General comments: * it's -> it is * don't -> do not On Fri, 22 Nov 2019 at 12:26, <glider@xxxxxxxxxx> wrote: [...] > diff --git a/Documentation/dev-tools/index.rst b/Documentation/dev-tools/index.rst > index b0522a4dd107..bc5e3fd87efa 100644 > --- a/Documentation/dev-tools/index.rst > +++ b/Documentation/dev-tools/index.rst > @@ -19,6 +19,7 @@ whole; patches welcome! > kcov > gcov > kasan > + kmsan > ubsan > kmemleak > gdb-kernel-debugging > diff --git a/Documentation/dev-tools/kmsan.rst b/Documentation/dev-tools/kmsan.rst > new file mode 100644 > index 000000000000..51f9c207cc2c > --- /dev/null > +++ b/Documentation/dev-tools/kmsan.rst > @@ -0,0 +1,418 @@ > +============================= > +KernelMemorySanitizer (KMSAN) > +============================= > + > +KMSAN is a dynamic memory error detector aimed at finding uses of uninitialized > +memory. > +It is based on compiler instrumentation, and is quite similar to the userspace > +MemorySanitizer tool (http://clang.llvm.org/docs/MemorySanitizer.html). These should be real links: `Memory sanitizer tool <...url...>`_. > +KMSAN and Clang > +=============== > + > +In order for KMSAN to work the kernel must be > +built with Clang, which is so far the only compiler that has KMSAN support. "is so far" -> "so far is" > +The kernel instrumentation pass is based on the userspace MemorySanitizer tool > +(http://clang.llvm.org/docs/MemorySanitizer.html). Because of the Should also be real link: `MemorySanitizer tool <..url..>`_ > +instrumentation complexity it's unlikely that any other compiler will support > +KMSAN soon. > + > +Right now the instrumentation pass supports x86_64 only. > + > +How to build > +============ > + > +In order to build a kernel with KMSAN you'll need a fresh Clang (10.0.0+, trunk > +version r365008 or greater). Please refer to > +https://llvm.org/docs/GettingStarted.html for the instructions on how to build > +Clang:: > + > + export KMSAN_CLANG_PATH=/path/to/clang > > + # Now configure and build the kernel with CONFIG_KMSAN enabled. > + make CC=$KMSAN_CLANG_PATH -j64 I don't think '-j64' is necessary to build. Also the 'export' is technically not required AFAIK, but I don't think it bothers anyone. > +How KMSAN works > +=============== > + > +KMSAN shadow memory > +------------------- > + > +KMSAN associates a so-called shadow byte with every byte of kernel memory. 'shadow' memory may not be a well-defined term. More intuitive would be saying that it's metadata associated with every byte of kernel memory. From then on you can say it's shadow 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's poisoned by default by > +instrumentation code inserted by the compiler (unless it's 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; > + 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 a creation stack, which lets > +the user figure out what's going on. > + > +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 = 0; > + 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. > + > +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 the debugging, KMSAN creates a new origin for every memory store. "the debugging" -> "debugging" > +The new origin references both its creation stack and the previous origin the > +memory location 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/kmsan_instr.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 *s, *o; > + } 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, u64 size) > + shadow_origin_ptr_t __msan_metadata_ptr_for_store_n(void *addr, u64 size) > + > +The function name depends on the memory access size. > +Each such function also checks if the shadow of the memory in the range > +[``addr``, ``addr + n``) is contiguous and reports an error otherwise. > + > +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(u64 address, u64 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_s { > + char param_tls[KMSAN_PARAM_SIZE]; > + char retval_tls[RETVAL_SIZE]; > + char va_arg_tls[KMSAN_PARAM_SIZE]; > + char va_arg_origin_tls[KMSAN_PARAM_SIZE]; > + u64 va_arg_overflow_size_tls; > + depot_stack_handle_t param_origin_tls[PARAM_ARRAY_SIZE]; > + depot_stack_handle_t retval_origin_tls; > + depot_stack_handle_t 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, u64 n) > + void *__msan_memmove(void *dst, void *src, u64 n) > + void *__msan_memset(void *dst, int c, size_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(u64 addr, u64 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 don't point to valid memory. > +In such cases they are ignored at runtime. > + > +Disabling the instrumentation > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > +A function can be marked with ``__no_sanitize_memory``. > +Doing so doesn't remove KMSAN instrumentation from it, however it makes the > +compiler ignore the uninitialized values coming from the function's inputs, > +and initialize the function's outputs. > +The compiler won't inline functions marked with this attribute into functions > +not marked with it, and vice versa. > + > +It's 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. This comes at a cost however: stack allocations from such files > +and parameters of instrumented functions called from them will have incorrect > +shadow/origin values. As a rule of thumb, avoid using KMSAN_SANITIZE. > + > +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_task_state { > + ... > + bool allow_reporting; > + struct kmsan_context_state cstate; > + ... > + } > + > + struct task_struct { > + ... > + struct kmsan_task_state 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(kmsan_context_state[KMSAN_NESTED_CONTEXT_MAX], > + kmsan_percpu_cstate); > + > +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; > + ... > + }; > + > +Every time a ``struct page`` is allocated, the runtime library allocates two > +additional pages to hold its shadow and origins. This is done by adding hooks > +to ``alloc_pages()``/``free_pages()`` in ``mm/page_alloc.c``. > +To avoid allocating the metadata for non-interesting pages (right now only the > +shadow/origin page themselves and stackdepot storage) the > +``__GFP_NO_KMSAN_SHADOW`` flag is used. > + > +There is a problem related to this allocation algorithm: when two contiguous > +memory blocks are allocated with two different ``alloc_pages()`` calls, their > +shadow 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. > + > +As a workaround, we check the access size in > +``__msan_metadata_ptr_for_XXX_YYY()`` and return a pointer to a fake shadow > +region in the case of an error:: > + > + 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. > + > +Unfortunately at boot time we need to allocate shadow and origin pages for the > +kernel data (``.data``, ``.bss`` etc.) and percpu memory regions, the size of > +which is not a power of 2. As a result, we have to allocate the metadata page by > +page, so that it is also non-contiguous, although it may be perfectly valid to > +access the corresponding kernel memory across page boundaries. > +This can be probably fixed by allocating 1<<N pages at once, splitting them and > +deallocating the rest. > + > +LSB of the ``shadow`` pointer in a ``struct page`` may be set to 1. In this case > +shadow and origin pages are allocated, but KMSAN ignores accesses to them by > +falling back to dummy pages. Allocating the metadata pages is still needed to > +support ``vmap()/vunmap()`` operations on this struct page. > + > +2. For vmalloc memory and modules, there's a direct mapping between the memory > +range, its shadow and origin. KMSAN lessens 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're 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); For some reason rst2html complains here that this is not a literal block. > +When calculating shadow and origin addresses for a given memory address, the > +runtime checks whether the address belongs to the physical page range, the > +virtual page range or CPU entry area. > + > +Handling ``pt_regs`` > +~~~~~~~~~~~~~~~~~~~ This is missing a '~' (I ran it through rst2html to find). > +Many functions receive a ``struct pt_regs`` holding the register state at a > +certain point. Registers don't have (easily calculatable) shadow or origin > +associated with them. > +We can assume that the registers are always initialized. > + > +Example report > +-------------- > +Here's an example of a real KMSAN report in ``packet_bind_spkt()``:: Shouldn't this section be somewhere at the top in a section such as "usage". A user of KMSAN doesn't really care how KMSAN works. > + ================================================================== > + BUG: KMSAN: uninit-value in strlen > + CPU: 0 PID: 1074 Comm: packet Not tainted 4.8.0-rc6+ #1891 > + Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011 > + 0000000000000000 ffff88006b6dfc08 ffffffff82559ae8 ffff88006b6dfb48 > + ffffffff818a7c91 ffffffff85b9c870 0000000000000092 ffffffff85b9c550 > + 0000000000000000 0000000000000092 00000000ec400911 0000000000000002 > + Call Trace: > + [< inline >] __dump_stack lib/dump_stack.c:15 > + [<ffffffff82559ae8>] dump_stack+0x238/0x290 lib/dump_stack.c:51 > + [<ffffffff818a6626>] kmsan_report+0x276/0x2e0 mm/kmsan/kmsan.c:1003 > + [<ffffffff818a783b>] __msan_warning+0x5b/0xb0 mm/kmsan/kmsan_instr.c:424 > + [< inline >] strlen lib/string.c:484 > + [<ffffffff8259b58d>] strlcpy+0x9d/0x200 lib/string.c:144 > + [<ffffffff84b2eca4>] packet_bind_spkt+0x144/0x230 net/packet/af_packet.c:3132 > + [<ffffffff84242e4d>] SYSC_bind+0x40d/0x5f0 net/socket.c:1370 > + [<ffffffff84242a22>] SyS_bind+0x82/0xa0 net/socket.c:1356 > + [<ffffffff8515991b>] entry_SYSCALL_64_fastpath+0x13/0x8f arch/x86/entry/entry_64.o:? > + chained origin: > + [<ffffffff810bb787>] save_stack_trace+0x27/0x50 arch/x86/kernel/stacktrace.c:67 > + [< inline >] kmsan_save_stack_with_flags mm/kmsan/kmsan.c:322 > + [< inline >] kmsan_save_stack mm/kmsan/kmsan.c:334 > + [<ffffffff818a59f8>] kmsan_internal_chain_origin+0x118/0x1e0 mm/kmsan/kmsan.c:527 > + [<ffffffff818a7773>] __msan_set_alloca_origin4+0xc3/0x130 mm/kmsan/kmsan_instr.c:380 > + [<ffffffff84242b69>] SYSC_bind+0x129/0x5f0 net/socket.c:1356 > + [<ffffffff84242a22>] SyS_bind+0x82/0xa0 net/socket.c:1356 > + [<ffffffff8515991b>] entry_SYSCALL_64_fastpath+0x13/0x8f arch/x86/entry/entry_64.o:? > + origin description: ----address@SYSC_bind (origin=00000000eb400911) > + ================================================================== > + > +The report tells that the local variable ``address`` was created uninitialized > +in ``SYSC_bind()`` (the ``bind`` system call implementation). The lower stack > +trace corresponds to the place where this variable was created. > + > +The upper stack shows where the uninit value was used - in ``strlen()``. > +It turned out that the contents of ``address`` were partially copied from the > +userspace, but the buffer wasn't zero-terminated and contained some trailing > +uninitialized bytes. > +``packet_bind_spkt()`` didn't check the length of the buffer, but called > +``strlcpy()`` on it, which called ``strlen()``, which started reading the > +buffer byte by byte till it hit the uninitialized memory. > + > + > +References > +========== > + > +E. Stepanov, K. Serebryany. MemorySanitizer: fast detector of uninitialized > +memory use in C++. > +In Proceedings of CGO 2015. This should be turned into a link.
