Documentation/dev-tools/kasan.rst

1 .. SPDX-License-Identifier: GPL-2.0
8 --------
11 designed to find out-of-bounds and use-after-free bugs.
15 1. Generic KASAN
16 2. Software Tag-Based KASAN
17 3. Hardware Tag-Based KASAN
23 Software Tag-Based KASAN or SW_TAGS KASAN, enabled with CONFIG_KASAN_SW_TAGS,
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
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
55 support for that. The Hardware Tag-Based mode relies on hardware to perform
62 Software Tag-Based KASAN requires GCC 11+
65 Hardware Tag-Based KASAN requires GCC 10+ or Clang 12+.
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
79 Hardware Tag-Based KASAN only supports SLUB.
82 -----
89 ``CONFIG_KASAN_SW_TAGS`` (to enable Software Tag-Based KASAN), and
90 ``CONFIG_KASAN_HW_TAGS`` (to enable Hardware Tag-Based KASAN).
113 - ``kasan.fault=report``, ``=panic``, or ``=panic_on_write`` controls whether
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
152   parameter (default: ``1``, or tag every such allocation).
155   Note that enabling this parameter makes Hardware Tag-Based KASAN skip checks
159 - ``kasan.page_alloc.sample.order=<minimum page order>`` specifies the minimum
162   than ``1``.
172     BUG: KASAN: slab-out-of-bounds in kmalloc_oob_right+0xa8/0xbc [test_kasan]
173     Write of size 1 at addr ffff8801f44ec37b by task insmod/2760
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
223      which belongs to the cache kmalloc-128 of size 128
225      128-byte region [ffff8801f44ec300, ffff8801f44ec380)
227     page:ffffea0007d13b00 count:1 mapcount:0 mapping:ffff8801f7001640 index:0x0
245 and a stack trace of where the object was freed (in case of a use-after-free
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
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
281 ----------------------
287 safe to access and use compile-time instrumentation to insert shadow memory
290 Generic KASAN dedicates 1/8th of kernel memory to its shadow memory (16TB
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
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
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
333 Software Tag-Based KASAN uses compile-time instrumentation to insert checks
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
346 Software Tag-Based KASAN uses 0xFF as a match-all pointer tag (accesses through
350 Hardware Tag-Based KASAN
353 Hardware Tag-Based KASAN is similar to the software mode in concept but uses
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
367 Hardware Tag-Based KASAN uses 0xFF as a match-all pointer tag (accesses through
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
382 -------------
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
407 lives in vmalloc space, it will be shadowed by the read-only page, and
438 This will require changes in arch-specific code.
444 --------------
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
477 Tag-Based KASAN, which does not use compiler instrumentation.
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``.
495 1. Tests that are integrated with the KUnit Test Framework. Enabled with
504 Each KUnit-compatible KASAN test prints one of multiple KASAN reports if an
509         ok 28 - kmalloc_double_kzfree
515         not ok 4 - kmalloc_large_oob_right
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.
534 1. Loadable module
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
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