traces point to places in code that interacted with the object but that are not
254directly present in the bad access stack trace. Currently, this includes
255call_rcu() and workqueue queuing.
256
257Implementation details
258----------------------
259
260Generic KASAN
261~~~~~~~~~~~~~
262
263Software KASAN modes use shadow memory to record whether each byte of memory is
264safe to access and use compile-time instrumentation to insert shadow memory
265checks before each memory access.
266
267Generic KASAN dedicates 1/8th of kernel memory to its shadow memory (16TB
268to cover 128TB on x86_64) and uses direct mapping with a scale and offset to
269translate a memory address to its corresponding shadow address.
270
271Here is the function which translates an address to its corresponding shadow
272address::
273
274 static inline void *kasan_mem_to_shadow(const void *addr)
275 {
276 return (void *)((unsigned long)addr >> KASAN_SHADOW_SCALE_SHIFT)
277 + KASAN_SHADOW_OFFSET;
278 }
279
280where ``KASAN_SHADOW_SCALE_SHIFT = 3``.
281
282Compile-time instrumentation is used to insert memory access checks. Compiler
283inserts function calls (``__asan_load*(addr)``, ``__asan_store*(addr)``) before
284each memory access of size 1, 2, 4, 8, or 16. These functions check whether
285memory accesses are valid or not by checking corresponding shadow memory.
286
287With inline instrumentation, instead of making function calls, the compiler
288directly inserts the code to check shadow memory. This option significantly
289enlarges the kernel, but it gives an x1.1-x2 performance boost over the
290outline-instrumented kernel.
291
292Generic KASAN is the only mode that delays the reuse of freed objects via
293quarantine (see mm/kasan/quarantine.c for implementation).
294
295Software Tag-Based KASAN
296~~~~~~~~~~~~~~~~~~~~~~~~
297
298Software Tag-Based KASAN uses a software memory tagging approach to checking
299access validity. It is currently only implemented for the arm64 architecture.
300
301Software Tag-Based KASAN uses the Top Byte Ignore (TBI) feature of arm64 CPUs
302to store a pointer tag in the top byte of kernel pointers. It uses shadow memory
303to store memory tags associated with each 16-byte memory cell (therefore, it
304dedicates 1/16th of the kernel memory for shadow memory).
305
306On each memory allocation, Software Tag-Based KASAN generates a random tag, tags
307the allocated memory with this tag, and embeds the same tag into the returned
308pointer.
309
310Software Tag-Based KASAN uses compile-time instrumentation to insert checks
311before each memory access. These checks make sure that the tag of the memory
312that is being accessed is equal to the tag of the pointer that is used to access
313this memory. In case of a tag mismatch, Software Tag-Based KASAN prints a bug
314report.
315
316Software Tag-Based KASAN also has two instrumentation modes (outline, which
317emits callbacks to check memory accesses; and inline, which performs the shadow
318memory checks inline). With outline instrumentation mode, a bug report is
319printed from the function that performs the access check. With inline
320instrumentation, a ``brk`` instruction is emitted by the compiler, and a
321dedicated ``brk`` handler is used to print bug reports.
322
323Software Tag-Based KASAN uses 0xFF as a match-all pointer tag (accesses through
324pointers with the 0xFF pointer tag are not checked). The value 0xFE is currently
325reserved to tag freed memory regions.
326
327Hardware Tag-Based KASAN
328~~~~~~~~~~~~~~~~~~~~~~~~
329
330Hardware Tag-Based KASAN is similar to the software mode in concept but uses
331hardware memory tagging support instead of compiler instrumentation and
332shadow memory.
333
334Hardware Tag-Based KASAN is currently only implemented for arm64 architecture
335and based on both arm64 Memory Tagging Extension (MTE) introduced in ARMv8.5
336Instruction Set Architecture and Top Byte Ignore (TBI).
337
338Special arm64 instructions are used to assign memory tags for each allocation.
339Same tags are assigned to pointers to those allocations. On every memory
340access, hardware makes sure that the tag of the memory that is being accessed is
341equal to the tag of the pointer that is used to access this memory. In case of a
342tag mismatch, a fault is generated, and a report is printed.
343
344Hardware Tag-Based KASAN uses 0xFF as a match-all pointer tag (accesses through
345pointers with the 0xFF pointer tag are not checked). The value 0xFE is currently
346reserved to tag freed memory regions.
347
348If the hardware does not support MTE (pre ARMv8.5), Hardware Tag-Based KASAN
349will not be enabled. In this case, all KASAN boot parameters are ignored.
350
351Note that enabling CONFIG_KASAN_HW_TAGS always results in in-kernel TBI being
352enabled. Even when ``kasan.mode=off`` is provided or when the hardware does not
353support MTE (but supports TBI).
354
355Hardware Tag-Based KASAN only reports the first found bug. After that, MTE tag
356checking gets disabled.
357
358Shadow memory
359-------------
360
361The contents of this section are only applicable to software KASAN modes.
362
363The kernel maps memory in several different parts of the address space.
364The range of kernel virtual addresses is large: there is not enough real
365memory to support a real shadow region for every address that could be
366accessed by the kernel. Therefore, KASAN only maps real shadow for certain
367parts of the address space.
368
369Default behaviour
370~~~~~~~~~~~~~~~~~
371
372By default, architectures only map real memory over the shadow region
373for the linear mapping (and potentially other small areas). For all
374other areas - such as vmalloc and vmemmap space - a single read-only
375page is mapped over the shadow area. This read-only shadow page
376declares all memory accesses as permitted.
377
378This presents a problem for modules: they do not live in the linear
379mapping but in a dedicated module space. By hooking into the module
380allocator, KASAN temporarily maps real shadow memory to cover them.
381This allows detection of invalid accesses to module globals, for example.
382
383This also creates an incompatibility with ``VMAP_STACK``: if the stack
384lives in vmalloc space, it will be shadowed by the read-only page, and
385the kernel will fault when trying to set up the shadow data for stack
386variables.
387
388CONFIG_KASAN_VMALLOC
389~~~~~~~~~~~~~~~~~~~~
390
391With ``CONFIG_KASAN_VMALLOC``, KASAN can cover vmalloc space at the
392cost of greater memory usage. Currently, this is supported on x86,
393arm64, riscv, s390, and powerpc.
394
395This works by hooking into vmalloc and vmap and dynamically
396allocating real shadow memory to back the mappings.
397
398Most mappings in vmalloc space are small, requiring less than a full
399page of shadow space. Allocating a full shadow page per mapping would
400therefore be wasteful. Furthermore, to ensure that different mappings
401use different shadow pages, mappings would have to be aligned to
402``KASAN_GRANULE_SIZE * PAGE_SIZE``.
403
404Instead, KASAN shares backing space across multiple mappings. It allocates
405a backing page when a mapping in vmalloc space uses a particular page
406of the shadow region. This page can be shared by other vmalloc
407mappings later on.
408
409KASAN hooks into the vmap infrastructure to lazily clean up unused shadow
410memory.
411
412To avoid the difficulties around swapping mappings around, KASAN expects
413that the part of the shadow region that covers the vmalloc space will
414not be covered by the early shadow page but will be left unmapped.
415This will require changes in arch-specific code.
416
417This allows ``VMAP_STACK`` support on x86 and can simplify support of
418architectures that do not have a fixed module region.
419
420For developers
421--------------
422
423Ignoring accesses
424~~~~~~~~~~~~~~~~~
425
426Software KASAN modes use compiler instrumentation to insert validity checks.
427Such instrumentation might be incompatible with some parts of the kernel, and
428therefore needs to be disabled.
429
430Other parts of the kernel might access metadata for allocated objects.
431Normally, KASAN detects and reports such accesses, but in some cases (e.g.,
432in memory allocators), these accesses are valid.
433
434For software KASAN modes, to disable instrumentation for a specific file or
435directory, add a ``KASAN_SANITIZE`` annotation to the respective kernel
436Makefile:
437
438- For a single file (e.g., main.o)::
439
440 KASAN_SANITIZE_main.o := n
441
442- For all files in one directory::
443
444 KASAN_SANITIZE := n
445
446For software KASAN modes, to disable instrumentation on a per-function basis,
447use the KASAN-specific ``__no_sanitize_address`` function attribute or the
448generic ``noinstr`` one.
449
450Note that disabling compiler instrumentation (either on a per-file or a
451per-function basis) makes KASAN ignore the accesses that happen directly in
452that code for software KASAN modes. It does not help when the accesses happen
453indirectly (through calls to instrumented functions) or with Hardware
454Tag-Based KASAN, which does not use compiler instrumentation.
455
456For software KASAN modes, to disable KASAN reports in a part of the kernel code
457for the current task, annotate this part of the code with a
458``kasan_disable_current()``/``kasan_enable_current()`` section. This also
459disables the reports for indirect accesses that happen through function calls.
460
461For tag-based KASAN modes, to disable access checking, use
462``kasan_reset_tag()`` or ``page_kasan_tag_reset()``. Note that temporarily
463disabling access checking via ``page_kasan_tag_reset()`` requires saving and
464restoring the per-page KASAN tag via ``page_kasan_tag``/``page_kasan_tag_set``.
465
466Tests
467~~~~~
468
469There are KASAN tests that allow verifying that KASAN works and can detect
470certain types of memory corruptions. The tests consist of two parts:
471
4721. Tests that are integrated with the KUnit Test Framework. Enabled with
473``CONFIG_KASAN_KUNIT_TEST``. These tests can be run and partially verified
474automatically in a few different ways; see the instructions below.
475
4762. Tests that are currently incompatible with KUnit. Enabled with
477``CONFIG_KASAN_MODULE_TEST`` and can only be run as a module. These tests can
478only be verified manually by loading the kernel module and inspecting the
479kernel log for KASAN reports.
480
481Each KUnit-compatible KASAN test prints one of multiple KASAN reports if an
482error is detected. Then the test prints its number and status.
483
484When a test passes::
485
486 ok 28 - kmalloc_double_kzfree
487
488When a test fails due to a failed ``kmalloc``::
489
490 # kmalloc_large_oob_right: ASSERTION FAILED at lib/test_kasan.c:163
491 Expected ptr is not null, but is
492 not ok 4 - kmalloc_large_oob_right
493
494When a test fails due to a missing KASAN report::
495
496 # kmalloc_double_kzfree: EXPECTATION FAILED at lib/test_kasan.c:974
497 KASAN failure expected in "kfree_sensitive(ptr)", but none occurred
498 not ok 44 - kmalloc_double_kzfree
499
500
501At the end the cumulative status of all KASAN tests is printed. On success::
502
503 ok 1 - kasan
504
505Or, if one of the tests failed::
506
507 not ok 1 - kasan
508
509There are a few ways to run KUnit-compatible KASAN tests.
510
5111. Loadable module
512
513 With ``CONFIG_KUNIT`` enabled, KASAN-KUnit tests can be built as a loadable
514 module and run by loading ``test_kasan.ko`` with ``insmod`` or ``modprobe``.
515
5162. Built-In
517
518 With ``CONFIG_KUNIT`` built-in, KASAN-KUnit tests can be built-in as well.
519 In this case, the tests will run at boot as a late-init call.
520
5213. Using kunit_tool
522
523 With ``CONFIG_KUNIT`` and ``CONFIG_KASAN_KUNIT_TEST`` built-in, it is also
524 possible to use ``kunit_tool`` to see the results of KUnit tests in a more
525 readable way. This will not print the KASAN reports of the tests that passed.
526 See `KUnit documentation <https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html>`_
527 for more up-to-date information on ``kunit_tool``.
528
529.. _KUnit: https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html