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Merge branch 'akpm' (patches from Andrew)

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Merge KASAN updates from Andrew Morton.

This adds a new hardware tag-based mode to KASAN.  The new mode is
similar to the existing software tag-based KASAN, but relies on arm64
Memory Tagging Extension (MTE) to perform memory and pointer tagging
(instead of shadow memory and compiler instrumentation).

By Andrey Konovalov and Vincenzo Frascino.

* emailed patches from Andrew Morton <akpm@linux-foundation.org>: (60 commits)
  kasan: update documentation
  kasan, mm: allow cache merging with no metadata
  kasan: sanitize objects when metadata doesn't fit
  kasan: clarify comment in __kasan_kfree_large
  kasan: simplify assign_tag and set_tag calls
  kasan: don't round_up too much
  kasan, mm: rename kasan_poison_kfree
  kasan, mm: check kasan_enabled in annotations
  kasan: add and integrate kasan boot parameters
  kasan: inline (un)poison_range and check_invalid_free
  kasan: open-code kasan_unpoison_slab
  kasan: inline random_tag for HW_TAGS
  kasan: inline kasan_reset_tag for tag-based modes
  kasan: remove __kasan_unpoison_stack
  kasan: allow VMAP_STACK for HW_TAGS mode
  kasan, arm64: unpoison stack only with CONFIG_KASAN_STACK
  kasan: introduce set_alloc_info
  kasan: rename get_alloc/free_info
  kasan: simplify quarantine_put call site
  kselftest/arm64: check GCR_EL1 after context switch
  ...
This commit is contained in:
Linus Torvalds
2020-12-22 13:38:17 -08:00
parent c45647f9f5 625d867347
commit 1375b9803e
72 changed files with 2786 additions and 1471 deletions
Download Patch File Download Diff File Expand all files Collapse all files

260
Documentation/dev-tools/kasan.rst
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@@ -4,13 +4,16 @@ The Kernel Address Sanitizer (KASAN)
Overview
--------
KernelAddressSANitizer (KASAN) is a dynamic memory error detector designed to
find out-of-bound and use-after-free bugs. KASAN has two modes: generic KASAN
(similar to userspace ASan) and software tag-based KASAN (similar to userspace
HWASan).
KernelAddressSANitizer (KASAN) is a dynamic memory safety error detector
designed to find out-of-bound and use-after-free bugs. KASAN has three modes:
KASAN uses compile-time instrumentation to insert validity checks before every
memory access, and therefore requires a compiler version that supports that.
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).
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 is supported in both GCC and Clang. With GCC it requires version
8.3.0 or later. Any supported Clang version is compatible, but detection of
@@ -19,7 +22,7 @@ out-of-bounds accesses for global variables is only supported since Clang 11.
Tag-based KASAN is only supported in Clang.
Currently generic KASAN is supported for the x86_64, arm, arm64, xtensa, s390
and riscv architectures, and tag-based KASAN is supported only for arm64.
and riscv architectures, and tag-based KASAN modes are supported only for arm64.
Usage
-----
@@ -28,30 +31,22 @@ To enable KASAN configure kernel with::
CONFIG_KASAN = y
and choose between CONFIG_KASAN_GENERIC (to enable generic KASAN) and
CONFIG_KASAN_SW_TAGS (to enable software 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).
You also need to choose between CONFIG_KASAN_OUTLINE and CONFIG_KASAN_INLINE.
Outline and inline are compiler instrumentation types. The former produces
smaller binary while the latter is 1.1 - 2 times faster.
For software modes, you also need to choose between CONFIG_KASAN_OUTLINE and
CONFIG_KASAN_INLINE. Outline and inline are compiler instrumentation types.
The former produces smaller binary while the latter is 1.1 - 2 times faster.
Both KASAN modes work with both SLUB and SLAB memory allocators.
For better bug detection and nicer reporting, enable CONFIG_STACKTRACE.
Both software KASAN modes work with both SLUB and SLAB memory allocators,
while the hardware tag-based KASAN currently only support SLUB.
For better error reports that include stack traces, enable CONFIG_STACKTRACE.
To augment reports with last allocation and freeing stack of the physical page,
it is recommended to enable also CONFIG_PAGE_OWNER and boot with page_owner=on.
To disable instrumentation for specific files or directories, add a line
similar to the following 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
Error reports
~~~~~~~~~~~~~
@@ -136,22 +131,75 @@ freed (in case of a use-after-free bug report). Next comes a description of
the accessed slab object and information about the accessed memory page.
In the last section the report shows memory state around the accessed address.
Reading this part requires some understanding of how KASAN works.
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.
The state of each 8 aligned bytes of memory is encoded in one shadow byte.
Those 8 bytes can be accessible, partially accessible, freed or be a redzone.
We use the following encoding for each shadow byte: 0 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.
We use different negative values to distinguish between different kinds of
inaccessible memory like redzones or freed memory (see mm/kasan/kasan.h).
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: 0 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 arrows point to the shadow byte 03, which means that
the accessed address is partially accessible.
For tag-based KASAN this last report section shows the memory tags around the
accessed address (see Implementation details section).
accessed address (see `Implementation details`_ section).
Boot parameters
~~~~~~~~~~~~~~~
Hardware tag-based KASAN mode (see the section about different mode below) is
intended for use in production as a security mitigation. Therefore it supports
boot parameters that allow to disable KASAN competely or otherwise control
particular KASAN features.
The things that can be controlled are:
1. Whether KASAN is enabled at all.
2. Whether KASAN collects and saves alloc/free stacks.
3. Whether KASAN panics on a detected bug or not.
The ``kasan.mode`` boot parameter allows to choose one of three main modes:
- ``kasan.mode=off`` - KASAN is disabled, no tag checks are performed
- ``kasan.mode=prod`` - only essential production features are enabled
- ``kasan.mode=full`` - all KASAN features are enabled
The chosen mode provides default control values for the features mentioned
above. However it's also possible to override the default values by providing:
- ``kasan.stacktrace=off`` or ``=on`` - enable alloc/free stack collection
(default: ``on`` for ``mode=full``,
otherwise ``off``)
- ``kasan.fault=report`` or ``=panic`` - only print KASAN report or also panic
(default: ``report``)
If ``kasan.mode`` parameter is not provided, it defaults to ``full`` when
``CONFIG_DEBUG_KERNEL`` is enabled, and to ``prod`` otherwise.
For developers
~~~~~~~~~~~~~~
Software KASAN modes use compiler instrumentation to insert validity checks.
Such instrumentation might be incompatible with some part of the kernel, and
therefore needs to be disabled. To disable instrumentation for specific files
or directories, add a line similar to the following 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
Implementation details
@@ -160,10 +208,10 @@ Implementation details
Generic KASAN
~~~~~~~~~~~~~
From a high level, our approach to memory error detection is similar to that
of kmemcheck: use shadow memory to record whether each byte of memory is safe
to access, and use compile-time instrumentation to insert checks of shadow
memory on each memory access.
From a high level perspective, KASAN's approach to memory error detection is
similar to that of kmemcheck: use shadow memory to record whether each byte of
memory is safe to access, and use compile-time instrumentation to insert checks
of shadow memory on each memory access.
Generic KASAN dedicates 1/8th of kernel memory to its shadow memory (e.g. 16TB
to cover 128TB on x86_64) and uses direct mapping with a scale and offset to
@@ -194,20 +242,30 @@ Generic KASAN also reports the last 2 call stacks to creation of work that
potentially has access to an object. Call stacks for the following are shown:
call_rcu() and workqueue queuing.
Generic KASAN is the only mode that delays the reuse of freed object via
quarantine (see mm/kasan/quarantine.c for implementation).
Software tag-based KASAN
~~~~~~~~~~~~~~~~~~~~~~~~
Tag-based KASAN uses the Top Byte Ignore (TBI) feature of modern arm64 CPUs to
store a pointer tag in the top byte of kernel pointers. Like generic KASAN it
uses shadow memory to store memory tags associated with each 16-byte memory
Software tag-based KASAN requires software memory tagging support in the form
of HWASan-like compiler instrumentation (see HWASan documentation for details).
Software tag-based KASAN is currently only implemented for 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. Like generic KASAN
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 tag-based KASAN generates a random tag, tags the
allocated memory with this tag, and embeds this tag into the returned pointer.
On each memory allocation software tag-based KASAN generates a random tag, tags
the allocated memory with this tag, and embeds this 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 tag of the memory that
is being accessed is equal to tag of the pointer that is used to access this
memory. In case of a tag mismatch tag-based KASAN prints a bug report.
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, that
emits callbacks to check memory accesses; and inline, that performs the shadow
@@ -216,9 +274,36 @@ simply 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.
A potential expansion of this mode is a hardware tag-based mode, which would
use hardware memory tagging support instead of compiler instrumentation and
manual shadow memory manipulation.
Software tag-based KASAN uses 0xFF as a match-all pointer tag (accesses through
pointers with 0xFF pointer tag aren't checked). The value 0xFE is currently
reserved to tag freed memory regions.
Software tag-based KASAN currently only supports tagging of
kmem_cache_alloc/kmalloc and page_alloc memory.
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 tag of the memory that is being accessed is
equal to 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 0xFF pointer tag aren't checked). The value 0xFE is currently
reserved to tag freed memory regions.
Hardware tag-based KASAN currently only supports tagging of
kmem_cache_alloc/kmalloc and page_alloc memory.
What memory accesses are sanitised by KASAN?
--------------------------------------------
@@ -265,17 +350,17 @@ 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_SHADOW_SCALE_SIZE * PAGE_SIZE``.
``KASAN_GRANULE_SIZE * PAGE_SIZE``.
Instead, we share backing space across multiple mappings. We allocate
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.
We hook in to the vmap infrastructure to lazily clean up unused shadow
KASAN hooks into the vmap infrastructure to lazily clean up unused shadow
memory.
To avoid the difficulties around swapping mappings around, we expect
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.
@@ -286,24 +371,31 @@ architectures that do not have a fixed module region.
CONFIG_KASAN_KUNIT_TEST & CONFIG_TEST_KASAN_MODULE
--------------------------------------------------
``CONFIG_KASAN_KUNIT_TEST`` utilizes the KUnit Test Framework for testing.
This means each test focuses on a small unit of functionality and
there are a few ways these tests can be run.
KASAN tests consist on two parts:
Each test will print the KASAN report if an error is detected and then
print the number of the test and the status of the test:
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.
pass::
2. Tests that are currently incompatible with KUnit. Enabled with
``CONFIG_TEST_KASAN_MODULE`` 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 a KASAN report if an error is detected.
Then the test prints its number and status.
When a test passes::
ok 28 - kmalloc_double_kzfree
or, if kmalloc failed::
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
or, if a KASAN report was expected, but not found::
When a test fails due to a missing KASAN report::
# kmalloc_double_kzfree: EXPECTATION FAILED at lib/test_kasan.c:629
Expected kasan_data->report_expected == kasan_data->report_found, but
@@ -311,46 +403,38 @@ or, if a KASAN report was expected, but not found::
kasan_data->report_found == 0
not ok 28 - kmalloc_double_kzfree
All test statuses are tracked as they run and an overall status will
be printed at the end::
At the end the cumulative status of all KASAN tests is printed. On success::
ok 1 - kasan
or::
Or, if one of the tests failed::
not ok 1 - kasan
(1) Loadable Module
~~~~~~~~~~~~~~~~~~~~
There are a few ways to run KUnit-compatible KASAN tests.
1. Loadable module
~~~~~~~~~~~~~~~~~~
With ``CONFIG_KUNIT`` enabled, ``CONFIG_KASAN_KUNIT_TEST`` can be built as
a loadable module and run on any architecture that supports KASAN
using something like insmod or modprobe. The module is called ``test_kasan``.
a loadable module and run on any architecture that supports KASAN by loading
the module with insmod or modprobe. The module is called ``test_kasan``.
(2) Built-In
~~~~~~~~~~~~~
2. Built-In
~~~~~~~~~~~
With ``CONFIG_KUNIT`` built-in, ``CONFIG_KASAN_KUNIT_TEST`` can be built-in
on any architecture that supports KASAN. These and any other KUnit
tests enabled will run and print the results at boot as a late-init
call.
on any architecure that supports KASAN. These and any other KUnit tests enabled
will run and print the results at boot as a late-init call.
(3) Using kunit_tool
~~~~~~~~~~~~~~~~~~~~~
3. Using kunit_tool
~~~~~~~~~~~~~~~~~~~
With ``CONFIG_KUNIT`` and ``CONFIG_KASAN_KUNIT_TEST`` built-in, we can also
use kunit_tool to see the results of these along with other KUnit
tests in a more readable way. This will not print the KASAN reports
of tests that passed. Use `KUnit documentation <https://www.kernel.org/doc/html/latest/dev-tools/kunit/index.html>`_ for more up-to-date
information on kunit_tool.
With ``CONFIG_KUNIT`` and ``CONFIG_KASAN_KUNIT_TEST`` built-in, it's also
possible use ``kunit_tool`` to see the results of these and other KUnit tests
in a more readable way. This will not print the KASAN reports of the tests that
passed. Use `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
``CONFIG_TEST_KASAN_MODULE`` is a set of KASAN tests that could not be
converted to KUnit. These tests can be run only as a module with
``CONFIG_TEST_KASAN_MODULE`` built as a loadable module and
``CONFIG_KASAN`` built-in. The type of error expected and the
function being run is printed before the expression expected to give
an error. Then the error is printed, if found, and that test
should be interpreted to pass only if the error was the one expected
by the test.