1 The Kernel Address Sanitizer (KASAN)
2 ====================================
7 KernelAddressSANitizer (KASAN) is a dynamic memory error detector designed to
8 find out-of-bound and use-after-free bugs. KASAN has two modes: generic KASAN
9 (similar to userspace ASan) and software tag-based KASAN (similar to userspace
12 KASAN uses compile-time instrumentation to insert validity checks before every
13 memory access, and therefore requires a compiler version that supports that.
15 Generic KASAN is supported in both GCC and Clang. With GCC it requires version
16 8.3.0 or later. Any supported Clang version is compatible, but detection of
17 out-of-bounds accesses for global variables is only supported since Clang 11.
19 Tag-based KASAN is only supported in Clang.
21 Currently generic KASAN is supported for the x86_64, arm64, xtensa, s390 and
22 riscv architectures, and tag-based KASAN is supported only for arm64.
27 To enable KASAN configure kernel with::
31 and choose between CONFIG_KASAN_GENERIC (to enable generic KASAN) and
32 CONFIG_KASAN_SW_TAGS (to enable software tag-based KASAN).
34 You also need to choose between CONFIG_KASAN_OUTLINE and CONFIG_KASAN_INLINE.
35 Outline and inline are compiler instrumentation types. The former produces
36 smaller binary while the latter is 1.1 - 2 times faster.
38 Both KASAN modes work with both SLUB and SLAB memory allocators.
39 For better bug detection and nicer reporting, enable CONFIG_STACKTRACE.
41 To augment reports with last allocation and freeing stack of the physical page,
42 it is recommended to enable also CONFIG_PAGE_OWNER and boot with page_owner=on.
44 To disable instrumentation for specific files or directories, add a line
45 similar to the following to the respective kernel Makefile:
47 - For a single file (e.g. main.o)::
49 KASAN_SANITIZE_main.o := n
51 - For all files in one directory::
58 A typical out-of-bounds access generic KASAN report looks like this::
60 ==================================================================
61 BUG: KASAN: slab-out-of-bounds in kmalloc_oob_right+0xa8/0xbc [test_kasan]
62 Write of size 1 at addr ffff8801f44ec37b by task insmod/2760
64 CPU: 1 PID: 2760 Comm: insmod Not tainted 4.19.0-rc3+ #698
65 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
68 print_address_description+0x73/0x280
69 kasan_report+0x144/0x187
70 __asan_report_store1_noabort+0x17/0x20
71 kmalloc_oob_right+0xa8/0xbc [test_kasan]
72 kmalloc_tests_init+0x16/0x700 [test_kasan]
73 do_one_initcall+0xa5/0x3ae
74 do_init_module+0x1b6/0x547
75 load_module+0x75df/0x8070
76 __do_sys_init_module+0x1c6/0x200
77 __x64_sys_init_module+0x6e/0xb0
78 do_syscall_64+0x9f/0x2c0
79 entry_SYSCALL_64_after_hwframe+0x44/0xa9
80 RIP: 0033:0x7f96443109da
81 RSP: 002b:00007ffcf0b51b08 EFLAGS: 00000202 ORIG_RAX: 00000000000000af
82 RAX: ffffffffffffffda RBX: 000055dc3ee521a0 RCX: 00007f96443109da
83 RDX: 00007f96445cff88 RSI: 0000000000057a50 RDI: 00007f9644992000
84 RBP: 000055dc3ee510b0 R08: 0000000000000003 R09: 0000000000000000
85 R10: 00007f964430cd0a R11: 0000000000000202 R12: 00007f96445cff88
86 R13: 000055dc3ee51090 R14: 0000000000000000 R15: 0000000000000000
88 Allocated by task 2760:
90 kasan_kmalloc+0xa7/0xd0
91 kmem_cache_alloc_trace+0xe1/0x1b0
92 kmalloc_oob_right+0x56/0xbc [test_kasan]
93 kmalloc_tests_init+0x16/0x700 [test_kasan]
94 do_one_initcall+0xa5/0x3ae
95 do_init_module+0x1b6/0x547
96 load_module+0x75df/0x8070
97 __do_sys_init_module+0x1c6/0x200
98 __x64_sys_init_module+0x6e/0xb0
99 do_syscall_64+0x9f/0x2c0
100 entry_SYSCALL_64_after_hwframe+0x44/0xa9
104 __kasan_slab_free+0x135/0x190
105 kasan_slab_free+0xe/0x10
107 umh_complete+0x6a/0xa0
108 call_usermodehelper_exec_async+0x4c3/0x640
109 ret_from_fork+0x35/0x40
111 The buggy address belongs to the object at ffff8801f44ec300
112 which belongs to the cache kmalloc-128 of size 128
113 The buggy address is located 123 bytes inside of
114 128-byte region [ffff8801f44ec300, ffff8801f44ec380)
115 The buggy address belongs to the page:
116 page:ffffea0007d13b00 count:1 mapcount:0 mapping:ffff8801f7001640 index:0x0
117 flags: 0x200000000000100(slab)
118 raw: 0200000000000100 ffffea0007d11dc0 0000001a0000001a ffff8801f7001640
119 raw: 0000000000000000 0000000080150015 00000001ffffffff 0000000000000000
120 page dumped because: kasan: bad access detected
122 Memory state around the buggy address:
123 ffff8801f44ec200: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
124 ffff8801f44ec280: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
125 >ffff8801f44ec300: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 03
127 ffff8801f44ec380: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
128 ffff8801f44ec400: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
129 ==================================================================
131 The header of the report provides a short summary of what kind of bug happened
132 and what kind of access caused it. It's followed by a stack trace of the bad
133 access, a stack trace of where the accessed memory was allocated (in case bad
134 access happens on a slab object), and a stack trace of where the object was
135 freed (in case of a use-after-free bug report). Next comes a description of
136 the accessed slab object and information about the accessed memory page.
138 In the last section the report shows memory state around the accessed address.
139 Reading this part requires some understanding of how KASAN works.
141 The state of each 8 aligned bytes of memory is encoded in one shadow byte.
142 Those 8 bytes can be accessible, partially accessible, freed or be a redzone.
143 We use the following encoding for each shadow byte: 0 means that all 8 bytes
144 of the corresponding memory region are accessible; number N (1 <= N <= 7) means
145 that the first N bytes are accessible, and other (8 - N) bytes are not;
146 any negative value indicates that the entire 8-byte word is inaccessible.
147 We use different negative values to distinguish between different kinds of
148 inaccessible memory like redzones or freed memory (see mm/kasan/kasan.h).
150 In the report above the arrows point to the shadow byte 03, which means that
151 the accessed address is partially accessible.
153 For tag-based KASAN this last report section shows the memory tags around the
154 accessed address (see Implementation details section).
157 Implementation details
158 ----------------------
163 From a high level, our approach to memory error detection is similar to that
164 of kmemcheck: use shadow memory to record whether each byte of memory is safe
165 to access, and use compile-time instrumentation to insert checks of shadow
166 memory on each memory access.
168 Generic KASAN dedicates 1/8th of kernel memory to its shadow memory (e.g. 16TB
169 to cover 128TB on x86_64) and uses direct mapping with a scale and offset to
170 translate a memory address to its corresponding shadow address.
172 Here is the function which translates an address to its corresponding shadow
175 static inline void *kasan_mem_to_shadow(const void *addr)
177 return ((unsigned long)addr >> KASAN_SHADOW_SCALE_SHIFT)
178 + KASAN_SHADOW_OFFSET;
181 where ``KASAN_SHADOW_SCALE_SHIFT = 3``.
183 Compile-time instrumentation is used to insert memory access checks. Compiler
184 inserts function calls (__asan_load*(addr), __asan_store*(addr)) before each
185 memory access of size 1, 2, 4, 8 or 16. These functions check whether memory
186 access is valid or not by checking corresponding shadow memory.
188 GCC 5.0 has possibility to perform inline instrumentation. Instead of making
189 function calls GCC directly inserts the code to check the shadow memory.
190 This option significantly enlarges kernel but it gives x1.1-x2 performance
191 boost over outline instrumented kernel.
193 Generic KASAN also reports the last 2 call stacks to creation of work that
194 potentially has access to an object. Call stacks for the following are shown:
195 call_rcu() and workqueue queuing.
197 Software tag-based KASAN
198 ~~~~~~~~~~~~~~~~~~~~~~~~
200 Tag-based KASAN uses the Top Byte Ignore (TBI) feature of modern arm64 CPUs to
201 store a pointer tag in the top byte of kernel pointers. Like generic KASAN it
202 uses shadow memory to store memory tags associated with each 16-byte memory
203 cell (therefore it dedicates 1/16th of the kernel memory for shadow memory).
205 On each memory allocation tag-based KASAN generates a random tag, tags the
206 allocated memory with this tag, and embeds this tag into the returned pointer.
207 Software tag-based KASAN uses compile-time instrumentation to insert checks
208 before each memory access. These checks make sure that tag of the memory that
209 is being accessed is equal to tag of the pointer that is used to access this
210 memory. In case of a tag mismatch tag-based KASAN prints a bug report.
212 Software tag-based KASAN also has two instrumentation modes (outline, that
213 emits callbacks to check memory accesses; and inline, that performs the shadow
214 memory checks inline). With outline instrumentation mode, a bug report is
215 simply printed from the function that performs the access check. With inline
216 instrumentation a brk instruction is emitted by the compiler, and a dedicated
217 brk handler is used to print bug reports.
219 A potential expansion of this mode is a hardware tag-based mode, which would
220 use hardware memory tagging support instead of compiler instrumentation and
221 manual shadow memory manipulation.
223 What memory accesses are sanitised by KASAN?
224 --------------------------------------------
226 The kernel maps memory in a number of different parts of the address
227 space. This poses something of a problem for KASAN, which requires
228 that all addresses accessed by instrumented code have a valid shadow
231 The range of kernel virtual addresses is large: there is not enough
232 real memory to support a real shadow region for every address that
233 could be accessed by the kernel.
238 By default, architectures only map real memory over the shadow region
239 for the linear mapping (and potentially other small areas). For all
240 other areas - such as vmalloc and vmemmap space - a single read-only
241 page is mapped over the shadow area. This read-only shadow page
242 declares all memory accesses as permitted.
244 This presents a problem for modules: they do not live in the linear
245 mapping, but in a dedicated module space. By hooking in to the module
246 allocator, KASAN can temporarily map real shadow memory to cover
247 them. This allows detection of invalid accesses to module globals, for
250 This also creates an incompatibility with ``VMAP_STACK``: if the stack
251 lives in vmalloc space, it will be shadowed by the read-only page, and
252 the kernel will fault when trying to set up the shadow data for stack
258 With ``CONFIG_KASAN_VMALLOC``, KASAN can cover vmalloc space at the
259 cost of greater memory usage. Currently this is only supported on x86.
261 This works by hooking into vmalloc and vmap, and dynamically
262 allocating real shadow memory to back the mappings.
264 Most mappings in vmalloc space are small, requiring less than a full
265 page of shadow space. Allocating a full shadow page per mapping would
266 therefore be wasteful. Furthermore, to ensure that different mappings
267 use different shadow pages, mappings would have to be aligned to
268 ``KASAN_SHADOW_SCALE_SIZE * PAGE_SIZE``.
270 Instead, we share backing space across multiple mappings. We allocate
271 a backing page when a mapping in vmalloc space uses a particular page
272 of the shadow region. This page can be shared by other vmalloc
275 We hook in to the vmap infrastructure to lazily clean up unused shadow
278 To avoid the difficulties around swapping mappings around, we expect
279 that the part of the shadow region that covers the vmalloc space will
280 not be covered by the early shadow page, but will be left
281 unmapped. This will require changes in arch-specific code.
283 This allows ``VMAP_STACK`` support on x86, and can simplify support of
284 architectures that do not have a fixed module region.
286 CONFIG_KASAN_KUNIT_TEST & CONFIG_TEST_KASAN_MODULE
287 --------------------------------------------------
289 ``CONFIG_KASAN_KUNIT_TEST`` utilizes the KUnit Test Framework for testing.
290 This means each test focuses on a small unit of functionality and
291 there are a few ways these tests can be run.
293 Each test will print the KASAN report if an error is detected and then
294 print the number of the test and the status of the test:
298 ok 28 - kmalloc_double_kzfree
300 or, if kmalloc failed::
302 # kmalloc_large_oob_right: ASSERTION FAILED at lib/test_kasan.c:163
303 Expected ptr is not null, but is
304 not ok 4 - kmalloc_large_oob_right
306 or, if a KASAN report was expected, but not found::
308 # kmalloc_double_kzfree: EXPECTATION FAILED at lib/test_kasan.c:629
309 Expected kasan_data->report_expected == kasan_data->report_found, but
310 kasan_data->report_expected == 1
311 kasan_data->report_found == 0
312 not ok 28 - kmalloc_double_kzfree
314 All test statuses are tracked as they run and an overall status will
315 be printed at the end::
326 With ``CONFIG_KUNIT`` enabled, ``CONFIG_KASAN_KUNIT_TEST`` can be built as
327 a loadable module and run on any architecture that supports KASAN
328 using something like insmod or modprobe. The module is called ``test_kasan``.
333 With ``CONFIG_KUNIT`` built-in, ``CONFIG_KASAN_KUNIT_TEST`` can be built-in
334 on any architecture that supports KASAN. These and any other KUnit
335 tests enabled will run and print the results at boot as a late-init
339 ~~~~~~~~~~~~~~~~~~~~~
341 With ``CONFIG_KUNIT`` and ``CONFIG_KASAN_KUNIT_TEST`` built-in, we can also
342 use kunit_tool to see the results of these along with other KUnit
343 tests in a more readable way. This will not print the KASAN reports
344 of tests that passed. Use `KUnit documentation <https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html>`_ for more up-to-date
345 information on kunit_tool.
347 .. _KUnit: https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html
349 ``CONFIG_TEST_KASAN_MODULE`` is a set of KASAN tests that could not be
350 converted to KUnit. These tests can be run only as a module with
351 ``CONFIG_TEST_KASAN_MODULE`` built as a loadable module and
352 ``CONFIG_KASAN`` built-in. The type of error expected and the
353 function being run is printed before the expression expected to give
354 an error. Then the error is printed, if found, and that test
355 should be interpreted to pass only if the error was the one expected