Lines Matching +full:memory +full:- +full:to +full:- +full:memory
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 [kasan_test]
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
283 Enabling CONFIG_KASAN_EXTRA_INFO allows KASAN to record and report more
287 extra memory to record more information (more cost details in the help text of
302 ----------------------
307 Software KASAN modes use shadow memory to record whether each byte of memory is
308 safe to access and use compile-time instrumentation to insert shadow memory
309 checks before each memory access.
311 Generic KASAN dedicates 1/8th of kernel memory to its shadow memory (16TB
312 to cover 128TB on x86_64) and uses direct mapping with a scale and offset to
313 translate a memory address to its corresponding shadow address.
315 Here is the function which translates an address to its corresponding shadow
326 Compile-time instrumentation is used to insert memory access checks. Compiler
328 each memory access of size 1, 2, 4, 8, or 16. These functions check whether
329 memory accesses are valid or not by checking corresponding shadow memory.
332 directly inserts the code to check shadow memory. This option significantly
333 enlarges the kernel, but it gives an x1.1-x2 performance boost over the
334 outline-instrumented kernel.
339 Software Tag-Based KASAN
342 Software Tag-Based KASAN uses a software memory tagging approach to checking
345 Software Tag-Based KASAN uses the Top Byte Ignore (TBI) feature of arm64 CPUs
346 to store a pointer tag in the top byte of kernel pointers. It uses shadow memory
347 to store memory tags associated with each 16-byte memory cell (therefore, it
348 dedicates 1/16th of the kernel memory for shadow memory).
350 On each memory allocation, Software Tag-Based KASAN generates a random tag, tags
351 the allocated memory with this tag, and embeds the same tag into the returned
354 Software Tag-Based KASAN uses compile-time instrumentation to insert checks
355 before each memory access. These checks make sure that the tag of the memory
356 that is being accessed is equal to the tag of the pointer that is used to access
357 this memory. In case of a tag mismatch, Software Tag-Based KASAN prints a bug
360 Software Tag-Based KASAN also has two instrumentation modes (outline, which
361 emits callbacks to check memory accesses; and inline, which performs the shadow
362 memory checks inline). With outline instrumentation mode, a bug report is
365 dedicated ``brk`` handler is used to print bug reports.
367 Software Tag-Based KASAN uses 0xFF as a match-all pointer tag (accesses through
369 reserved to tag freed memory regions.
371 Hardware Tag-Based KASAN
374 Hardware Tag-Based KASAN is similar to the software mode in concept but uses
375 hardware memory tagging support instead of compiler instrumentation and
376 shadow memory.
378 Hardware Tag-Based KASAN is currently only implemented for arm64 architecture
379 and based on both arm64 Memory Tagging Extension (MTE) introduced in ARMv8.5
382 Special arm64 instructions are used to assign memory tags for each allocation.
383 Same tags are assigned to pointers to those allocations. On every memory
384 access, hardware makes sure that the tag of the memory that is being accessed is
385 equal to the tag of the pointer that is used to access this memory. In case of a
388 Hardware Tag-Based KASAN uses 0xFF as a match-all pointer tag (accesses through
390 reserved to tag freed memory regions.
392 If the hardware does not support MTE (pre ARMv8.5), Hardware Tag-Based KASAN
395 Note that enabling CONFIG_KASAN_HW_TAGS always results in in-kernel TBI being
399 Hardware Tag-Based KASAN only reports the first found bug. After that, MTE tag
402 Shadow memory
403 -------------
405 The contents of this section are only applicable to software KASAN modes.
407 The kernel maps memory in several different parts of the address space.
409 memory to support a real shadow region for every address that could be
416 By default, architectures only map real memory over the shadow region
418 other areas - such as vmalloc and vmemmap space - a single read-only
419 page is mapped over the shadow area. This read-only shadow page
420 declares all memory accesses as permitted.
424 allocator, KASAN temporarily maps real shadow memory to cover them.
425 This allows detection of invalid accesses to module globals, for example.
428 lives in vmalloc space, it will be shadowed by the read-only page, and
429 the kernel will fault when trying to set up the shadow data for stack
436 cost of greater memory usage. Currently, this is supported on x86,
440 allocating real shadow memory to back the mappings.
444 therefore be wasteful. Furthermore, to ensure that different mappings
445 use different shadow pages, mappings would have to be aligned to
453 KASAN hooks into the vmap infrastructure to lazily clean up unused shadow
454 memory.
456 To avoid the difficulties around swapping mappings around, KASAN expects
459 This will require changes in arch-specific code.
465 --------------
470 Software KASAN modes use compiler instrumentation to insert validity checks.
472 therefore needs to be disabled.
476 in memory allocators), these accesses are valid.
478 For software KASAN modes, to disable instrumentation for a specific file or
479 directory, add a ``KASAN_SANITIZE`` annotation to the respective kernel
482 - For a single file (e.g., main.o)::
486 - For all files in one directory::
490 For software KASAN modes, to disable instrumentation on a per-function basis,
491 use the KASAN-specific ``__no_sanitize_address`` function attribute or the
494 Note that disabling compiler instrumentation (either on a per-file or a
495 per-function basis) makes KASAN ignore the accesses that happen directly in
497 indirectly (through calls to instrumented functions) or with Hardware
498 Tag-Based KASAN, which does not use compiler instrumentation.
500 For software KASAN modes, to disable KASAN reports in a part of the kernel code
505 For tag-based KASAN modes, to disable access checking, use
508 restoring the per-page KASAN tag via ``page_kasan_tag``/``page_kasan_tag_set``.
514 certain types of memory corruptions. The tests consist of two parts:
525 Each KUnit-compatible KASAN test prints one of multiple KASAN reports if an
530 ok 28 - kmalloc_double_kzfree
532 When a test fails due to a failed ``kmalloc``::
536 not ok 5 - kmalloc_large_oob_right
538 When a test fails due to a missing KASAN report::
542 not ok 28 - kmalloc_double_kzfree
547 ok 1 - kasan
551 not ok 1 - kasan
553 There are a few ways to run KUnit-compatible KASAN tests.
557 With ``CONFIG_KUNIT`` enabled, KASAN-KUnit tests can be built as a loadable
560 2. Built-In
562 With ``CONFIG_KUNIT`` built-in, KASAN-KUnit tests can be built-in as well.
563 In this case, the tests will run at boot as a late-init call.
567 With ``CONFIG_KUNIT`` and ``CONFIG_KASAN_KUNIT_TEST`` built-in, it is also
568 possible to use ``kunit_tool`` to see the results of KUnit tests in a more
570 See `KUnit documentation <https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html>`_
571 for more up-to-date information on ``kunit_tool``.
573 .. _KUnit: https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html