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kasan: update documentation

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Do assorted clean-ups and improvements to KASAN documentation, including:

- Describe each mode in a dedicated paragraph.
- Split out a Support section that describes in details which compilers,
  architectures and memory types each mode requires/supports.
- Capitalize the first letter in the names of each KASAN mode.

[andreyknvl@google.com: rewording, per Marco]
  Link: https://lkml.kernel.org/r/896b2d914d6b50d677fd7b38f76967cc705c01ba.1652203271.git.andreyknvl@google.com
Link: https://lkml.kernel.org/r/5bd58ebebf066593ce0e1d265d60278b5f5a1874.1652123204.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Andrey Konovalov 2022-05-12 20:23:09 -07:00 committed by Andrew Morton
parent 06bc4cf6cd
commit c2ec0c8f68
1 changed files with 90 additions and 60 deletions
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150
Documentation/dev-tools/kasan.rst
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@ -4,39 +4,76 @@ The Kernel Address Sanitizer (KASAN)
Overview
--------
KernelAddressSANitizer (KASAN) is a dynamic memory safety error detector
designed to find out-of-bound and use-after-free bugs. KASAN has three modes:
Kernel Address Sanitizer (KASAN) is a dynamic memory safety error detector
designed to find out-of-bounds and use-after-free bugs.
1. generic KASAN (similar to userspace ASan),
2. software tag-based KASAN (similar to userspace HWASan),
3. hardware tag-based KASAN (based on hardware memory tagging).
KASAN has three modes:
Generic KASAN is mainly used for debugging due to a large memory overhead.
Software tag-based KASAN can be used for dogfood testing as it has a lower
memory overhead that allows using it with real workloads. Hardware tag-based
KASAN comes with low memory and performance overheads and, therefore, can be
used in production. Either as an in-field memory bug detector or as a security
mitigation.
1. Generic KASAN
2. Software Tag-Based KASAN
3. Hardware Tag-Based KASAN
Software KASAN modes (#1 and #2) use compile-time instrumentation to insert
validity checks before every memory access and, therefore, require a compiler
version that supports that.
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.
Generic KASAN is supported in GCC and Clang. With GCC, it requires version
8.3.0 or later. Any supported Clang version is compatible, but detection of
out-of-bounds accesses for global variables is only supported since Clang 11.
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.
Software tag-based KASAN mode is only supported in Clang.
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.
The hardware KASAN mode (#3) relies on hardware to perform the checks but
still requires a compiler version that supports memory tagging instructions.
This mode is supported in GCC 10+ and Clang 12+.
For details about the memory and performance impact of each KASAN mode, see the
descriptions of the corresponding Kconfig options.
Both software KASAN modes work with SLUB and SLAB memory allocators,
while the hardware tag-based KASAN currently only supports SLUB.
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.
Currently, generic KASAN is supported for the x86_64, arm, arm64, xtensa, s390,
and riscv architectures, and tag-based KASAN modes are supported only for arm64.
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
-----
@ -45,13 +82,13 @@ 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).
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 software modes, also choose between ``CONFIG_KASAN_OUTLINE`` and
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 1.1-2 times faster.
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
@ -146,7 +183,7 @@ 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
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
@ -181,14 +218,14 @@ 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
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
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:
@ -250,49 +287,46 @@ 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
~~~~~~~~~~~~~~~~~~~~~~~~
Software tag-based KASAN uses a software memory tagging approach to checking
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
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
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
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
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
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
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.
Software tag-based KASAN currently only supports tagging of slab, page_alloc,
and vmalloc memory.
Hardware tag-based KASAN
Hardware Tag-Based KASAN
~~~~~~~~~~~~~~~~~~~~~~~~
Hardware tag-based KASAN is similar to the software mode in concept but uses
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
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).
@ -302,21 +336,18 @@ 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
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.
Hardware tag-based KASAN currently only supports tagging of slab, page_alloc,
and VM_ALLOC-based vmalloc memory.
If the hardware does not support MTE (pre ARMv8.5), hardware tag-based KASAN
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
Hardware Tag-Based KASAN only reports the first found bug. After that, MTE tag
checking gets disabled.
Shadow memory
@ -414,19 +445,18 @@ 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 the hardware
tag-based mode that does not use compiler instrumentation.
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 (include the hardware one), 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``.
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
~~~~~