1 The Kernel Address Sanitizer (KASAN)
2 ====================================
7 Kernel Address Sanitizer (KASAN) is a dynamic memory safety error detector
8 designed to find out-of-bounds and use-after-free bugs.
10 KASAN has three modes:
13 2. Software Tag-Based KASAN
14 3. Hardware Tag-Based KASAN
16 Generic KASAN, enabled with CONFIG_KASAN_GENERIC, is the mode intended for
17 debugging, similar to userspace ASan. This mode is supported on many CPU
18 architectures, but it has significant performance and memory overheads.
20 Software Tag-Based KASAN or SW_TAGS KASAN, enabled with CONFIG_KASAN_SW_TAGS,
21 can be used for both debugging and dogfood testing, similar to userspace HWASan.
22 This mode is only supported for arm64, but its moderate memory overhead allows
23 using it for testing on memory-restricted devices with real workloads.
25 Hardware Tag-Based KASAN or HW_TAGS KASAN, enabled with CONFIG_KASAN_HW_TAGS,
26 is the mode intended to be used as an in-field memory bug detector or as a
27 security mitigation. This mode only works on arm64 CPUs that support MTE
28 (Memory Tagging Extension), but it has low memory and performance overheads and
29 thus can be used in production.
31 For details about the memory and performance impact of each KASAN mode, see the
32 descriptions of the corresponding Kconfig options.
34 The Generic and the Software Tag-Based modes are commonly referred to as the
35 software modes. The Software Tag-Based and the Hardware Tag-Based modes are
36 referred to as the tag-based modes.
44 Generic KASAN is supported on x86_64, arm, arm64, powerpc, riscv, s390, and
45 xtensa, and the tag-based KASAN modes are supported only on arm64.
50 Software KASAN modes use compile-time instrumentation to insert validity checks
51 before every memory access and thus require a compiler version that provides
52 support for that. The Hardware Tag-Based mode relies on hardware to perform
53 these checks but still requires a compiler version that supports the memory
56 Generic KASAN requires GCC version 8.3.0 or later
57 or any Clang version supported by the kernel.
59 Software Tag-Based KASAN requires GCC 11+
60 or any Clang version supported by the kernel.
62 Hardware Tag-Based KASAN requires GCC 10+ or Clang 12+.
67 Generic KASAN supports finding bugs in all of slab, page_alloc, vmap, vmalloc,
68 stack, and global memory.
70 Software Tag-Based KASAN supports slab, page_alloc, vmalloc, and stack memory.
72 Hardware Tag-Based KASAN supports slab, page_alloc, and non-executable vmalloc
75 For slab, both software KASAN modes support SLUB and SLAB allocators, while
76 Hardware Tag-Based KASAN only supports SLUB.
81 To enable KASAN, configure the kernel with::
85 and 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).
89 For the software modes, also choose between ``CONFIG_KASAN_OUTLINE`` and
90 ``CONFIG_KASAN_INLINE``. Outline and inline are compiler instrumentation types.
91 The former produces a smaller binary while the latter is up to 2 times faster.
93 To include alloc and free stack traces of affected slab objects into reports,
94 enable ``CONFIG_STACKTRACE``. To include alloc and free stack traces of affected
95 physical pages, enable ``CONFIG_PAGE_OWNER`` and boot with ``page_owner=on``.
100 KASAN is affected by the generic ``panic_on_warn`` command line parameter.
101 When it is enabled, KASAN panics the kernel after printing a bug report.
103 By default, KASAN prints a bug report only for the first invalid memory access.
104 With ``kasan_multi_shot``, KASAN prints a report on every invalid access. This
105 effectively disables ``panic_on_warn`` for KASAN reports.
107 Alternatively, independent of ``panic_on_warn``, the ``kasan.fault=`` boot
108 parameter can be used to control panic and reporting behaviour:
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.
114 Software and Hardware Tag-Based KASAN modes (see the section about various
115 modes below) support altering stack trace collection behavior:
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``).
122 Hardware Tag-Based KASAN mode is intended for use in production as a security
123 mitigation. Therefore, it supports additional boot parameters that allow
124 disabling KASAN altogether or controlling its features:
126 - ``kasan=off`` or ``=on`` controls whether KASAN is enabled (default: ``on``).
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
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.
140 - ``kasan.vmalloc=off`` or ``=on`` disables or enables tagging of vmalloc
141 allocations (default: ``on``).
146 A typical KASAN report looks like this::
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
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
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
176 Allocated by task 2760:
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
192 __kasan_slab_free+0x135/0x190
193 kasan_slab_free+0xe/0x10
195 umh_complete+0x6a/0xa0
196 call_usermodehelper_exec_async+0x4c3/0x640
197 ret_from_fork+0x35/0x40
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
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
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 ==================================================================
219 The report header summarizes what kind of bug happened and what kind of access
220 caused it. It is followed by a stack trace of the bad access, a stack trace of
221 where the accessed memory was allocated (in case a slab object was accessed),
222 and a stack trace of where the object was freed (in case of a use-after-free
223 bug report). Next comes a description of the accessed slab object and the
224 information about the accessed memory page.
226 In the end, the report shows the memory state around the accessed address.
227 Internally, KASAN tracks memory state separately for each memory granule, which
228 is either 8 or 16 aligned bytes depending on KASAN mode. Each number in the
229 memory state section of the report shows the state of one of the memory
230 granules that surround the accessed address.
232 For Generic KASAN, the size of each memory granule is 8. The state of each
233 granule is encoded in one shadow byte. Those 8 bytes can be accessible,
234 partially accessible, freed, or be a part of a redzone. KASAN uses the following
235 encoding for each shadow byte: 00 means that all 8 bytes of the corresponding
236 memory region are accessible; number N (1 <= N <= 7) means that the first N
237 bytes are accessible, and other (8 - N) bytes are not; any negative value
238 indicates that the entire 8-byte word is inaccessible. KASAN uses different
239 negative values to distinguish between different kinds of inaccessible memory
240 like redzones or freed memory (see mm/kasan/kasan.h).
242 In the report above, the arrow points to the shadow byte ``03``, which means
243 that the accessed address is partially accessible.
245 For tag-based KASAN modes, this last report section shows the memory tags around
246 the accessed address (see the `Implementation details`_ section).
248 Note that KASAN bug titles (like ``slab-out-of-bounds`` or ``use-after-free``)
249 are best-effort: KASAN prints the most probable bug type based on the limited
250 information it has. The actual type of the bug might be different.
252 Generic KASAN also reports up to two auxiliary call stack traces. These stack
253 traces point to places in code that interacted with the object but that are not
254 directly present in the bad access stack trace. Currently, this includes
255 call_rcu() and workqueue queuing.
257 Implementation details
258 ----------------------
263 Software KASAN modes use shadow memory to record whether each byte of memory is
264 safe to access and use compile-time instrumentation to insert shadow memory
265 checks before each memory access.
267 Generic KASAN dedicates 1/8th of kernel memory to its shadow memory (16TB
268 to cover 128TB on x86_64) and uses direct mapping with a scale and offset to
269 translate a memory address to its corresponding shadow address.
271 Here is the function which translates an address to its corresponding shadow
274 static inline void *kasan_mem_to_shadow(const void *addr)
276 return (void *)((unsigned long)addr >> KASAN_SHADOW_SCALE_SHIFT)
277 + KASAN_SHADOW_OFFSET;
280 where ``KASAN_SHADOW_SCALE_SHIFT = 3``.
282 Compile-time instrumentation is used to insert memory access checks. Compiler
283 inserts function calls (``__asan_load*(addr)``, ``__asan_store*(addr)``) before
284 each memory access of size 1, 2, 4, 8, or 16. These functions check whether
285 memory accesses are valid or not by checking corresponding shadow memory.
287 With inline instrumentation, instead of making function calls, the compiler
288 directly inserts the code to check shadow memory. This option significantly
289 enlarges the kernel, but it gives an x1.1-x2 performance boost over the
290 outline-instrumented kernel.
292 Generic KASAN is the only mode that delays the reuse of freed objects via
293 quarantine (see mm/kasan/quarantine.c for implementation).
295 Software Tag-Based KASAN
296 ~~~~~~~~~~~~~~~~~~~~~~~~
298 Software Tag-Based KASAN uses a software memory tagging approach to checking
299 access validity. It is currently only implemented for the arm64 architecture.
301 Software Tag-Based KASAN uses the Top Byte Ignore (TBI) feature of arm64 CPUs
302 to store a pointer tag in the top byte of kernel pointers. It uses shadow memory
303 to store memory tags associated with each 16-byte memory cell (therefore, it
304 dedicates 1/16th of the kernel memory for shadow memory).
306 On each memory allocation, Software Tag-Based KASAN generates a random tag, tags
307 the allocated memory with this tag, and embeds the same tag into the returned
310 Software Tag-Based KASAN uses compile-time instrumentation to insert checks
311 before each memory access. These checks make sure that the tag of the memory
312 that is being accessed is equal to the tag of the pointer that is used to access
313 this memory. In case of a tag mismatch, Software Tag-Based KASAN prints a bug
316 Software Tag-Based KASAN also has two instrumentation modes (outline, which
317 emits callbacks to check memory accesses; and inline, which performs the shadow
318 memory checks inline). With outline instrumentation mode, a bug report is
319 printed from the function that performs the access check. With inline
320 instrumentation, a ``brk`` instruction is emitted by the compiler, and a
321 dedicated ``brk`` handler is used to print bug reports.
323 Software Tag-Based KASAN uses 0xFF as a match-all pointer tag (accesses through
324 pointers with the 0xFF pointer tag are not checked). The value 0xFE is currently
325 reserved to tag freed memory regions.
327 Hardware Tag-Based KASAN
328 ~~~~~~~~~~~~~~~~~~~~~~~~
330 Hardware Tag-Based KASAN is similar to the software mode in concept but uses
331 hardware memory tagging support instead of compiler instrumentation and
334 Hardware Tag-Based KASAN is currently only implemented for arm64 architecture
335 and based on both arm64 Memory Tagging Extension (MTE) introduced in ARMv8.5
336 Instruction Set Architecture and Top Byte Ignore (TBI).
338 Special arm64 instructions are used to assign memory tags for each allocation.
339 Same tags are assigned to pointers to those allocations. On every memory
340 access, hardware makes sure that the tag of the memory that is being accessed is
341 equal to the tag of the pointer that is used to access this memory. In case of a
342 tag mismatch, a fault is generated, and a report is printed.
344 Hardware Tag-Based KASAN uses 0xFF as a match-all pointer tag (accesses through
345 pointers with the 0xFF pointer tag are not checked). The value 0xFE is currently
346 reserved to tag freed memory regions.
348 If the hardware does not support MTE (pre ARMv8.5), Hardware Tag-Based KASAN
349 will not be enabled. In this case, all KASAN boot parameters are ignored.
351 Note that enabling CONFIG_KASAN_HW_TAGS always results in in-kernel TBI being
352 enabled. Even when ``kasan.mode=off`` is provided or when the hardware does not
353 support MTE (but supports TBI).
355 Hardware Tag-Based KASAN only reports the first found bug. After that, MTE tag
356 checking gets disabled.
361 The contents of this section are only applicable to software KASAN modes.
363 The kernel maps memory in several different parts of the address space.
364 The range of kernel virtual addresses is large: there is not enough real
365 memory to support a real shadow region for every address that could be
366 accessed by the kernel. Therefore, KASAN only maps real shadow for certain
367 parts of the address space.
372 By default, architectures only map real memory over the shadow region
373 for the linear mapping (and potentially other small areas). For all
374 other areas - such as vmalloc and vmemmap space - a single read-only
375 page is mapped over the shadow area. This read-only shadow page
376 declares all memory accesses as permitted.
378 This presents a problem for modules: they do not live in the linear
379 mapping but in a dedicated module space. By hooking into the module
380 allocator, KASAN temporarily maps real shadow memory to cover them.
381 This allows detection of invalid accesses to module globals, for example.
383 This also creates an incompatibility with ``VMAP_STACK``: if the stack
384 lives in vmalloc space, it will be shadowed by the read-only page, and
385 the kernel will fault when trying to set up the shadow data for stack
391 With ``CONFIG_KASAN_VMALLOC``, KASAN can cover vmalloc space at the
392 cost of greater memory usage. Currently, this is supported on x86,
393 arm64, riscv, s390, and powerpc.
395 This works by hooking into vmalloc and vmap and dynamically
396 allocating real shadow memory to back the mappings.
398 Most mappings in vmalloc space are small, requiring less than a full
399 page of shadow space. Allocating a full shadow page per mapping would
400 therefore be wasteful. Furthermore, to ensure that different mappings
401 use different shadow pages, mappings would have to be aligned to
402 ``KASAN_GRANULE_SIZE * PAGE_SIZE``.
404 Instead, KASAN shares backing space across multiple mappings. It allocates
405 a backing page when a mapping in vmalloc space uses a particular page
406 of the shadow region. This page can be shared by other vmalloc
409 KASAN hooks into the vmap infrastructure to lazily clean up unused shadow
412 To avoid the difficulties around swapping mappings around, KASAN expects
413 that the part of the shadow region that covers the vmalloc space will
414 not be covered by the early shadow page but will be left unmapped.
415 This will require changes in arch-specific code.
417 This allows ``VMAP_STACK`` support on x86 and can simplify support of
418 architectures that do not have a fixed module region.
426 Software KASAN modes use compiler instrumentation to insert validity checks.
427 Such instrumentation might be incompatible with some parts of the kernel, and
428 therefore needs to be disabled.
430 Other parts of the kernel might access metadata for allocated objects.
431 Normally, KASAN detects and reports such accesses, but in some cases (e.g.,
432 in memory allocators), these accesses are valid.
434 For software KASAN modes, to disable instrumentation for a specific file or
435 directory, add a ``KASAN_SANITIZE`` annotation to the respective kernel
438 - For a single file (e.g., main.o)::
440 KASAN_SANITIZE_main.o := n
442 - For all files in one directory::
446 For software KASAN modes, to disable instrumentation on a per-function basis,
447 use the KASAN-specific ``__no_sanitize_address`` function attribute or the
448 generic ``noinstr`` one.
450 Note that disabling compiler instrumentation (either on a per-file or a
451 per-function basis) makes KASAN ignore the accesses that happen directly in
452 that code for software KASAN modes. It does not help when the accesses happen
453 indirectly (through calls to instrumented functions) or with Hardware
454 Tag-Based KASAN, which does not use compiler instrumentation.
456 For software KASAN modes, to disable KASAN reports in a part of the kernel code
457 for the current task, annotate this part of the code with a
458 ``kasan_disable_current()``/``kasan_enable_current()`` section. This also
459 disables the reports for indirect accesses that happen through function calls.
461 For tag-based KASAN modes, to disable access checking, use
462 ``kasan_reset_tag()`` or ``page_kasan_tag_reset()``. Note that temporarily
463 disabling access checking via ``page_kasan_tag_reset()`` requires saving and
464 restoring the per-page KASAN tag via ``page_kasan_tag``/``page_kasan_tag_set``.
469 There are KASAN tests that allow verifying that KASAN works and can detect
470 certain types of memory corruptions. The tests consist of two parts:
472 1. Tests that are integrated with the KUnit Test Framework. Enabled with
473 ``CONFIG_KASAN_KUNIT_TEST``. These tests can be run and partially verified
474 automatically in a few different ways; see the instructions below.
476 2. 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
478 only be verified manually by loading the kernel module and inspecting the
479 kernel log for KASAN reports.
481 Each KUnit-compatible KASAN test prints one of multiple KASAN reports if an
482 error is detected. Then the test prints its number and status.
486 ok 28 - kmalloc_double_kzfree
488 When a test fails due to a failed ``kmalloc``::
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
494 When a test fails due to a missing KASAN report::
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
501 At the end the cumulative status of all KASAN tests is printed. On success::
505 Or, if one of the tests failed::
509 There are a few ways to run KUnit-compatible KASAN tests.
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``.
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.
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``.
529 .. _KUnit: https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html