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 The Kernel Address Sanitizer (KASAN)
 ====================================
 
 Overview
 --------
 
 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:
 
 1. Generic KASAN
 2. Software Tag-Based KASAN
 3. 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.
 
 Support
 -------
 
 Architectures
 ~~~~~~~~~~~~~
 
 Generic KASAN is supported on x86_64, arm, arm64, powerpc, riscv, s390, and
 xtensa, and the tag-based KASAN modes are supported only on arm64.
 
 Compilers
 ~~~~~~~~~
 
 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+.
 
 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 vmalloc
 memory.
 
 For slab, both software KASAN modes support SLUB and SLAB allocators, while
 Hardware Tag-Based KASAN only supports SLUB.
 
 Usage
 -----
 
 To enable KASAN, configure the kernel with::
 
           CONFIG_KASAN=y
 
 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_OUTLINE`` and
 ``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,
 enable ``CONFIG_STACKTRACE``. To include alloc and free stack traces of affected
 physical pages, enable ``CONFIG_PAGE_OWNER`` and boot with ``page_owner=on``.
 
 Boot parameters
 ~~~~~~~~~~~~~~~
 
 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.
 With ``kasan_multi_shot``, KASAN prints a report on every invalid access. This
 effectively disables ``panic_on_warn`` for KASAN reports.
 
 Alternatively, independent of ``panic_on_warn``, the ``kasan.fault=`` boot
 parameter can be used to control panic and reporting behaviour:
 
 - ``kasan.fault=report`` or ``=panic`` controls whether to only print a KASAN
   report or also panic the kernel (default: ``report``). The panic happens even
   if ``kasan_multi_shot`` is enabled.
 
 Hardware Tag-Based KASAN mode (see the section about various modes below) is
 intended for use in production as a security mitigation. Therefore, it supports
 additional boot parameters that allow disabling KASAN or controlling features:
 
 - ``kasan=off`` or ``=on`` controls whether KASAN is enabled (default: ``on``).
 
 - ``kasan.mode=sync``, ``=async`` or ``=asymm`` controls 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.
 
 - ``kasan.vmalloc=off`` or ``=on`` disables or enables tagging of vmalloc
   allocations (default: ``on``).
 
 - ``kasan.stacktrace=off`` or ``=on`` disables or enables alloc and free stack
   traces collection (default: ``on``).
 
 Error reports
 ~~~~~~~~~~~~~
 
 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
     Call 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 [test_kasan]
      kmalloc_tests_init+0x16/0x700 [test_kasan]
      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+0x44/0xa9
     RIP: 0033:0x7f96443109da
     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:
      save_stack+0x43/0xd0
      kasan_kmalloc+0xa7/0xd0
      kmem_cache_alloc_trace+0xe1/0x1b0
      kmalloc_oob_right+0x56/0xbc [test_kasan]
      kmalloc_tests_init+0x16/0x700 [test_kasan]
      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+0x44/0xa9
 
     Freed 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+0x35/0x40
 
     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
     flags: 0x200000000000100(slab)
     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 ``slab-out-of-bounds`` or ``use-after-free``)
 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.
 
 Implementation details
 ----------------------
 
 Generic KASAN
 ~~~~~~~~~~~~~
 
 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_OFFSET;
     }
 
 where ``KASAN_SHADOW_SCALE_SHIFT = 3``.
 
 Compile-time instrumentation is used to insert memory access checks. Compiler
 inserts function calls (``__asan_load*(addr)``, ``__asan_store*(addr)``) before
 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
 instrumentation, a ``brk`` instruction is emitted by the compiler, and a
 dedicated ``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.
 
 Shadow memory
 -------------
 
 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.
 
 Default behaviour
 ~~~~~~~~~~~~~~~~~
 
 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
 variables.
 
 CONFIG_KASAN_VMALLOC
 ~~~~~~~~~~~~~~~~~~~~
 
 With ``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.
 
 This allows ``VMAP_STACK`` support on x86 and can simplify support of
 architectures that do not have a fixed module region.
 
 For developers
 --------------
 
 Ignoring accesses
 ~~~~~~~~~~~~~~~~~
 
 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
 Makefile:
 
 - 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
 generic ``noinstr`` one.
 
 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_disable_current()``/``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
 ``kasan_reset_tag()`` or ``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 ``page_kasan_tag``/``page_kasan_tag_set``.
 
 Tests
 ~~~~~
 
 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``::
 
         # 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.
 
 1. Loadable module
 
    With ``CONFIG_KUNIT`` enabled, KASAN-KUnit tests can be built as a loadable
    module and run by loading ``test_kasan.ko`` with ``insmod`` or ``modprobe``.
 
 2. Built-In
 
    With ``CONFIG_KUNIT`` built-in, KASAN-KUnit 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
 
    With ``CONFIG_KUNIT`` and ``CONFIG_KASAN_KUNIT_TEST`` built-in, it is also
    possible to use ``kunit_tool`` to 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 <https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html>`_
    for more up-to-date information on ``kunit_tool``.
 
 .. _KUnit: https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html

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