Architectures¶

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.

Compilers¶

Software KASAN modes use compile-time instrumentation to insert validity checksbefore every memory access and thus require a compiler version that providessupport for that. The Hardware Tag-Based mode relies on hardware to performthese checks but still requires a compiler version that supports the memorytagging instructions.

Generic KASAN requires GCC version 8.3.0 or lateror 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+.

Memory types¶

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 vmallocmemory.

For slab, both software KASAN modes support SLUB and SLAB allocators, whileHardware Tag-Based KASAN only supports SLUB.

Boot parameters¶

KASAN is affected by the genericpanic_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.Withkasan_multi_shot, KASAN prints a report on every invalid access. Thiseffectively disablespanic_on_warn for KASAN reports.

Alternatively, independent ofpanic_on_warn, thekasan.fault= bootparameter can be used to control panic and reporting behaviour:

• kasan.fault=report,=panic, or=panic_on_write controls whetherto only print a KASAN report, panic the kernel, or panic the kernel oninvalid writes only (default:report). The panic happens even ifkasan_multi_shot is enabled. Note that when using asynchronous mode ofHardware Tag-Based KASAN,kasan.fault=panic_on_write always panics onasynchronously checked accesses (including reads).

Software and Hardware Tag-Based KASAN modes (see the section about variousmodes below) support altering stack trace collection behavior:

• kasan.stacktrace=off or=on disables or enables alloc and free stacktraces collection (default:on).

• kasan.stack_ring_size=<numberofentries> specifies the number of entriesin the stack ring (default:32768).

Hardware Tag-Based KASAN mode is intended for use in production as a securitymitigation. Therefore, it supports additional boot parameters that allowdisabling KASAN altogether or controlling its features:

• kasan=off or=on controls whether KASAN is enabled (default:on).

• kasan.mode=sync,=async or=asymm controls whether KASANis configured in synchronous, asynchronous or asymmetric mode ofexecution (default:sync).Synchronous mode: a bad access is detected immediately when a tagcheck fault occurs.Asynchronous mode: a bad access detection is delayed. When a tag checkfault occurs, the information is stored in hardware (in the TFSR_EL1register for arm64). The kernel periodically checks the hardware andonly reports tag faults during these checks.Asymmetric mode: a bad access is detected synchronously on reads andasynchronously on writes.

• kasan.write_only=off orkasan.write_only=on controls whether KASANchecks the write (store) accesses only or all accesses (default:off).

• kasan.vmalloc=off or=on disables or enables tagging of vmallocallocations (default:on).

• kasan.page_alloc.sample=<samplinginterval> makes KASAN tag only everyNth page_alloc allocation with the order equal or greater thankasan.page_alloc.sample.order, where N is the value of thesampleparameter (default:1, or tag every such allocation).This parameter is intended to mitigate the performance overhead introducedby KASAN.Note that enabling this parameter makes Hardware Tag-Based KASAN skip checksof allocations chosen by sampling and thus miss bad accesses to theseallocations. Use the default value for accurate bug detection.

• kasan.page_alloc.sample.order=<minimumpageorder> specifies the minimumorder of allocations that are affected by sampling (default:3).Only applies whenkasan.page_alloc.sample is set to a value greaterthan1.This parameter is intended to allow sampling only large page_allocallocations, which is the biggest source of the performance overhead.

Error reports¶

A typical KASAN report looks like this:

==================================================================BUG: KASAN: slab-out-of-bounds in kmalloc_oob_right+0xa8/0xbc [kasan_test]Write of size 1 at addr ffff8801f44ec37b by task insmod/2760CPU: 1 PID: 2760 Comm: insmod Not tainted 4.19.0-rc3+ #698Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014Call Trace: dump_stack+0x94/0xd8 print_address_description+0x73/0x280 kasan_report+0x144/0x187 __asan_report_store1_noabort+0x17/0x20 kmalloc_oob_right+0xa8/0xbc [kasan_test] kmalloc_tests_init+0x16/0x700 [kasan_test] do_one_initcall+0xa5/0x3ae do_init_module+0x1b6/0x547 load_module+0x75df/0x8070 __do_sys_init_module+0x1c6/0x200 __x64_sys_init_module+0x6e/0xb0 do_syscall_64+0x9f/0x2c0 entry_SYSCALL_64_after_hwframe+0x44/0xa9RIP: 0033:0x7f96443109daRSP: 002b:00007ffcf0b51b08 EFLAGS: 00000202 ORIG_RAX: 00000000000000afRAX: ffffffffffffffda RBX: 000055dc3ee521a0 RCX: 00007f96443109daRDX: 00007f96445cff88 RSI: 0000000000057a50 RDI: 00007f9644992000RBP: 000055dc3ee510b0 R08: 0000000000000003 R09: 0000000000000000R10: 00007f964430cd0a R11: 0000000000000202 R12: 00007f96445cff88R13: 000055dc3ee51090 R14: 0000000000000000 R15: 0000000000000000Allocated by task 2760: save_stack+0x43/0xd0 kasan_kmalloc+0xa7/0xd0 kmem_cache_alloc_trace+0xe1/0x1b0 kmalloc_oob_right+0x56/0xbc [kasan_test] kmalloc_tests_init+0x16/0x700 [kasan_test] do_one_initcall+0xa5/0x3ae do_init_module+0x1b6/0x547 load_module+0x75df/0x8070 __do_sys_init_module+0x1c6/0x200 __x64_sys_init_module+0x6e/0xb0 do_syscall_64+0x9f/0x2c0 entry_SYSCALL_64_after_hwframe+0x44/0xa9Freed by task 815: save_stack+0x43/0xd0 __kasan_slab_free+0x135/0x190 kasan_slab_free+0xe/0x10 kfree+0x93/0x1a0 umh_complete+0x6a/0xa0 call_usermodehelper_exec_async+0x4c3/0x640 ret_from_fork+0x35/0x40The buggy address belongs to the object at ffff8801f44ec300 which belongs to the cache kmalloc-128 of size 128The 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:0x0flags: 0x200000000000100(slab)raw: 0200000000000100 ffffea0007d11dc0 0000001a0000001a ffff8801f7001640raw: 0000000000000000 0000000080150015 00000001ffffffff 0000000000000000page dumped because: kasan: bad access detectedMemory 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 accesscaused it. It is followed by a stack trace of the bad access, a stack trace ofwhere 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-freebug report). Next comes a description of the accessed slab object and theinformation 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, whichis either 8 or 16 aligned bytes depending on KASAN mode. Each number in thememory state section of the report shows the state of one of the memorygranules that surround the accessed address.

For Generic KASAN, the size of each memory granule is 8. The state of eachgranule 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 followingencoding for each shadow byte: 00 means that all 8 bytes of the correspondingmemory region are accessible; number N (1 <= N <= 7) means that the first Nbytes are accessible, and other (8 - N) bytes are not; any negative valueindicates that the entire 8-byte word is inaccessible. KASAN uses differentnegative values to distinguish between different kinds of inaccessible memorylike redzones or freed memory (see mm/kasan/kasan.h).

In the report above, the arrow points to the shadow byte03, which meansthat the accessed address is partially accessible.

For tag-based KASAN modes, this last report section shows the memory tags aroundthe accessed address (see theImplementation details section).

Note that KASAN bug titles (likeslab-out-of-bounds oruse-after-free)are best-effort: KASAN prints the most probable bug type based on the limitedinformation it has. The actual type of the bug might be different.

Generic KASAN also reports up to two auxiliary call stack traces. These stacktraces point to places in code that interacted with the object but that are notdirectly present in the bad access stack trace. Currently, this includescall_rcu() and workqueue queuing.

CONFIG_KASAN_EXTRA_INFO¶

Enabling CONFIG_KASAN_EXTRA_INFO allows KASAN to record and report moreinformation. The extra information currently supported is the CPU number andtimestamp at allocation and free. More information can help find the cause ofthe bug and correlate the error with other system events, at the cost of usingextra memory to record more information (more cost details in the help text ofCONFIG_KASAN_EXTRA_INFO).

Here is the report with CONFIG_KASAN_EXTRA_INFO enabled (only thedifferent parts are shown):

==================================================================...Allocated by task 134 on cpu 5 at 229.133855s:...Freed by task 136 on cpu 3 at 230.199335s:...==================================================================

Generic KASAN¶

Software KASAN modes use shadow memory to record whether each byte of memory issafe to access and use compile-time instrumentation to insert shadow memorychecks before each memory access.

Generic KASAN dedicates 1/8th of kernel memory to its shadow memory (16TBto cover 128TB on x86_64) and uses direct mapping with a scale and offset totranslate a memory address to its corresponding shadow address.

Here is the function which translates an address to its corresponding shadowaddress:

static inline void *kasan_mem_to_shadow(const void *addr){    return (void *)((unsigned long)addr >> KASAN_SHADOW_SCALE_SHIFT)            + KASAN_SHADOW_OFFSET;}

whereKASAN_SHADOW_SCALE_SHIFT=3.

Compile-time instrumentation is used to insert memory access checks. Compilerinserts function calls (__asan_load*(addr),__asan_store*(addr)) beforeeach memory access of size 1, 2, 4, 8, or 16. These functions check whethermemory accesses are valid or not by checking corresponding shadow memory.

With inline instrumentation, instead of making function calls, the compilerdirectly inserts the code to check shadow memory. This option significantlyenlarges the kernel, but it gives an x1.1-x2 performance boost over theoutline-instrumented kernel.

Generic KASAN is the only mode that delays the reuse of freed objects viaquarantine (see mm/kasan/quarantine.c for implementation).

Software Tag-Based KASAN¶

Software Tag-Based KASAN uses a software memory tagging approach to checkingaccess validity. It is currently only implemented for the arm64 architecture.

Software Tag-Based KASAN uses the Top Byte Ignore (TBI) feature of arm64 CPUsto store a pointer tag in the top byte of kernel pointers. It uses shadow memoryto store memory tags associated with each 16-byte memory cell (therefore, itdedicates 1/16th of the kernel memory for shadow memory).

On each memory allocation, Software Tag-Based KASAN generates a random tag, tagsthe allocated memory with this tag, and embeds the same tag into the returnedpointer.

Software Tag-Based KASAN uses compile-time instrumentation to insert checksbefore each memory access. These checks make sure that the tag of the memorythat is being accessed is equal to the tag of the pointer that is used to accessthis memory. In case of a tag mismatch, Software Tag-Based KASAN prints a bugreport.

Software Tag-Based KASAN also has two instrumentation modes (outline, whichemits callbacks to check memory accesses; and inline, which performs the shadowmemory checks inline). With outline instrumentation mode, a bug report isprinted from the function that performs the access check. With inlineinstrumentation, abrk instruction is emitted by the compiler, and adedicatedbrk handler is used to print bug reports.

Software Tag-Based KASAN uses 0xFF as a match-all pointer tag (accesses throughpointers with the 0xFF pointer tag are not checked). The value 0xFE is currentlyreserved to tag freed memory regions.

Hardware Tag-Based KASAN¶

Hardware Tag-Based KASAN is similar to the software mode in concept but useshardware memory tagging support instead of compiler instrumentation andshadow memory.

Hardware Tag-Based KASAN is currently only implemented for arm64 architectureand based on both arm64 Memory Tagging Extension (MTE) introduced in ARMv8.5Instruction 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 memoryaccess, hardware makes sure that the tag of the memory that is being accessed isequal to the tag of the pointer that is used to access this memory. In case of atag mismatch, a fault is generated, and a report is printed.

Hardware Tag-Based KASAN uses 0xFF as a match-all pointer tag (accesses throughpointers with the 0xFF pointer tag are not checked). The value 0xFE is currentlyreserved to tag freed memory regions.

If the hardware does not support MTE (pre ARMv8.5), Hardware Tag-Based KASANwill 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 beingenabled. Even whenkasan.mode=off is provided or when the hardware does notsupport MTE (but supports TBI).

Hardware Tag-Based KASAN only reports the first found bug. After that, MTE tagchecking gets disabled.

Default behaviour¶

By default, architectures only map real memory over the shadow regionfor the linear mapping (and potentially other small areas). For allother areas - such as vmalloc and vmemmap space - a single read-onlypage is mapped over the shadow area. This read-only shadow pagedeclares all memory accesses as permitted.

This presents a problem for modules: they do not live in the linearmapping but in a dedicated module space. By hooking into the moduleallocator, 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 withVMAP_STACK: if the stacklives in vmalloc space, it will be shadowed by the read-only page, andthe kernel will fault when trying to set up the shadow data for stackvariables.

CONFIG_KASAN_VMALLOC¶

WithCONFIG_KASAN_VMALLOC, KASAN can cover vmalloc space at thecost of greater memory usage. Currently, this is supported on x86,arm64, riscv, s390, and powerpc.

This works by hooking into vmalloc and vmap and dynamicallyallocating real shadow memory to back the mappings.

Most mappings in vmalloc space are small, requiring less than a fullpage of shadow space. Allocating a full shadow page per mapping wouldtherefore be wasteful. Furthermore, to ensure that different mappingsuse different shadow pages, mappings would have to be aligned toKASAN_GRANULE_SIZE*PAGE_SIZE.

Instead, KASAN shares backing space across multiple mappings. It allocatesa backing page when a mapping in vmalloc space uses a particular pageof the shadow region. This page can be shared by other vmallocmappings later on.

KASAN hooks into the vmap infrastructure to lazily clean up unused shadowmemory.

To avoid the difficulties around swapping mappings around, KASAN expectsthat the part of the shadow region that covers the vmalloc space willnot be covered by the early shadow page but will be left unmapped.This will require changes in arch-specific code.

This allowsVMAP_STACK support on x86 and can simplify support ofarchitectures that do not have a fixed module region.

Ignoring accesses¶

Software KASAN modes use compiler instrumentation to insert validity checks.Such instrumentation might be incompatible with some parts of the kernel, andtherefore 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 ordirectory, add aKASAN_SANITIZE annotation to the respective kernelMakefile:

• 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 thegenericnoinstr one.

Note that disabling compiler instrumentation (either on a per-file or aper-function basis) makes KASAN ignore the accesses that happen directly inthat code for software KASAN modes. It does not help when the accesses happenindirectly (through calls to instrumented functions) or with HardwareTag-Based KASAN, which does not use compiler instrumentation.

For software KASAN modes, to disable KASAN reports in a part of the kernel codefor the current task, annotate this part of the code with akasan_disable_current()/kasan_enable_current() section. This alsodisables the reports for indirect accesses that happen through function calls.

For tag-based KASAN modes, to disable access checking, usekasan_reset_tag() orpage_kasan_tag_reset(). Note that temporarilydisabling access checking viapage_kasan_tag_reset() requires saving andrestoring the per-page KASAN tag viapage_kasan_tag/page_kasan_tag_set.

Tests¶

There are KASAN tests that allow verifying that KASAN works and can detectcertain types of memory corruptions.

All KASAN tests are integrated with the KUnit Test Framework and can be enabledviaCONFIG_KASAN_KUNIT_TEST. The tests can be run and partially verifiedautomatically in a few different ways; see the instructions below.

Each 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 failedkmalloc:

# kmalloc_large_oob_right: ASSERTION FAILED at mm/kasan/kasan_test.c:245Expected ptr is not null, but isnot ok 5 - kmalloc_large_oob_right

When a test fails due to a missing KASAN report:

# kmalloc_double_kzfree: EXPECTATION FAILED at mm/kasan/kasan_test.c:709KASAN failure expected in "kfree_sensitive(ptr)", but none occurrednot ok 28 - 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 the KASAN tests.

1. Loadable module

   WithCONFIG_KUNIT enabled, the tests can be built as a loadable moduleand run by loadingkasan_test.ko withinsmod ormodprobe.

2. Built-In

   WithCONFIG_KUNIT built-in, the tests can be built-in as well.In this case, the tests will run at boot as a late-init call.

3. Using kunit_tool

   WithCONFIG_KUNIT andCONFIG_KASAN_KUNIT_TEST built-in, it is alsopossible to usekunit_tool to see the results of KUnit tests in a morereadable way. This will not print the KASAN reports of the tests that passed.SeeKUnit documentationfor more up-to-date information onkunit_tool.