kasan: update documentation

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).

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

~~~~~~~~~~~~~

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

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

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

~~~~~~~~~~~~~~~~~~~~~~~~

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.

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

        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.