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Commit 792524ff authored by Andrey Konovalov's avatar Andrey Konovalov Committed by Vincenzo Frascino
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kasan: update documentation 



This change updates KASAN documentation to reflect the addition of boot
parameters and also reworks and clarifies some of the existing sections,
in particular: defines what a memory granule is, mentions quarantine,
makes Kunit section more readable.
Signed-off-by: default avatarAndrey Konovalov <andreyknvl@google.com>
Link: https://linux-review.googlesource.com/id/Ib1f83e91be273264b25f42b04448ac96b858849f
parent b00a5660
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Documentation/dev-tools/kasan.rst
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......@@ -4,8 +4,9 @@ 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 three modes:
KernelAddressSANitizer (KASAN) is a dynamic memory safety error detector
designed to find out-of-bound and use-after-free bugs. KASAN has three modes:
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).
......@@ -39,23 +40,13 @@ 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 software KASAN modes work with both SLUB and SLAB memory allocators,
hardware tag-based KASAN currently only support SLUB.
For better bug detection and nicer reporting, enable CONFIG_STACKTRACE.
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
~~~~~~~~~~~~~
......@@ -140,22 +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.
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).
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: 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
......@@ -164,10 +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
......@@ -194,7 +238,10 @@ function calls GCC directly inserts the code to check the shadow memory.
This option significantly enlarges kernel but it gives x1.1-x2 performance
boost over outline instrumented kernel.
Generic KASAN prints up to 2 call_rcu() call stacks in reports, the last one
Generic KASAN is the only mode that delays the reuse of freed object via
quarantine (see mm/kasan/quarantine.c for implementation).
Generic KASAN prints up to two call_rcu() call stacks in reports, the last one
and the second to last.
Software tag-based KASAN
......@@ -304,15 +351,15 @@ 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, 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.
......@@ -323,24 +370,31 @@ 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:
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.
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:
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.
pass::
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
......@@ -348,46 +402,38 @@ 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 architecure 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 interpretted to pass only if the error was the one expected
by the test.
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