Documentation/dev-tools/kasan.rst

5 --------
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.
17 out-of-bounds accesses for global variables is only supported since Clang 11.
19 Tag-based KASAN is only supported in Clang.
22 riscv architectures, and tag-based KASAN is supported only for arm64.
25 -----
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.
36 smaller binary while the latter is 1.1 - 2 times faster.
38 Both KASAN modes work with both SLUB and SLAB memory allocators.
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)::
51 - For all files in one directory::
58 A typical out-of-bounds access generic KASAN report looks like this::
61     BUG: KASAN: slab-out-of-bounds in kmalloc_oob_right+0xa8/0xbc [test_kasan]
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
111     The buggy address belongs to the object at ffff8801f44ec300
112      which belongs to the cache kmalloc-128 of size 128
114      128-byte region [ffff8801f44ec300, ffff8801f44ec380)
115     The buggy address belongs to the page:
122     Memory state around the buggy address:
133 access, a stack trace of where the accessed memory was allocated (in case bad
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.
141 The state of each 8 aligned bytes of memory is encoded in one shadow byte.
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
153 For tag-based KASAN this last report section shows the memory tags around the
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
183 Compile-time instrumentation is used to insert memory access checks. Compiler
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
193 Generic KASAN prints up to 2 call_rcu() call stacks in reports, the last one
194 and the second to last.
196 Software tag-based KASAN
199 Tag-based KASAN uses the Top Byte Ignore (TBI) feature of modern arm64 CPUs to
201 uses shadow memory to store memory tags associated with each 16-byte memory
202 cell (therefore it dedicates 1/16th of the kernel memory for shadow memory).
204 On each memory allocation tag-based KASAN generates a random tag, tags the
205 allocated memory with this tag, and embeds this tag into the returned pointer.
206 Software tag-based KASAN uses compile-time instrumentation to insert checks
207 before each memory access. These checks make sure that tag of the memory that
208 is being accessed is equal to tag of the pointer that is used to access this
209 memory. In case of a tag mismatch tag-based KASAN prints a bug report.
211 Software tag-based KASAN also has two instrumentation modes (outline, that
212 emits callbacks to check memory accesses; and inline, that performs the shadow
213 memory checks inline). With outline instrumentation mode, a bug report is
216 brk handler is used to print bug reports.
218 A potential expansion of this mode is a hardware tag-based mode, which would
219 use hardware memory tagging support instead of compiler instrumentation and
220 manual shadow memory manipulation.
222 What memory accesses are sanitised by KASAN?
223 --------------------------------------------
225 The kernel maps memory in a number of different parts of the address
231 real memory to support a real shadow region for every address that
237 By default, architectures only map real memory over the shadow region
239 other areas - such as vmalloc and vmemmap space - a single read-only
240 page is mapped over the shadow area. This read-only shadow page
241 declares all memory accesses as permitted.
244 mapping, but in a dedicated module space. By hooking in to the module
245 allocator, KASAN can temporarily map real shadow memory to cover
246 them. This allows detection of invalid accesses to module globals, for
250 lives in vmalloc space, it will be shadowed by the read-only page, and
251 the kernel will fault when trying to set up the shadow data for stack
258 cost of greater memory usage. Currently this is only supported on x86.
261 allocating real shadow memory to back the mappings.
265 therefore be wasteful. Furthermore, to ensure that different mappings
266 use different shadow pages, mappings would have to be aligned to
274 We hook in to the vmap infrastructure to lazily clean up unused shadow
275 memory.
277 To avoid the difficulties around swapping mappings around, we expect
280 unmapped. This will require changes in arch-specific code.
286 --------------------------------------------------
297         ok 28 - kmalloc_double_kzfree
303         not ok 4 - kmalloc_large_oob_right
308         Expected kasan_data->report_expected == kasan_data->report_found, but
309         kasan_data->report_expected == 1
310         kasan_data->report_found == 0
311         not ok 28 - kmalloc_double_kzfree
316         ok 1 - kasan
320         not ok 1 - kasan
329 (2) Built-In
332 With ``CONFIG_KUNIT`` built-in, ``CONFIG_KASAN_KUNIT_TEST`` can be built-in
334 tests enabled will run and print the results at boot as a late-init
340 With ``CONFIG_KUNIT`` and ``CONFIG_KASAN_KUNIT_TEST`` built-in, we can also
341 use kunit_tool to see the results of these along with other KUnit
343 …cumentation <https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html>`_ for more up-to-d…
346 .. _KUnit: https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html
349 converted to KUnit. These tests can be run only as a module with
351 ``CONFIG_KASAN`` built-in. The type of error expected and the
352 function being run is printed before the expression expected to give
354 should be interpretted to pass only if the error was the one expected