On Tue, Nov 10, 2020 at 11:20PM +0100, Andrey Konovalov wrote: > 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: Andrey Konovalov <andreyknvl@xxxxxxxxxx> > --- > Documentation/dev-tools/kasan.rst | 180 +++++++++++++++++++----------- > 1 file changed, 113 insertions(+), 67 deletions(-) > > diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst > index 422f8ee1bb17..f2da2b09e5c7 100644 > --- a/Documentation/dev-tools/kasan.rst > +++ b/Documentation/dev-tools/kasan.rst > @@ -6,6 +6,7 @@ Overview > > KernelAddressSANitizer (KASAN) is a dynamic memory error detector designed to s/memory error/memory safety error/ to be precise and consistent with various other docs and literature we have, if you deem it appropriate to change in this patch. > 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 > ~~~~~~~~~~~~~ > > @@ -140,16 +131,20 @@ 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. > @@ -157,6 +152,55 @@ 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). I think ReST automatically creates a link if you write it as ... (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 ``kasam.mode`` boot parameter allows to choose one of three main modes: s/kasam/kasan/ > +- ``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``) This is indented with tabs instead of spaces. > + > +If ``kasan.mode parameter`` is not provided, it defaults to ``full`` when s/``kasan.mode parameter``/``kasan.mode`` parameter/ ? > +``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 > @@ -302,15 +349,15 @@ 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 in to the vmap infrastructure to lazily clean up unused shadow s/in to/into/ > 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. > @@ -321,24 +368,31 @@ 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 s/Kunit/KUnit/ > +``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. s/KUNIT/KUnit/ like elsewhere. > +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 > @@ -346,46 +400,38 @@ 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. s/Kunit/KUnit/ > + > +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. > -- > 2.29.2.222.g5d2a92d10f8-goog >
