From 01573e231f18eb2d99162747186f59511f56b64d Mon Sep 17 00:00:00 2001 From: hc <hc@nodka.com> Date: Fri, 08 Dec 2023 10:40:48 +0000 Subject: [PATCH] 移去rt --- kernel/Documentation/dev-tools/kasan.rst | 336 ++++++++++++++++++++++++++++++++++++++++++++++--------- 1 files changed, 281 insertions(+), 55 deletions(-) diff --git a/kernel/Documentation/dev-tools/kasan.rst b/kernel/Documentation/dev-tools/kasan.rst index b72d07d..075ef8e 100644 --- a/kernel/Documentation/dev-tools/kasan.rst +++ b/kernel/Documentation/dev-tools/kasan.rst @@ -4,25 +4,25 @@ 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 -4.9.2 or later for basic support and version 5.0 or later for detection of -out-of-bounds accesses for stack and global variables and for inline -instrumentation mode (see the Usage section). With Clang it requires version -7.0.0 or later and it doesn't support detection of out-of-bounds accesses for -global variables yet. +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. -Tag-based KASAN is only supported in Clang and requires version 7.0.0 or later. +Tag-based KASAN is only supported in Clang. -Currently generic KASAN is supported for the x86_64, arm64, xtensa and s390 -architectures, and tag-based KASAN is supported only for arm64. +Currently generic KASAN is supported for the x86_64, arm64, xtensa, s390 and +and riscv architectures, and tag-based KASAN modes are supported only for arm64. Usage ----- @@ -31,26 +31,21 @@ 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. -To disable instrumentation for specific files or directories, add a line -similar to the following to the respective kernel Makefile: +For better error reports that include stack traces, enable CONFIG_STACKTRACE. -- For a single file (e.g. main.o):: - - KASAN_SANITIZE_main.o := n - -- For all files in one directory:: - - KASAN_SANITIZE := n +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. Error reports ~~~~~~~~~~~~~ @@ -136,22 +131,68 @@ 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. +the accessed address is partially accessible. For tag-based KASAN modes this +last report section shows the memory tags around the accessed address +(see the `Implementation details`_ section). -For tag-based KASAN this last report section shows the memory tags around the -accessed address (see Implementation details section). +Boot parameters +~~~~~~~~~~~~~~~ + +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 +boot parameters that allow to disable KASAN competely or otherwise control +particular KASAN features. + +- ``kasan=off`` or ``=on`` controls whether KASAN is enabled (default: ``on``). + +- ``kasan.mode=sync`` or ``=async`` controls whether KASAN is configured in + synchronous or asynchronous mode of execution (default: ``sync``). + Synchronous mode: a bad access is detected immediately when a tag + check fault occurs. + Asynchronous mode: a bad access detection is delayed. When a tag check + fault occurs, the information is stored in hardware (in the TFSR_EL1 + register for arm64). The kernel periodically checks the hardware and + only reports tag faults during these checks. + +- ``kasan.stacktrace=off`` or ``=on`` disables or enables alloc and free stack + traces collection (default: ``on``). + +- ``kasan.fault=report`` or ``=panic`` controls whether to only print a KASAN + report or also panic the kernel (default: ``report``). Note, that tag + checking gets disabled after the first reported bug. + +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 +201,10 @@ 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 @@ -190,20 +231,34 @@ This option significantly enlarges kernel but it gives x1.1-x2 performance boost over outline instrumented kernel. +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 @@ -212,6 +267,177 @@ 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. + +If the hardware doesn't support MTE (pre ARMv8.5), hardware tag-based KASAN +won't be enabled. In this case all 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 doesn't +support MTE (but supports TBI). + +Hardware tag-based KASAN only reports the first found bug. After that MTE tag +checking gets disabled. + +What memory accesses are sanitised by KASAN? +-------------------------------------------- + +The kernel maps memory in a number of different parts of the address +space. This poses something of a problem for KASAN, which requires +that all addresses accessed by instrumented code have a valid shadow +region. + +The range of kernel virtual addresses is large: there is not enough +real memory to support a real shadow region for every address that +could be accessed by the kernel. + +By default +~~~~~~~~~~ + +By default, architectures only map real memory over the shadow region +for the linear mapping (and potentially other small areas). For all +other areas - such as vmalloc and vmemmap space - a single read-only +page is mapped over the shadow area. This read-only shadow page +declares all memory accesses as permitted. + +This presents a problem for modules: they do not live in the linear +mapping, but in a dedicated module space. By hooking in to the module +allocator, KASAN can temporarily map real shadow memory to cover +them. This allows detection of invalid accesses to module globals, for +example. + +This also creates an incompatibility with ``VMAP_STACK``: if the stack +lives in vmalloc space, it will be shadowed by the read-only page, and +the kernel will fault when trying to set up the shadow data for stack +variables. + +CONFIG_KASAN_VMALLOC +~~~~~~~~~~~~~~~~~~~~ + +With ``CONFIG_KASAN_VMALLOC``, KASAN can cover vmalloc space at the +cost of greater memory usage. Currently this is only supported on x86. + +This works by hooking into vmalloc and vmap, and dynamically +allocating real shadow memory to back the mappings. + +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_GRANULE_SIZE * PAGE_SIZE``. + +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. + +KASAN hooks into the vmap infrastructure to lazily clean up unused shadow +memory. + +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. + +This allows ``VMAP_STACK`` support on x86, and can simplify support of +architectures that do not have a fixed module region. + +CONFIG_KASAN_KUNIT_TEST and CONFIG_KASAN_MODULE_TEST +---------------------------------------------------- + +KASAN tests consist of 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. + +2. Tests that are currently incompatible with KUnit. Enabled with +``CONFIG_KASAN_MODULE_TEST`` 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 + +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 + +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_expected == 1 + kasan_data->report_found == 0 + not ok 28 - kmalloc_double_kzfree + +At the end the cumulative status of all KASAN tests is printed. On success:: + + ok 1 - kasan + +Or, if one of the tests failed:: + + not ok 1 - kasan + + +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 by loading +the module with insmod or modprobe. The module is called ``test_kasan``. + +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. + +3. Using 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 -- Gitblit v1.6.2