Add Documentation/dev-tools/kmsan.rst and reference it in the dev-tools index. Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx> --- v2: -- added a note that KMSAN is not intended for production use Link: https://linux-review.googlesource.com/id/I751586f79418b95550a83c6035c650b5b01567cc --- Documentation/dev-tools/index.rst | 1 + Documentation/dev-tools/kmsan.rst | 414 ++++++++++++++++++++++++++++++ 2 files changed, 415 insertions(+) create mode 100644 Documentation/dev-tools/kmsan.rst diff --git a/Documentation/dev-tools/index.rst b/Documentation/dev-tools/index.rst index 4621eac290f46..6b0663075dc04 100644 --- a/Documentation/dev-tools/index.rst +++ b/Documentation/dev-tools/index.rst @@ -24,6 +24,7 @@ Documentation/dev-tools/testing-overview.rst kcov gcov kasan + kmsan ubsan kmemleak kcsan diff --git a/Documentation/dev-tools/kmsan.rst b/Documentation/dev-tools/kmsan.rst new file mode 100644 index 0000000000000..e116889da79d5 --- /dev/null +++ b/Documentation/dev-tools/kmsan.rst @@ -0,0 +1,414 @@ +============================= +KernelMemorySanitizer (KMSAN) +============================= + +KMSAN is a dynamic error detector aimed at finding uses of uninitialized +values. It is based on compiler instrumentation, and is quite similar to the +userspace `MemorySanitizer tool`_. + +An important note is that KMSAN is not intended for production use, because it +drastically increases kernel memory footprint and slows the whole system down. + +Example report +============== + +Here is an example of a KMSAN report:: + + ===================================================== + BUG: KMSAN: uninit-value in test_uninit_kmsan_check_memory+0x1be/0x380 [kmsan_test] + test_uninit_kmsan_check_memory+0x1be/0x380 mm/kmsan/kmsan_test.c:273 + kunit_run_case_internal lib/kunit/test.c:333 + kunit_try_run_case+0x206/0x420 lib/kunit/test.c:374 + kunit_generic_run_threadfn_adapter+0x6d/0xc0 lib/kunit/try-catch.c:28 + kthread+0x721/0x850 kernel/kthread.c:327 + ret_from_fork+0x1f/0x30 ??:? + + Uninit was stored to memory at: + do_uninit_local_array+0xfa/0x110 mm/kmsan/kmsan_test.c:260 + test_uninit_kmsan_check_memory+0x1a2/0x380 mm/kmsan/kmsan_test.c:271 + kunit_run_case_internal lib/kunit/test.c:333 + kunit_try_run_case+0x206/0x420 lib/kunit/test.c:374 + kunit_generic_run_threadfn_adapter+0x6d/0xc0 lib/kunit/try-catch.c:28 + kthread+0x721/0x850 kernel/kthread.c:327 + ret_from_fork+0x1f/0x30 ??:? + + Local variable uninit created at: + do_uninit_local_array+0x4a/0x110 mm/kmsan/kmsan_test.c:256 + test_uninit_kmsan_check_memory+0x1a2/0x380 mm/kmsan/kmsan_test.c:271 + + Bytes 4-7 of 8 are uninitialized + Memory access of size 8 starts at ffff888083fe3da0 + + CPU: 0 PID: 6731 Comm: kunit_try_catch Tainted: G B E 5.16.0-rc3+ #104 + Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014 + ===================================================== + + +The report says that the local variable ``uninit`` was created uninitialized in +``do_uninit_local_array()``. The lower stack trace corresponds to the place +where this variable was created. + +The upper stack shows where the uninit value was used - in +``test_uninit_kmsan_check_memory()``. The tool shows the bytes which were left +uninitialized in the local variable, as well as the stack where the value was +copied to another memory location before use. + +Please note that KMSAN only reports an error when an uninitialized value is +actually used (e.g. in a condition or pointer dereference). A lot of +uninitialized values in the kernel are never used, and reporting them would +result in too many false positives. + +KMSAN and Clang +=============== + +In order for KMSAN to work the kernel must be built with Clang, which so far is +the only compiler that has KMSAN support. The kernel instrumentation pass is +based on the userspace `MemorySanitizer tool`_. + +How to build +============ + +In order to build a kernel with KMSAN you will need a fresh Clang (14.0.0+). +Please refer to `LLVM documentation`_ for the instructions on how to build Clang. + +Now configure and build the kernel with CONFIG_KMSAN enabled. + +How KMSAN works +=============== + +KMSAN shadow memory +------------------- + +KMSAN associates a metadata byte (also called shadow byte) with every byte of +kernel memory. A bit in the shadow byte is set iff the corresponding bit of the +kernel memory byte is uninitialized. Marking the memory uninitialized (i.e. +setting its shadow bytes to ``0xff``) is called poisoning, marking it +initialized (setting the shadow bytes to ``0x00``) is called unpoisoning. + +When a new variable is allocated on the stack, it is poisoned by default by +instrumentation code inserted by the compiler (unless it is a stack variable +that is immediately initialized). Any new heap allocation done without +``__GFP_ZERO`` is also poisoned. + +Compiler instrumentation also tracks the shadow values with the help from the +runtime library in ``mm/kmsan/``. + +The shadow value of a basic or compound type is an array of bytes of the same +length. When a constant value is written into memory, that memory is unpoisoned. +When a value is read from memory, its shadow memory is also obtained and +propagated into all the operations which use that value. For every instruction +that takes one or more values the compiler generates code that calculates the +shadow of the result depending on those values and their shadows. + +Example:: + + int a = 0xff; // i.e. 0x000000ff + int b; + int c = a | b; + +In this case the shadow of ``a`` is ``0``, shadow of ``b`` is ``0xffffffff``, +shadow of ``c`` is ``0xffffff00``. This means that the upper three bytes of +``c`` are uninitialized, while the lower byte is initialized. + + +Origin tracking +--------------- + +Every four bytes of kernel memory also have a so-called origin assigned to +them. This origin describes the point in program execution at which the +uninitialized value was created. Every origin is associated with either the +full allocation stack (for heap-allocated memory), or the function containing +the uninitialized variable (for locals). + +When an uninitialized variable is allocated on stack or heap, a new origin +value is created, and that variable's origin is filled with that value. +When a value is read from memory, its origin is also read and kept together +with the shadow. For every instruction that takes one or more values the origin +of the result is one of the origins corresponding to any of the uninitialized +inputs. If a poisoned value is written into memory, its origin is written to the +corresponding storage as well. + +Example 1:: + + int a = 42; + int b; + int c = a + b; + +In this case the origin of ``b`` is generated upon function entry, and is +stored to the origin of ``c`` right before the addition result is written into +memory. + +Several variables may share the same origin address, if they are stored in the +same four-byte chunk. In this case every write to either variable updates the +origin for all of them. We have to sacrifice precision in this case, because +storing origins for individual bits (and even bytes) would be too costly. + +Example 2:: + + int combine(short a, short b) { + union ret_t { + int i; + short s[2]; + } ret; + ret.s[0] = a; + ret.s[1] = b; + return ret.i; + } + +If ``a`` is initialized and ``b`` is not, the shadow of the result would be +0xffff0000, and the origin of the result would be the origin of ``b``. +``ret.s[0]`` would have the same origin, but it will be never used, because +that variable is initialized. + +If both function arguments are uninitialized, only the origin of the second +argument is preserved. + +Origin chaining +~~~~~~~~~~~~~~~ + +To ease debugging, KMSAN creates a new origin for every store of an +uninitialized value to memory. The new origin references both its creation stack +and the previous origin the value had. This may cause increased memory +consumption, so we limit the length of origin chains in the runtime. + +Clang instrumentation API +------------------------- + +Clang instrumentation pass inserts calls to functions defined in +``mm/kmsan/instrumentation.c`` into the kernel code. + +Shadow manipulation +~~~~~~~~~~~~~~~~~~~ + +For every memory access the compiler emits a call to a function that returns a +pair of pointers to the shadow and origin addresses of the given memory:: + + typedef struct { + void *shadow, *origin; + } shadow_origin_ptr_t + + shadow_origin_ptr_t __msan_metadata_ptr_for_load_{1,2,4,8}(void *addr) + shadow_origin_ptr_t __msan_metadata_ptr_for_store_{1,2,4,8}(void *addr) + shadow_origin_ptr_t __msan_metadata_ptr_for_load_n(void *addr, uintptr_t size) + shadow_origin_ptr_t __msan_metadata_ptr_for_store_n(void *addr, uintptr_t size) + +The function name depends on the memory access size. + +The compiler makes sure that for every loaded value its shadow and origin +values are read from memory. When a value is stored to memory, its shadow and +origin are also stored using the metadata pointers. + +Origin tracking +~~~~~~~~~~~~~~~ + +A special function is used to create a new origin value for a local variable and +set the origin of that variable to that value:: + + void __msan_poison_alloca(void *addr, uintptr_t size, char *descr) + +Access to per-task data +~~~~~~~~~~~~~~~~~~~~~~~~~ + +At the beginning of every instrumented function KMSAN inserts a call to +``__msan_get_context_state()``:: + + kmsan_context_state *__msan_get_context_state(void) + +``kmsan_context_state`` is declared in ``include/linux/kmsan.h``:: + + struct kmsan_context_state { + char param_tls[KMSAN_PARAM_SIZE]; + char retval_tls[KMSAN_RETVAL_SIZE]; + char va_arg_tls[KMSAN_PARAM_SIZE]; + char va_arg_origin_tls[KMSAN_PARAM_SIZE]; + u64 va_arg_overflow_size_tls; + char param_origin_tls[KMSAN_PARAM_SIZE]; + depot_stack_handle_t retval_origin_tls; + }; + +This structure is used by KMSAN to pass parameter shadows and origins between +instrumented functions. + +String functions +~~~~~~~~~~~~~~~~ + +The compiler replaces calls to ``memcpy()``/``memmove()``/``memset()`` with the +following functions. These functions are also called when data structures are +initialized or copied, making sure shadow and origin values are copied alongside +with the data:: + + void *__msan_memcpy(void *dst, void *src, uintptr_t n) + void *__msan_memmove(void *dst, void *src, uintptr_t n) + void *__msan_memset(void *dst, int c, uintptr_t n) + +Error reporting +~~~~~~~~~~~~~~~ + +For each pointer dereference and each condition the compiler emits a shadow +check that calls ``__msan_warning()`` in the case a poisoned value is being +used:: + + void __msan_warning(u32 origin) + +``__msan_warning()`` causes KMSAN runtime to print an error report. + +Inline assembly instrumentation +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +KMSAN instruments every inline assembly output with a call to:: + + void __msan_instrument_asm_store(void *addr, uintptr_t size) + +, which unpoisons the memory region. + +This approach may mask certain errors, but it also helps to avoid a lot of +false positives in bitwise operations, atomics etc. + +Sometimes the pointers passed into inline assembly do not point to valid memory. +In such cases they are ignored at runtime. + +Disabling the instrumentation +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +A function can be marked with ``__no_kmsan_checks``. Doing so makes KMSAN +ignore uninitialized values in that function and mark its output as initialized. +As a result, the user will not get KMSAN reports related to that function. + +Another function attribute supported by KMSAN is ``__no_sanitize_memory``. +Applying this attribute to a function will result in KMSAN not instrumenting it, +which can be helpful if we do not want the compiler to mess up some low-level +code (e.g. that marked with ``noinstr``). + +This however comes at a cost: stack allocations from such functions will have +incorrect shadow/origin values, likely leading to false positives. Functions +called from non-instrumented code may also receive incorrect metadata for their +parameters. + +As a rule of thumb, avoid using ``__no_sanitize_memory`` explicitly. + +It is also possible to disable KMSAN for a single file (e.g. main.o):: + + KMSAN_SANITIZE_main.o := n + +or for the whole directory:: + + KMSAN_SANITIZE := n + +in the Makefile. Think of this as applying ``__no_sanitize_memory`` to every +function in the file or directory. Most users won't need KMSAN_SANITIZE, unless +their code gets broken by KMSAN (e.g. runs at early boot time). + +Runtime library +--------------- + +The code is located in ``mm/kmsan/``. + +Per-task KMSAN state +~~~~~~~~~~~~~~~~~~~~ + +Every task_struct has an associated KMSAN task state that holds the KMSAN +context (see above) and a per-task flag disallowing KMSAN reports:: + + struct kmsan_context { + ... + bool allow_reporting; + struct kmsan_context_state cstate; + ... + } + + struct task_struct { + ... + struct kmsan_context kmsan; + ... + } + + +KMSAN contexts +~~~~~~~~~~~~~~ + +When running in a kernel task context, KMSAN uses ``current->kmsan.cstate`` to +hold the metadata for function parameters and return values. + +But in the case the kernel is running in the interrupt, softirq or NMI context, +where ``current`` is unavailable, KMSAN switches to per-cpu interrupt state:: + + DEFINE_PER_CPU(struct kmsan_ctx, kmsan_percpu_ctx); + +Metadata allocation +~~~~~~~~~~~~~~~~~~~ + +There are several places in the kernel for which the metadata is stored. + +1. Each ``struct page`` instance contains two pointers to its shadow and +origin pages:: + + struct page { + ... + struct page *shadow, *origin; + ... + }; + +At boot-time, the kernel allocates shadow and origin pages for every available +kernel page. This is done quite late, when the kernel address space is already +fragmented, so normal data pages may arbitrarily interleave with the metadata +pages. + +This means that in general for two contiguous memory pages their shadow/origin +pages may not be contiguous. So, if a memory access crosses the boundary +of a memory block, accesses to shadow/origin memory may potentially corrupt +other pages or read incorrect values from them. + +In practice, contiguous memory pages returned by the same ``alloc_pages()`` +call will have contiguous metadata, whereas if these pages belong to two +different allocations their metadata pages can be fragmented. + +For the kernel data (``.data``, ``.bss`` etc.) and percpu memory regions +there also are no guarantees on metadata contiguity. + +In the case ``__msan_metadata_ptr_for_XXX_YYY()`` hits the border between two +pages with non-contiguous metadata, it returns pointers to fake shadow/origin regions:: + + char dummy_load_page[PAGE_SIZE] __attribute__((aligned(PAGE_SIZE))); + char dummy_store_page[PAGE_SIZE] __attribute__((aligned(PAGE_SIZE))); + +``dummy_load_page`` is zero-initialized, so reads from it always yield zeroes. +All stores to ``dummy_store_page`` are ignored. + +2. For vmalloc memory and modules, there is a direct mapping between the memory +range, its shadow and origin. KMSAN reduces the vmalloc area by 3/4, making only +the first quarter available to ``vmalloc()``. The second quarter of the vmalloc +area contains shadow memory for the first quarter, the third one holds the +origins. A small part of the fourth quarter contains shadow and origins for the +kernel modules. Please refer to ``arch/x86/include/asm/pgtable_64_types.h`` for +more details. + +When an array of pages is mapped into a contiguous virtual memory space, their +shadow and origin pages are similarly mapped into contiguous regions. + +3. For CPU entry area there are separate per-CPU arrays that hold its +metadata:: + + DEFINE_PER_CPU(char[CPU_ENTRY_AREA_SIZE], cpu_entry_area_shadow); + DEFINE_PER_CPU(char[CPU_ENTRY_AREA_SIZE], cpu_entry_area_origin); + +When calculating shadow and origin addresses for a given memory address, KMSAN +checks whether the address belongs to the physical page range, the virtual page +range or CPU entry area. + +Handling ``pt_regs`` +~~~~~~~~~~~~~~~~~~~~ + +Many functions receive a ``struct pt_regs`` holding the register state at a +certain point. Registers do not have (easily calculatable) shadow or origin +associated with them, so we assume they are always initialized. + +References +========== + +E. Stepanov, K. Serebryany. `MemorySanitizer: fast detector of uninitialized +memory use in C++ +<https://static.googleusercontent.com/media/research.google.com/en//pubs/archive/43308.pdf>`_. +In Proceedings of CGO 2015. + +.. _MemorySanitizer tool: https://clang.llvm.org/docs/MemorySanitizer.html +.. _LLVM documentation: https://llvm.org/docs/GettingStarted.html -- 2.35.1.1021.g381101b075-goog
