1 The Kernel Address Sanitizer (KASAN)
2 ====================================
7 KernelAddressSANitizer (KASAN) is a dynamic memory safety error detector
8 designed to find out-of-bound and use-after-free bugs. KASAN has three modes:
10 1. generic KASAN (similar to userspace ASan),
11 2. software tag-based KASAN (similar to userspace HWASan),
12 3. hardware tag-based KASAN (based on hardware memory tagging).
14 Generic KASAN is mainly used for debugging due to a large memory overhead.
15 Software tag-based KASAN can be used for dogfood testing as it has a lower
16 memory overhead that allows using it with real workloads. Hardware tag-based
17 KASAN comes with low memory and performance overheads and, therefore, can be
18 used in production. Either as an in-field memory bug detector or as a security
21 Software KASAN modes (#1 and #2) use compile-time instrumentation to insert
22 validity checks before every memory access and, therefore, require a compiler
23 version that supports that.
25 Generic KASAN is supported in GCC and Clang. With GCC, it requires version
26 8.3.0 or later. Any supported Clang version is compatible, but detection of
27 out-of-bounds accesses for global variables is only supported since Clang 11.
29 Software tag-based KASAN mode is only supported in Clang.
31 The hardware KASAN mode (#3) relies on hardware to perform the checks but
32 still requires a compiler version that supports memory tagging instructions.
33 This mode is supported in GCC 10+ and Clang 11+.
35 Both software KASAN modes work with SLUB and SLAB memory allocators,
36 while the hardware tag-based KASAN currently only supports SLUB.
38 Currently, generic KASAN is supported for the x86_64, arm, arm64, xtensa, s390,
39 and riscv architectures, and tag-based KASAN modes are supported only for arm64.
44 To enable KASAN, configure the kernel with::
48 and choose between ``CONFIG_KASAN_GENERIC`` (to enable generic KASAN),
49 ``CONFIG_KASAN_SW_TAGS`` (to enable software tag-based KASAN), and
50 ``CONFIG_KASAN_HW_TAGS`` (to enable hardware tag-based KASAN).
52 For software modes, also choose between ``CONFIG_KASAN_OUTLINE`` and
53 ``CONFIG_KASAN_INLINE``. Outline and inline are compiler instrumentation types.
54 The former produces a smaller binary while the latter is 1.1-2 times faster.
56 To include alloc and free stack traces of affected slab objects into reports,
57 enable ``CONFIG_STACKTRACE``. To include alloc and free stack traces of affected
58 physical pages, enable ``CONFIG_PAGE_OWNER`` and boot with ``page_owner=on``.
63 A typical KASAN report looks like this::
65 ==================================================================
66 BUG: KASAN: slab-out-of-bounds in kmalloc_oob_right+0xa8/0xbc [test_kasan]
67 Write of size 1 at addr ffff8801f44ec37b by task insmod/2760
69 CPU: 1 PID: 2760 Comm: insmod Not tainted 4.19.0-rc3+ #698
70 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
73 print_address_description+0x73/0x280
74 kasan_report+0x144/0x187
75 __asan_report_store1_noabort+0x17/0x20
76 kmalloc_oob_right+0xa8/0xbc [test_kasan]
77 kmalloc_tests_init+0x16/0x700 [test_kasan]
78 do_one_initcall+0xa5/0x3ae
79 do_init_module+0x1b6/0x547
80 load_module+0x75df/0x8070
81 __do_sys_init_module+0x1c6/0x200
82 __x64_sys_init_module+0x6e/0xb0
83 do_syscall_64+0x9f/0x2c0
84 entry_SYSCALL_64_after_hwframe+0x44/0xa9
85 RIP: 0033:0x7f96443109da
86 RSP: 002b:00007ffcf0b51b08 EFLAGS: 00000202 ORIG_RAX: 00000000000000af
87 RAX: ffffffffffffffda RBX: 000055dc3ee521a0 RCX: 00007f96443109da
88 RDX: 00007f96445cff88 RSI: 0000000000057a50 RDI: 00007f9644992000
89 RBP: 000055dc3ee510b0 R08: 0000000000000003 R09: 0000000000000000
90 R10: 00007f964430cd0a R11: 0000000000000202 R12: 00007f96445cff88
91 R13: 000055dc3ee51090 R14: 0000000000000000 R15: 0000000000000000
93 Allocated by task 2760:
95 kasan_kmalloc+0xa7/0xd0
96 kmem_cache_alloc_trace+0xe1/0x1b0
97 kmalloc_oob_right+0x56/0xbc [test_kasan]
98 kmalloc_tests_init+0x16/0x700 [test_kasan]
99 do_one_initcall+0xa5/0x3ae
100 do_init_module+0x1b6/0x547
101 load_module+0x75df/0x8070
102 __do_sys_init_module+0x1c6/0x200
103 __x64_sys_init_module+0x6e/0xb0
104 do_syscall_64+0x9f/0x2c0
105 entry_SYSCALL_64_after_hwframe+0x44/0xa9
109 __kasan_slab_free+0x135/0x190
110 kasan_slab_free+0xe/0x10
112 umh_complete+0x6a/0xa0
113 call_usermodehelper_exec_async+0x4c3/0x640
114 ret_from_fork+0x35/0x40
116 The buggy address belongs to the object at ffff8801f44ec300
117 which belongs to the cache kmalloc-128 of size 128
118 The buggy address is located 123 bytes inside of
119 128-byte region [ffff8801f44ec300, ffff8801f44ec380)
120 The buggy address belongs to the page:
121 page:ffffea0007d13b00 count:1 mapcount:0 mapping:ffff8801f7001640 index:0x0
122 flags: 0x200000000000100(slab)
123 raw: 0200000000000100 ffffea0007d11dc0 0000001a0000001a ffff8801f7001640
124 raw: 0000000000000000 0000000080150015 00000001ffffffff 0000000000000000
125 page dumped because: kasan: bad access detected
127 Memory state around the buggy address:
128 ffff8801f44ec200: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
129 ffff8801f44ec280: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
130 >ffff8801f44ec300: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 03
132 ffff8801f44ec380: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
133 ffff8801f44ec400: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
134 ==================================================================
136 The report header summarizes what kind of bug happened and what kind of access
137 caused it. It is followed by a stack trace of the bad access, a stack trace of
138 where the accessed memory was allocated (in case a slab object was accessed),
139 and a stack trace of where the object was freed (in case of a use-after-free
140 bug report). Next comes a description of the accessed slab object and the
141 information about the accessed memory page.
143 In the end, the report shows the memory state around the accessed address.
144 Internally, KASAN tracks memory state separately for each memory granule, which
145 is either 8 or 16 aligned bytes depending on KASAN mode. Each number in the
146 memory state section of the report shows the state of one of the memory
147 granules that surround the accessed address.
149 For generic KASAN, the size of each memory granule is 8. The state of each
150 granule is encoded in one shadow byte. Those 8 bytes can be accessible,
151 partially accessible, freed, or be a part of a redzone. KASAN uses the following
152 encoding for each shadow byte: 00 means that all 8 bytes of the corresponding
153 memory region are accessible; number N (1 <= N <= 7) means that the first N
154 bytes are accessible, and other (8 - N) bytes are not; any negative value
155 indicates that the entire 8-byte word is inaccessible. KASAN uses different
156 negative values to distinguish between different kinds of inaccessible memory
157 like redzones or freed memory (see mm/kasan/kasan.h).
159 In the report above, the arrow points to the shadow byte ``03``, which means
160 that the accessed address is partially accessible.
162 For tag-based KASAN modes, this last report section shows the memory tags around
163 the accessed address (see the `Implementation details`_ section).
165 Note that KASAN bug titles (like ``slab-out-of-bounds`` or ``use-after-free``)
166 are best-effort: KASAN prints the most probable bug type based on the limited
167 information it has. The actual type of the bug might be different.
169 Generic KASAN also reports up to two auxiliary call stack traces. These stack
170 traces point to places in code that interacted with the object but that are not
171 directly present in the bad access stack trace. Currently, this includes
172 call_rcu() and workqueue queuing.
177 KASAN is affected by the generic ``panic_on_warn`` command line parameter.
178 When it is enabled, KASAN panics the kernel after printing a bug report.
180 By default, KASAN prints a bug report only for the first invalid memory access.
181 With ``kasan_multi_shot``, KASAN prints a report on every invalid access. This
182 effectively disables ``panic_on_warn`` for KASAN reports.
184 Hardware tag-based KASAN mode (see the section about various modes below) is
185 intended for use in production as a security mitigation. Therefore, it supports
186 boot parameters that allow disabling KASAN or controlling its features.
188 - ``kasan=off`` or ``=on`` controls whether KASAN is enabled (default: ``on``).
190 - ``kasan.mode=sync`` or ``=async`` controls whether KASAN is configured in
191 synchronous or asynchronous mode of execution (default: ``sync``).
192 Synchronous mode: a bad access is detected immediately when a tag
194 Asynchronous mode: a bad access detection is delayed. When a tag check
195 fault occurs, the information is stored in hardware (in the TFSR_EL1
196 register for arm64). The kernel periodically checks the hardware and
197 only reports tag faults during these checks.
199 - ``kasan.stacktrace=off`` or ``=on`` disables or enables alloc and free stack
200 traces collection (default: ``on``).
202 - ``kasan.fault=report`` or ``=panic`` controls whether to only print a KASAN
203 report or also panic the kernel (default: ``report``). The panic happens even
204 if ``kasan_multi_shot`` is enabled.
206 Implementation details
207 ----------------------
212 Software KASAN modes use shadow memory to record whether each byte of memory is
213 safe to access and use compile-time instrumentation to insert shadow memory
214 checks before each memory access.
216 Generic KASAN dedicates 1/8th of kernel memory to its shadow memory (16TB
217 to cover 128TB on x86_64) and uses direct mapping with a scale and offset to
218 translate a memory address to its corresponding shadow address.
220 Here is the function which translates an address to its corresponding shadow
223 static inline void *kasan_mem_to_shadow(const void *addr)
225 return (void *)((unsigned long)addr >> KASAN_SHADOW_SCALE_SHIFT)
226 + KASAN_SHADOW_OFFSET;
229 where ``KASAN_SHADOW_SCALE_SHIFT = 3``.
231 Compile-time instrumentation is used to insert memory access checks. Compiler
232 inserts function calls (``__asan_load*(addr)``, ``__asan_store*(addr)``) before
233 each memory access of size 1, 2, 4, 8, or 16. These functions check whether
234 memory accesses are valid or not by checking corresponding shadow memory.
236 With inline instrumentation, instead of making function calls, the compiler
237 directly inserts the code to check shadow memory. This option significantly
238 enlarges the kernel, but it gives an x1.1-x2 performance boost over the
239 outline-instrumented kernel.
241 Generic KASAN is the only mode that delays the reuse of freed objects via
242 quarantine (see mm/kasan/quarantine.c for implementation).
244 Software tag-based KASAN
245 ~~~~~~~~~~~~~~~~~~~~~~~~
247 Software tag-based KASAN requires software memory tagging support in the form
248 of HWASan-like compiler instrumentation (see HWASan documentation for details).
250 Software tag-based KASAN is currently only implemented for arm64 architecture.
252 Software tag-based KASAN uses the Top Byte Ignore (TBI) feature of arm64 CPUs
253 to store a pointer tag in the top byte of kernel pointers. Like generic KASAN
254 it uses shadow memory to store memory tags associated with each 16-byte memory
255 cell (therefore it dedicates 1/16th of the kernel memory for shadow memory).
257 On each memory allocation software tag-based KASAN generates a random tag, tags
258 the allocated memory with this tag, and embeds this tag into the returned
261 Software tag-based KASAN uses compile-time instrumentation to insert checks
262 before each memory access. These checks make sure that tag of the memory that
263 is being accessed is equal to tag of the pointer that is used to access this
264 memory. In case of a tag mismatch software tag-based KASAN prints a bug report.
266 Software tag-based KASAN also has two instrumentation modes (outline, that
267 emits callbacks to check memory accesses; and inline, that performs the shadow
268 memory checks inline). With outline instrumentation mode, a bug report is
269 simply printed from the function that performs the access check. With inline
270 instrumentation a brk instruction is emitted by the compiler, and a dedicated
271 brk handler is used to print bug reports.
273 Software tag-based KASAN uses 0xFF as a match-all pointer tag (accesses through
274 pointers with 0xFF pointer tag aren't checked). The value 0xFE is currently
275 reserved to tag freed memory regions.
277 Software tag-based KASAN currently only supports tagging of
278 kmem_cache_alloc/kmalloc and page_alloc memory.
280 Hardware tag-based KASAN
281 ~~~~~~~~~~~~~~~~~~~~~~~~
283 Hardware tag-based KASAN is similar to the software mode in concept, but uses
284 hardware memory tagging support instead of compiler instrumentation and
287 Hardware tag-based KASAN is currently only implemented for arm64 architecture
288 and based on both arm64 Memory Tagging Extension (MTE) introduced in ARMv8.5
289 Instruction Set Architecture, and Top Byte Ignore (TBI).
291 Special arm64 instructions are used to assign memory tags for each allocation.
292 Same tags are assigned to pointers to those allocations. On every memory
293 access, hardware makes sure that tag of the memory that is being accessed is
294 equal to tag of the pointer that is used to access this memory. In case of a
295 tag mismatch a fault is generated and a report is printed.
297 Hardware tag-based KASAN uses 0xFF as a match-all pointer tag (accesses through
298 pointers with 0xFF pointer tag aren't checked). The value 0xFE is currently
299 reserved to tag freed memory regions.
301 Hardware tag-based KASAN currently only supports tagging of
302 kmem_cache_alloc/kmalloc and page_alloc memory.
304 If the hardware doesn't support MTE (pre ARMv8.5), hardware tag-based KASAN
305 won't be enabled. In this case all boot parameters are ignored.
307 Note, that enabling CONFIG_KASAN_HW_TAGS always results in in-kernel TBI being
308 enabled. Even when kasan.mode=off is provided, or when the hardware doesn't
309 support MTE (but supports TBI).
311 Hardware tag-based KASAN only reports the first found bug. After that MTE tag
312 checking gets disabled.
317 The kernel maps memory in a number of different parts of the address
318 space. This poses something of a problem for KASAN, which requires
319 that all addresses accessed by instrumented code have a valid shadow
322 The range of kernel virtual addresses is large: there is not enough
323 real memory to support a real shadow region for every address that
324 could be accessed by the kernel.
329 By default, architectures only map real memory over the shadow region
330 for the linear mapping (and potentially other small areas). For all
331 other areas - such as vmalloc and vmemmap space - a single read-only
332 page is mapped over the shadow area. This read-only shadow page
333 declares all memory accesses as permitted.
335 This presents a problem for modules: they do not live in the linear
336 mapping, but in a dedicated module space. By hooking in to the module
337 allocator, KASAN can temporarily map real shadow memory to cover
338 them. This allows detection of invalid accesses to module globals, for
341 This also creates an incompatibility with ``VMAP_STACK``: if the stack
342 lives in vmalloc space, it will be shadowed by the read-only page, and
343 the kernel will fault when trying to set up the shadow data for stack
349 With ``CONFIG_KASAN_VMALLOC``, KASAN can cover vmalloc space at the
350 cost of greater memory usage. Currently this is only supported on x86.
352 This works by hooking into vmalloc and vmap, and dynamically
353 allocating real shadow memory to back the mappings.
355 Most mappings in vmalloc space are small, requiring less than a full
356 page of shadow space. Allocating a full shadow page per mapping would
357 therefore be wasteful. Furthermore, to ensure that different mappings
358 use different shadow pages, mappings would have to be aligned to
359 ``KASAN_GRANULE_SIZE * PAGE_SIZE``.
361 Instead, KASAN shares backing space across multiple mappings. It allocates
362 a backing page when a mapping in vmalloc space uses a particular page
363 of the shadow region. This page can be shared by other vmalloc
366 KASAN hooks into the vmap infrastructure to lazily clean up unused shadow
369 To avoid the difficulties around swapping mappings around, KASAN expects
370 that the part of the shadow region that covers the vmalloc space will
371 not be covered by the early shadow page, but will be left
372 unmapped. This will require changes in arch-specific code.
374 This allows ``VMAP_STACK`` support on x86, and can simplify support of
375 architectures that do not have a fixed module region.
383 Software KASAN modes use compiler instrumentation to insert validity checks.
384 Such instrumentation might be incompatible with some part of the kernel, and
385 therefore needs to be disabled. To disable instrumentation for specific files
386 or directories, add a line similar to the following to the respective kernel
389 - For a single file (e.g. main.o)::
391 KASAN_SANITIZE_main.o := n
393 - For all files in one directory::
401 KASAN tests consist of two parts:
403 1. Tests that are integrated with the KUnit Test Framework. Enabled with
404 ``CONFIG_KASAN_KUNIT_TEST``. These tests can be run and partially verified
405 automatically in a few different ways, see the instructions below.
407 2. Tests that are currently incompatible with KUnit. Enabled with
408 ``CONFIG_KASAN_MODULE_TEST`` and can only be run as a module. These tests can
409 only be verified manually, by loading the kernel module and inspecting the
410 kernel log for KASAN reports.
412 Each KUnit-compatible KASAN test prints a KASAN report if an error is detected.
413 Then the test prints its number and status.
417 ok 28 - kmalloc_double_kzfree
419 When a test fails due to a failed ``kmalloc``::
421 # kmalloc_large_oob_right: ASSERTION FAILED at lib/test_kasan.c:163
422 Expected ptr is not null, but is
423 not ok 4 - kmalloc_large_oob_right
425 When a test fails due to a missing KASAN report::
427 # kmalloc_double_kzfree: EXPECTATION FAILED at lib/test_kasan.c:629
428 Expected kasan_data->report_expected == kasan_data->report_found, but
429 kasan_data->report_expected == 1
430 kasan_data->report_found == 0
431 not ok 28 - kmalloc_double_kzfree
433 At the end the cumulative status of all KASAN tests is printed. On success::
437 Or, if one of the tests failed::
442 There are a few ways to run KUnit-compatible KASAN tests.
446 With ``CONFIG_KUNIT`` enabled, ``CONFIG_KASAN_KUNIT_TEST`` can be built as
447 a loadable module and run on any architecture that supports KASAN by loading
448 the module with insmod or modprobe. The module is called ``test_kasan``.
452 With ``CONFIG_KUNIT`` built-in, ``CONFIG_KASAN_KUNIT_TEST`` can be built-in
453 on any architecure that supports KASAN. These and any other KUnit tests enabled
454 will run and print the results at boot as a late-init call.
458 With ``CONFIG_KUNIT`` and ``CONFIG_KASAN_KUNIT_TEST`` built-in, it's also
459 possible use ``kunit_tool`` to see the results of these and other KUnit tests
460 in a more readable way. This will not print the KASAN reports of the tests that
461 passed. Use `KUnit documentation <https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html>`_
462 for more up-to-date information on ``kunit_tool``.
464 .. _KUnit: https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html