Kernel Address Sanitizer (KASAN)¶
Kernel Address Sanitizer (KASAN) is a dynamic memory safety error detector designed to find out-of-bounds and use-after-free bugs.
KASAN has three modes:
Software Tag-Based KASAN
Hardware Tag-Based KASAN
Generic KASAN, enabled with CONFIG_KASAN_GENERIC, is the mode intended for debugging, similar to userspace ASan. This mode is supported on many CPU architectures, but it has significant performance and memory overheads.
Software Tag-Based KASAN or SW_TAGS KASAN, enabled with CONFIG_KASAN_SW_TAGS, can be used for both debugging and dogfood testing, similar to userspace HWASan. This mode is only supported for arm64, but its moderate memory overhead allows using it for testing on memory-restricted devices with real workloads.
Hardware Tag-Based KASAN or HW_TAGS KASAN, enabled with CONFIG_KASAN_HW_TAGS, is the mode intended to be used as an in-field memory bug detector or as a security mitigation. This mode only works on arm64 CPUs that support MTE (Memory Tagging Extension), but it has low memory and performance overheads and thus can be used in production.
For details about the memory and performance impact of each KASAN mode, see the descriptions of the corresponding Kconfig options.
The Generic and the Software Tag-Based modes are commonly referred to as the software modes. The Software Tag-Based and the Hardware Tag-Based modes are referred to as the tag-based modes.
Generic KASAN is supported on x86_64, arm, arm64, powerpc, riscv, s390, xtensa, and loongarch, and the tag-based KASAN modes are supported only on arm64.
Software KASAN modes use compile-time instrumentation to insert validity checks before every memory access and thus require a compiler version that provides support for that. The Hardware Tag-Based mode relies on hardware to perform these checks but still requires a compiler version that supports the memory tagging instructions.
Generic KASAN requires GCC version 8.3.0 or later or any Clang version supported by the kernel.
Software Tag-Based KASAN requires GCC 11+ or any Clang version supported by the kernel.
Hardware Tag-Based KASAN requires GCC 10+ or Clang 12+.
Generic KASAN supports finding bugs in all of slab, page_alloc, vmap, vmalloc, stack, and global memory.
Software Tag-Based KASAN supports slab, page_alloc, vmalloc, and stack memory.
Hardware Tag-Based KASAN supports slab, page_alloc, and non-executable vmalloc memory.
For slab, both software KASAN modes support SLUB and SLAB allocators, while Hardware Tag-Based KASAN only supports SLUB.
To enable KASAN, configure the kernel with:
and choose between
CONFIG_KASAN_GENERIC (to enable Generic KASAN),
CONFIG_KASAN_SW_TAGS (to enable Software Tag-Based KASAN), and
CONFIG_KASAN_HW_TAGS (to enable Hardware Tag-Based KASAN).
For the software modes, also choose between
CONFIG_KASAN_INLINE. Outline and inline are compiler instrumentation types.
The former produces a smaller binary while the latter is up to 2 times faster.
To include alloc and free stack traces of affected slab objects into reports,
CONFIG_STACKTRACE. To include alloc and free stack traces of affected
physical pages, enable
CONFIG_PAGE_OWNER and boot with
KASAN is affected by the generic
panic_on_warn command line parameter.
When it is enabled, KASAN panics the kernel after printing a bug report.
By default, KASAN prints a bug report only for the first invalid memory access.
kasan_multi_shot, KASAN prints a report on every invalid access. This
panic_on_warn for KASAN reports.
Alternatively, independent of
parameter can be used to control panic and reporting behaviour:
=panic_on_writecontrols whether to only print a KASAN report, panic the kernel, or panic the kernel on invalid writes only (default:
report). The panic happens even if
kasan_multi_shotis enabled. Note that when using asynchronous mode of Hardware Tag-Based KASAN,
kasan.fault=panic_on_writealways panics on asynchronously checked accesses (including reads).
Software and Hardware Tag-Based KASAN modes (see the section about various modes below) support altering stack trace collection behavior:
=ondisables or enables alloc and free stack traces collection (default:
kasan.stack_ring_size=<number of entries>specifies the number of entries in the stack ring (default:
Hardware Tag-Based KASAN mode is intended for use in production as a security mitigation. Therefore, it supports additional boot parameters that allow disabling KASAN altogether or controlling its features:
=oncontrols whether KASAN is enabled (default:
=asymmcontrols whether KASAN is configured in synchronous, asynchronous or asymmetric mode of execution (default:
sync). Synchronous mode: a bad access is detected immediately when a tag check fault occurs. Asynchronous mode: a bad access detection is delayed. When a tag check fault occurs, the information is stored in hardware (in the TFSR_EL1 register for arm64). The kernel periodically checks the hardware and only reports tag faults during these checks. Asymmetric mode: a bad access is detected synchronously on reads and asynchronously on writes.
=ondisables or enables tagging of vmalloc allocations (default:
kasan.page_alloc.sample=<sampling interval>makes KASAN tag only every Nth page_alloc allocation with the order equal or greater than
kasan.page_alloc.sample.order, where N is the value of the
1, or tag every such allocation). This parameter is intended to mitigate the performance overhead introduced by KASAN. Note that enabling this parameter makes Hardware Tag-Based KASAN skip checks of allocations chosen by sampling and thus miss bad accesses to these allocations. Use the default value for accurate bug detection.
kasan.page_alloc.sample.order=<minimum page order>specifies the minimum order of allocations that are affected by sampling (default:
3). Only applies when
kasan.page_alloc.sampleis set to a value greater than
1. This parameter is intended to allow sampling only large page_alloc allocations, which is the biggest source of the performance overhead.
A typical KASAN report looks like this:
BUG: KASAN: slab-out-of-bounds in kmalloc_oob_right+0xa8/0xbc [test_kasan]
Write of size 1 at addr ffff8801f44ec37b by task insmod/2760
CPU: 1 PID: 2760 Comm: insmod Not tainted 4.19.0-rc3+ #698
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
RSP: 002b:00007ffcf0b51b08 EFLAGS: 00000202 ORIG_RAX: 00000000000000af
RAX: ffffffffffffffda RBX: 000055dc3ee521a0 RCX: 00007f96443109da
RDX: 00007f96445cff88 RSI: 0000000000057a50 RDI: 00007f9644992000
RBP: 000055dc3ee510b0 R08: 0000000000000003 R09: 0000000000000000
R10: 00007f964430cd0a R11: 0000000000000202 R12: 00007f96445cff88
R13: 000055dc3ee51090 R14: 0000000000000000 R15: 0000000000000000
Allocated by task 2760:
Freed by task 815:
The buggy address belongs to the object at ffff8801f44ec300
which belongs to the cache kmalloc-128 of size 128
The buggy address is located 123 bytes inside of
128-byte region [ffff8801f44ec300, ffff8801f44ec380)
The buggy address belongs to the page:
page:ffffea0007d13b00 count:1 mapcount:0 mapping:ffff8801f7001640 index:0x0
raw: 0200000000000100 ffffea0007d11dc0 0000001a0000001a ffff8801f7001640
raw: 0000000000000000 0000000080150015 00000001ffffffff 0000000000000000
page dumped because: kasan: bad access detected
Memory state around the buggy address:
ffff8801f44ec200: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
ffff8801f44ec280: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
>ffff8801f44ec300: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 03
ffff8801f44ec380: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
ffff8801f44ec400: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
The report header summarizes what kind of bug happened and what kind of access caused it. It is followed by a stack trace of the bad access, a stack trace of where the accessed memory was allocated (in case a slab object was accessed), and a stack trace of where the object was freed (in case of a use-after-free bug report). Next comes a description of the accessed slab object and the information about the accessed memory page.
In the end, the report shows the memory state around the accessed address. Internally, KASAN tracks memory state separately for each memory granule, which is either 8 or 16 aligned bytes depending on KASAN mode. Each number in the memory state section of the report shows the state of one of the memory granules that surround the accessed address.
For Generic KASAN, the size of each memory granule is 8. The state of each granule is encoded in one shadow byte. Those 8 bytes can be accessible, partially accessible, freed, or be a part of a redzone. KASAN uses the following encoding for each shadow byte: 00 means that all 8 bytes of the corresponding memory region are accessible; number N (1 <= N <= 7) means that the first N bytes are accessible, and other (8 - N) bytes are not; any negative value indicates that the entire 8-byte word is inaccessible. KASAN uses different negative values to distinguish between different kinds of inaccessible memory like redzones or freed memory (see mm/kasan/kasan.h).
In the report above, the arrow points to the shadow byte
03, which means
that the accessed address is partially accessible.
For tag-based KASAN modes, this last report section shows the memory tags around the accessed address (see the Implementation details section).
Note that KASAN bug titles (like
are best-effort: KASAN prints the most probable bug type based on the limited
information it has. The actual type of the bug might be different.
Generic KASAN also reports up to two auxiliary call stack traces. These stack
traces point to places in code that interacted with the object but that are not
directly present in the bad access stack trace. Currently, this includes
call_rcu() and workqueue queuing.
Software KASAN modes use shadow memory to record whether each byte of memory is safe to access and use compile-time instrumentation to insert shadow memory checks before each memory access.
Generic KASAN dedicates 1/8th of kernel memory to its shadow memory (16TB to cover 128TB on x86_64) and uses direct mapping with a scale and offset to translate a memory address to its corresponding shadow address.
Here is the function which translates an address to its corresponding shadow address:
static inline void *kasan_mem_to_shadow(const void *addr)
return (void *)((unsigned long)addr >> KASAN_SHADOW_SCALE_SHIFT)
KASAN_SHADOW_SCALE_SHIFT = 3.
Compile-time instrumentation is used to insert memory access checks. Compiler
inserts function calls (
each memory access of size 1, 2, 4, 8, or 16. These functions check whether
memory accesses are valid or not by checking corresponding shadow memory.
With inline instrumentation, instead of making function calls, the compiler directly inserts the code to check shadow memory. This option significantly enlarges the kernel, but it gives an x1.1-x2 performance boost over the outline-instrumented kernel.
Generic KASAN is the only mode that delays the reuse of freed objects via quarantine (see mm/kasan/quarantine.c for implementation).
Software Tag-Based KASAN¶
Software Tag-Based KASAN uses a software memory tagging approach to checking access validity. It is currently only implemented for the arm64 architecture.
Software Tag-Based KASAN uses the Top Byte Ignore (TBI) feature of arm64 CPUs to store a pointer tag in the top byte of kernel pointers. It uses shadow memory to store memory tags associated with each 16-byte memory cell (therefore, it dedicates 1/16th of the kernel memory for shadow memory).
On each memory allocation, Software Tag-Based KASAN generates a random tag, tags the allocated memory with this tag, and embeds the same tag into the returned pointer.
Software Tag-Based KASAN uses compile-time instrumentation to insert checks before each memory access. These checks make sure that the tag of the memory that is being accessed is equal to the tag of the pointer that is used to access this memory. In case of a tag mismatch, Software Tag-Based KASAN prints a bug report.
Software Tag-Based KASAN also has two instrumentation modes (outline, which
emits callbacks to check memory accesses; and inline, which performs the shadow
memory checks inline). With outline instrumentation mode, a bug report is
printed from the function that performs the access check. With inline
brk instruction is emitted by the compiler, and a
brk handler is used to print bug reports.
Software Tag-Based KASAN uses 0xFF as a match-all pointer tag (accesses through pointers with the 0xFF pointer tag are not checked). The value 0xFE is currently reserved to tag freed memory regions.
Hardware Tag-Based KASAN¶
Hardware Tag-Based KASAN is similar to the software mode in concept but uses hardware memory tagging support instead of compiler instrumentation and shadow memory.
Hardware Tag-Based KASAN is currently only implemented for arm64 architecture and based on both arm64 Memory Tagging Extension (MTE) introduced in ARMv8.5 Instruction Set Architecture and Top Byte Ignore (TBI).
Special arm64 instructions are used to assign memory tags for each allocation. Same tags are assigned to pointers to those allocations. On every memory access, hardware makes sure that the tag of the memory that is being accessed is equal to the tag of the pointer that is used to access this memory. In case of a tag mismatch, a fault is generated, and a report is printed.
Hardware Tag-Based KASAN uses 0xFF as a match-all pointer tag (accesses through pointers with the 0xFF pointer tag are not checked). The value 0xFE is currently reserved to tag freed memory regions.
If the hardware does not support MTE (pre ARMv8.5), Hardware Tag-Based KASAN will not be enabled. In this case, all KASAN boot parameters are ignored.
Note that enabling CONFIG_KASAN_HW_TAGS always results in in-kernel TBI being
enabled. Even when
kasan.mode=off is provided or when the hardware does not
support MTE (but supports TBI).
Hardware Tag-Based KASAN only reports the first found bug. After that, MTE tag checking gets disabled.
The contents of this section are only applicable to software KASAN modes.
The kernel maps memory in several different parts of the address space. The range of kernel virtual addresses is large: there is not enough real memory to support a real shadow region for every address that could be accessed by the kernel. Therefore, KASAN only maps real shadow for certain parts of the address space.
By default, architectures only map real memory over the shadow region for the linear mapping (and potentially other small areas). For all other areas - such as vmalloc and vmemmap space - a single read-only page is mapped over the shadow area. This read-only shadow page declares all memory accesses as permitted.
This presents a problem for modules: they do not live in the linear mapping but in a dedicated module space. By hooking into the module allocator, KASAN temporarily maps real shadow memory to cover them. This allows detection of invalid accesses to module globals, for example.
This also creates an incompatibility with
VMAP_STACK: if the stack
lives in vmalloc space, it will be shadowed by the read-only page, and
the kernel will fault when trying to set up the shadow data for stack
CONFIG_KASAN_VMALLOC, KASAN can cover vmalloc space at the
cost of greater memory usage. Currently, this is supported on x86,
arm64, riscv, s390, and powerpc.
This works by hooking into vmalloc and vmap and dynamically allocating real shadow memory to back the mappings.
Most mappings in vmalloc space are small, requiring less than a full
page of shadow space. Allocating a full shadow page per mapping would
therefore be wasteful. Furthermore, to ensure that different mappings
use different shadow pages, mappings would have to be aligned to
KASAN_GRANULE_SIZE * PAGE_SIZE.
Instead, KASAN shares backing space across multiple mappings. It allocates a backing page when a mapping in vmalloc space uses a particular page of the shadow region. This page can be shared by other vmalloc mappings later on.
KASAN hooks into the vmap infrastructure to lazily clean up unused shadow memory.
To avoid the difficulties around swapping mappings around, KASAN expects that the part of the shadow region that covers the vmalloc space will not be covered by the early shadow page but will be left unmapped. This will require changes in arch-specific code.
VMAP_STACK support on x86 and can simplify support of
architectures that do not have a fixed module region.
Software KASAN modes use compiler instrumentation to insert validity checks. Such instrumentation might be incompatible with some parts of the kernel, and therefore needs to be disabled.
Other parts of the kernel might access metadata for allocated objects. Normally, KASAN detects and reports such accesses, but in some cases (e.g., in memory allocators), these accesses are valid.
For software KASAN modes, to disable instrumentation for a specific file or
directory, add a
KASAN_SANITIZE annotation to the respective kernel
For a single file (e.g., main.o):
KASAN_SANITIZE_main.o := n
For all files in one directory:
KASAN_SANITIZE := n
For software KASAN modes, to disable instrumentation on a per-function basis,
use the KASAN-specific
__no_sanitize_address function attribute or the
Note that disabling compiler instrumentation (either on a per-file or a per-function basis) makes KASAN ignore the accesses that happen directly in that code for software KASAN modes. It does not help when the accesses happen indirectly (through calls to instrumented functions) or with Hardware Tag-Based KASAN, which does not use compiler instrumentation.
For software KASAN modes, to disable KASAN reports in a part of the kernel code
for the current task, annotate this part of the code with a
kasan_enable_current() section. This also
disables the reports for indirect accesses that happen through function calls.
For tag-based KASAN modes, to disable access checking, use
page_kasan_tag_reset(). Note that temporarily
disabling access checking via
page_kasan_tag_reset() requires saving and
restoring the per-page KASAN tag via
There are KASAN tests that allow verifying that KASAN works and can detect certain types of memory corruptions. The tests consist of two parts:
1. Tests that are integrated with the KUnit Test Framework. Enabled with
CONFIG_KASAN_KUNIT_TEST. These tests can be run and partially verified
automatically in a few different ways; see the instructions below.
2. Tests that are currently incompatible with KUnit. Enabled with
CONFIG_KASAN_MODULE_TEST and can only be run as a module. These tests can
only be verified manually by loading the kernel module and inspecting the
kernel log for KASAN reports.
Each KUnit-compatible KASAN test prints one of multiple KASAN reports if an error is detected. Then the test prints its number and status.
When a test passes:
ok 28 - kmalloc_double_kzfree
When a test fails due to a failed
# kmalloc_large_oob_right: ASSERTION FAILED at lib/test_kasan.c:163
Expected ptr is not null, but is
not ok 4 - kmalloc_large_oob_right
When a test fails due to a missing KASAN report:
# kmalloc_double_kzfree: EXPECTATION FAILED at lib/test_kasan.c:974
KASAN failure expected in "kfree_sensitive(ptr)", but none occurred
not ok 44 - kmalloc_double_kzfree
At the end the cumulative status of all KASAN tests is printed. On success:
ok 1 - kasan
Or, if one of the tests failed:
not ok 1 - kasan
There are a few ways to run KUnit-compatible KASAN tests.
CONFIG_KUNITenabled, KASAN-KUnit tests can be built as a loadable module and run by loading
CONFIG_KUNITbuilt-in, KASAN-KUnit tests can be built-in as well. In this case, the tests will run at boot as a late-init call.
CONFIG_KASAN_KUNIT_TESTbuilt-in, it is also possible to use
kunit_toolto see the results of KUnit tests in a more readable way. This will not print the KASAN reports of the tests that passed. See KUnit documentation for more up-to-date information on