Documentation/dev-tools/kasan.rst

1 .. SPDX-License-Identifier: GPL-2.0
8 --------
10 Kernel Address Sanitizer (KASAN) is a dynamic memory safety error detector
11 designed to find out-of-bounds and use-after-free bugs.
16 2. Software Tag-Based KASAN
17 3. Hardware Tag-Based KASAN
20 debugging, similar to userspace ASan. This mode is supported on many CPU
21 architectures, but it has significant performance and memory overheads.
23 Software Tag-Based KASAN or SW_TAGS KASAN, enabled with CONFIG_KASAN_SW_TAGS,
24 can be used for both debugging and dogfood testing, similar to userspace HWASan.
25 This mode is only supported for arm64, but its moderate memory overhead allows
26 using it for testing on memory-restricted devices with real workloads.
28 Hardware Tag-Based KASAN or HW_TAGS KASAN, enabled with CONFIG_KASAN_HW_TAGS,
29 is the mode intended to be used as an in-field memory bug detector or as a
31 (Memory Tagging Extension), but it has low memory and performance overheads and
34 For details about the memory and performance impact of each KASAN mode, see the
37 The Generic and the Software Tag-Based modes are commonly referred to as the
38 software modes. The Software Tag-Based and the Hardware Tag-Based modes are
39 referred to as the tag-based modes.
42 -------
48 and loongarch, and the tag-based KASAN modes are supported only on arm64.
53 Software KASAN modes use compile-time instrumentation to insert validity checks
54 before every memory access and thus require a compiler version that provides
55 support for that. The Hardware Tag-Based mode relies on hardware to perform
56 these checks but still requires a compiler version that supports the memory
62 Software Tag-Based KASAN requires GCC 11+
65 Hardware Tag-Based KASAN requires GCC 10+ or Clang 12+.
67 Memory types
71 stack, and global memory.
73 Software Tag-Based KASAN supports slab, page_alloc, vmalloc, and stack memory.
75 Hardware Tag-Based KASAN supports slab, page_alloc, and non-executable vmalloc
76 memory.
79 Hardware Tag-Based KASAN only supports SLUB.
82 -----
84 To enable KASAN, configure the kernel with::
88 and 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).
94 The former produces a smaller binary while the latter is up to 2 times faster.
96 To include alloc and free stack traces of affected slab objects into reports,
97 enable ``CONFIG_STACKTRACE``. To include alloc and free stack traces of affected
106 By default, KASAN prints a bug report only for the first invalid memory access.
111 parameter can be used to control panic and reporting behaviour:
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
117   Hardware Tag-Based KASAN, ``kasan.fault=panic_on_write`` always panics on
120 Software and Hardware Tag-Based KASAN modes (see the section about various
123 - ``kasan.stacktrace=off`` or ``=on`` disables or enables alloc and free stack
125 - ``kasan.stack_ring_size=<number of entries>`` specifies the number of entries
128 Hardware Tag-Based KASAN mode is intended for use in production as a security
132 - ``kasan=off`` or ``=on`` controls whether KASAN is enabled (default: ``on``).
134 - ``kasan.mode=sync``, ``=async`` or ``=asymm`` controls whether KASAN
146 - ``kasan.vmalloc=off`` or ``=on`` disables or enables tagging of vmalloc
149 - ``kasan.page_alloc.sample=<sampling interval>`` makes KASAN tag only every
153   This parameter is intended to mitigate the performance overhead introduced
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
159 - ``kasan.page_alloc.sample.order=<minimum page order>`` specifies the minimum
161   Only applies when ``kasan.page_alloc.sample`` is set to a value greater
163   This parameter is intended to allow sampling only large page_alloc
172     BUG: KASAN: slab-out-of-bounds in kmalloc_oob_right+0xa8/0xbc [test_kasan]
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
222     The buggy address belongs to the object at ffff8801f44ec300
223      which belongs to the cache kmalloc-128 of size 128
225      128-byte region [ffff8801f44ec300, ffff8801f44ec380)
226     The buggy address belongs to the page:
233     Memory state around the buggy address:
244 where the accessed memory was allocated (in case a slab object was accessed),
245 and a stack trace of where the object was freed (in case of a use-after-free
247 information about the accessed memory page.
249 In the end, the report shows the memory state around the accessed address.
250 Internally, KASAN tracks memory state separately for each memory granule, which
252 memory state section of the report shows the state of one of the memory
255 For Generic KASAN, the size of each memory granule is 8. The state of each
259 memory region are accessible; number N (1 <= N <= 7) means that the first N
260 bytes are accessible, and other (8 - N) bytes are not; any negative value
261 indicates that the entire 8-byte word is inaccessible. KASAN uses different
262 negative values to distinguish between different kinds of inaccessible memory
263 like redzones or freed memory (see mm/kasan/kasan.h).
265 In the report above, the arrow points to the shadow byte ``03``, which means
268 For tag-based KASAN modes, this last report section shows the memory tags around
271 Note that KASAN bug titles (like ``slab-out-of-bounds`` or ``use-after-free``)
272 are best-effort: KASAN prints the most probable bug type based on the limited
275 Generic KASAN also reports up to two auxiliary call stack traces. These stack
276 traces point to places in code that interacted with the object but that are not
281 ----------------------
286 Software KASAN modes use shadow memory to record whether each byte of memory is
287 safe to access and use compile-time instrumentation to insert shadow memory
288 checks before each memory access.
290 Generic KASAN dedicates 1/8th of kernel memory to its shadow memory (16TB
291 to cover 128TB on x86_64) and uses direct mapping with a scale and offset to
292 translate a memory address to its corresponding shadow address.
294 Here is the function which translates an address to its corresponding shadow
305 Compile-time instrumentation is used to insert memory access checks. Compiler
307 each memory access of size 1, 2, 4, 8, or 16. These functions check whether
308 memory accesses are valid or not by checking corresponding shadow memory.
311 directly inserts the code to check shadow memory. This option significantly
312 enlarges the kernel, but it gives an x1.1-x2 performance boost over the
313 outline-instrumented kernel.
318 Software Tag-Based KASAN
321 Software Tag-Based KASAN uses a software memory tagging approach to checking
324 Software Tag-Based KASAN uses the Top Byte Ignore (TBI) feature of arm64 CPUs
325 to store a pointer tag in the top byte of kernel pointers. It uses shadow memory
326 to store memory tags associated with each 16-byte memory cell (therefore, it
327 dedicates 1/16th of the kernel memory for shadow memory).
329 On each memory allocation, Software Tag-Based KASAN generates a random tag, tags
330 the allocated memory with this tag, and embeds the same tag into the returned
333 Software Tag-Based KASAN uses compile-time instrumentation to insert checks
334 before each memory access. These checks make sure that the tag of the memory
335 that is being accessed is equal to the tag of the pointer that is used to access
336 this memory. In case of a tag mismatch, Software Tag-Based KASAN prints a bug
339 Software Tag-Based KASAN also has two instrumentation modes (outline, which
340 emits callbacks to check memory accesses; and inline, which performs the shadow
341 memory checks inline). With outline instrumentation mode, a bug report is
344 dedicated ``brk`` handler is used to print bug reports.
346 Software Tag-Based KASAN uses 0xFF as a match-all pointer tag (accesses through
348 reserved to tag freed memory regions.
350 Hardware Tag-Based KASAN
353 Hardware Tag-Based KASAN is similar to the software mode in concept but uses
354 hardware memory tagging support instead of compiler instrumentation and
355 shadow memory.
357 Hardware Tag-Based KASAN is currently only implemented for arm64 architecture
358 and based on both arm64 Memory Tagging Extension (MTE) introduced in ARMv8.5
361 Special arm64 instructions are used to assign memory tags for each allocation.
362 Same tags are assigned to pointers to those allocations. On every memory
363 access, hardware makes sure that the tag of the memory that is being accessed is
364 equal to the tag of the pointer that is used to access this memory. In case of a
367 Hardware Tag-Based KASAN uses 0xFF as a match-all pointer tag (accesses through
369 reserved to tag freed memory regions.
371 If the hardware does not support MTE (pre ARMv8.5), Hardware Tag-Based KASAN
374 Note that enabling CONFIG_KASAN_HW_TAGS always results in in-kernel TBI being
378 Hardware Tag-Based KASAN only reports the first found bug. After that, MTE tag
381 Shadow memory
382 -------------
384 The contents of this section are only applicable to software KASAN modes.
386 The kernel maps memory in several different parts of the address space.
388 memory to support a real shadow region for every address that could be
395 By default, architectures only map real memory over the shadow region
397 other areas - such as vmalloc and vmemmap space - a single read-only
398 page is mapped over the shadow area. This read-only shadow page
399 declares all memory accesses as permitted.
403 allocator, KASAN temporarily maps real shadow memory to cover them.
404 This allows detection of invalid accesses to module globals, for example.
407 lives in vmalloc space, it will be shadowed by the read-only page, and
408 the kernel will fault when trying to set up the shadow data for stack
415 cost of greater memory usage. Currently, this is supported on x86,
419 allocating real shadow memory to back the mappings.
423 therefore be wasteful. Furthermore, to ensure that different mappings
424 use different shadow pages, mappings would have to be aligned to
432 KASAN hooks into the vmap infrastructure to lazily clean up unused shadow
433 memory.
435 To avoid the difficulties around swapping mappings around, KASAN expects
438 This will require changes in arch-specific code.
444 --------------
449 Software KASAN modes use compiler instrumentation to insert validity checks.
451 therefore needs to be disabled.
455 in memory allocators), these accesses are valid.
457 For software KASAN modes, to disable instrumentation for a specific file or
458 directory, add a ``KASAN_SANITIZE`` annotation to the respective kernel
461 - For a single file (e.g., main.o)::
465 - For all files in one directory::
469 For software KASAN modes, to disable instrumentation on a per-function basis,
470 use the KASAN-specific ``__no_sanitize_address`` function attribute or the
473 Note that disabling compiler instrumentation (either on a per-file or a
474 per-function basis) makes KASAN ignore the accesses that happen directly in
476 indirectly (through calls to instrumented functions) or with Hardware
477 Tag-Based KASAN, which does not use compiler instrumentation.
479 For software KASAN modes, to disable KASAN reports in a part of the kernel code
484 For tag-based KASAN modes, to disable access checking, use
487 restoring the per-page KASAN tag via ``page_kasan_tag``/``page_kasan_tag_set``.
493 certain types of memory corruptions. The tests consist of two parts:
504 Each KUnit-compatible KASAN test prints one of multiple KASAN reports if an
509         ok 28 - kmalloc_double_kzfree
511 When a test fails due to a failed ``kmalloc``::
515         not ok 4 - kmalloc_large_oob_right
517 When a test fails due to a missing KASAN report::
521         not ok 44 - kmalloc_double_kzfree
526         ok 1 - kasan
530         not ok 1 - kasan
532 There are a few ways to run KUnit-compatible KASAN tests.
536    With ``CONFIG_KUNIT`` enabled, KASAN-KUnit tests can be built as a loadable
539 2. Built-In
541    With ``CONFIG_KUNIT`` built-in, KASAN-KUnit tests can be built-in as well.
542    In this case, the tests will run at boot as a late-init call.
546    With ``CONFIG_KUNIT`` and ``CONFIG_KASAN_KUNIT_TEST`` built-in, it is also
547    possible to use ``kunit_tool`` to see the results of KUnit tests in a more
549    See `KUnit documentation <https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html>`_
550    for more up-to-date information on ``kunit_tool``.
552 .. _KUnit: https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html