Messages in this thread Patch in this message | | | Date | Mon, 22 Nov 2021 21:16:58 -0800 | Subject | [PATCH v2 5/5] Documentation: document uaccess logging | From | Peter Collingbourne <> |
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Add documentation for the uaccess logging feature.
Link: https://linux-review.googlesource.com/id/Ia626c0ca91bc0a3d8067d7f28406aa40693b65a2 Signed-off-by: Peter Collingbourne <pcc@google.com> --- Documentation/admin-guide/index.rst | 1 + Documentation/admin-guide/uaccess-logging.rst | 149 ++++++++++++++++++ 2 files changed, 150 insertions(+) create mode 100644 Documentation/admin-guide/uaccess-logging.rst
diff --git a/Documentation/admin-guide/index.rst b/Documentation/admin-guide/index.rst index 1bedab498104..4f6ee447ab2f 100644 --- a/Documentation/admin-guide/index.rst +++ b/Documentation/admin-guide/index.rst @@ -54,6 +54,7 @@ ABI will be found here. :maxdepth: 1 sysfs-rules + uaccess-logging The rest of this manual consists of various unordered guides on how to configure specific aspects of kernel behavior to your liking. diff --git a/Documentation/admin-guide/uaccess-logging.rst b/Documentation/admin-guide/uaccess-logging.rst new file mode 100644 index 000000000000..4b2b297afc00 --- /dev/null +++ b/Documentation/admin-guide/uaccess-logging.rst @@ -0,0 +1,149 @@ +.. SPDX-License-Identifier: GPL-2.0 + +=============== +Uaccess Logging +=============== + +Background +---------- + +Userspace tools such as sanitizers (ASan, MSan, HWASan) and tools +making use of the ARM Memory Tagging Extension (MTE) need to +monitor all memory accesses in a program so that they can detect +memory errors. Furthermore, fuzzing tools such as syzkaller need to +monitor all memory accesses so that they know which parts of memory +to fuzz. For accesses made purely in userspace, this is achieved +via compiler instrumentation, or for MTE, via direct hardware +support. However, accesses made by the kernel on behalf of the user +program via syscalls (i.e. uaccesses) are normally invisible to +these tools. + +Traditionally, the sanitizers have handled this by interposing the libc +syscall stubs with a wrapper that checks the memory based on what we +believe the uaccesses will be. However, this creates a maintenance +burden: each syscall must be annotated with its uaccesses in order +to be recognized by the sanitizer, and these annotations must be +continuously updated as the kernel changes. + +The kernel's uaccess logging feature provides userspace tools with +the address and size of each userspace access, thereby allowing these +tools to report memory errors involving these accesses without needing +annotations for every syscall. + +By relying on the kernel's actual uaccesses, rather than a +reimplementation of them, the userspace memory safety tools may +play a dual role of verifying the validity of kernel accesses. Even +a sanitizer whose syscall wrappers have complete knowledge of the +kernel's intended API may vary from the kernel's actual uaccesses due +to kernel bugs. A sanitizer with knowledge of the kernel's actual +uaccesses may produce more accurate error reports that reveal such +bugs. For example, a kernel that accesses more memory than expected +by the userspace program could indicate that either userspace or the +kernel has the wrong idea about which kernel functionality is being +requested -- either way, there is a bug. + +Interface +--------- + +The feature may be used via the following prctl: + +.. code-block:: c + + uint64_t addr = 0; /* Generally will be a TLS slot or equivalent */ + prctl(PR_SET_UACCESS_DESCRIPTOR_ADDR_ADDR, &addr, 0, 0, 0); + +Supplying a non-zero address as the second argument to ``prctl`` +will cause the kernel to read an address (referred to as the *uaccess +descriptor address*) from that address on each kernel entry. + +When entering the kernel with a non-zero uaccess descriptor address +to handle a syscall, the kernel will read a data structure of type +``struct uaccess_descriptor`` from the uaccess descriptor address, +which is defined as follows: + +.. code-block:: c + + struct uaccess_descriptor { + uint64_t addr, size; + }; + +This data structure contains the address and size (in array elements) +of a *uaccess buffer*, which is an array of data structures of type +``struct uaccess_buffer_entry``. Before returning to userspace, the +kernel will log information about uaccesses to sequential entries +in the uaccess buffer. It will also store ``NULL`` to the uaccess +descriptor address, and store the address and size of the unused +portion of the uaccess buffer to the uaccess descriptor. + +The format of a uaccess buffer entry is defined as follows: + +.. code-block:: c + + struct uaccess_buffer_entry { + uint64_t addr, size, flags; + }; + +The meaning of ``addr`` and ``size`` should be obvious. On arm64, +tag bits are preserved in the ``addr`` field. There is currently +one flag bit assignment for the ``flags`` field: + +.. code-block:: c + + #define UACCESS_BUFFER_FLAG_WRITE 1 + +This flag is set if the access was a write, or clear if it was a +read. The meaning of all other flag bits is reserved. + +When entering the kernel with a non-zero uaccess descriptor +address for a reason other than a syscall (for example, when +IPI'd due to an incoming asynchronous signal), any signals other +than ``SIGKILL`` and ``SIGSTOP`` are masked as if by calling +``sigprocmask(SIG_SETMASK, set, NULL)`` where ``set`` has been +initialized with ``sigfillset(set)``. This is to prevent incoming +signals from interfering with uaccess logging. + +Example +------- + +Here is an example of a code snippet that will enumerate the accesses +performed by a ``uname(2)`` syscall: + +.. code-block:: c + + struct uaccess_buffer_entry entries[64]; + struct uaccess_descriptor desc; + uint64_t desc_addr = 0; + prctl(PR_SET_UACCESS_DESCRIPTOR_ADDR_ADDR, &desc_addr, 0, 0, 0); + + desc.addr = (uint64_t)&entries; + desc.size = 64; + desc_addr = (uint64_t)&desc; + + struct utsname un; + uname(&un); + + struct uaccess_buffer_entry* entries_end = (struct uaccess_buffer_entry*)desc.addr; + for (struct uaccess_buffer_entry* entry = entries; entry != entries_end; ++entry) { + printf("%s at 0x%lx size 0x%lx\n", entry->flags & UACCESS_BUFFER_FLAG_WRITE ? "WRITE" : "READ", + (unsigned long)entry->addr, (unsigned long)entry->size); + } + +Limitations +----------- + +This feature is currently only supported on the arm64, s390 and x86 +architectures. + +Uaccess buffers are a "best-effort" mechanism for logging uaccesses. Of +course, not all of the accesses may fit in the buffer, but aside from +that, not all internal kernel APIs that access userspace memory are +covered. Therefore, userspace programs should tolerate unreported +accesses. + +On the other hand, the kernel guarantees that it will not +(intentionally) report accessing more data than it is specified +to read. For example, if the kernel implements a syscall that is +specified to read a data structure of size ``N`` bytes by first +reading a page's worth of data and then only using the first ``N`` +bytes from it, the kernel will either report reading ``N`` bytes or +not report the access at all. -- 2.34.0.rc2.393.gf8c9666880-goog
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