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![Catalin Marinas](/assets/img/avatar_default.png)
On exception entry, the kernel explicitly resets the PSTATE.TCO (tag
check override) so that any kernel memory accesses will be checked (the
bit is restored on exception return). This has the side-effect that the
uaccess routines will not honour the PSTATE.TCO that may have been set
by the user prior to a syscall.
There is no issue in practice since PSTATE.TCO is expected to be used
only for brief periods in specific routines (e.g. garbage collection).
To control the tag checking mode of the uaccess routines, the user will
have to invoke a corresponding prctl() call.
Document the kernel behaviour w.r.t. PSTATE.TCO accordingly.
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Fixes: df9d7a22dd
("arm64: mte: Add Memory Tagging Extension documentation")
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Szabolcs Nagy <szabolcs.nagy@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>
307 lines
11 KiB
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307 lines
11 KiB
ReStructuredText
===============================================
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Memory Tagging Extension (MTE) in AArch64 Linux
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===============================================
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Authors: Vincenzo Frascino <vincenzo.frascino@arm.com>
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Catalin Marinas <catalin.marinas@arm.com>
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Date: 2020-02-25
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This document describes the provision of the Memory Tagging Extension
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functionality in AArch64 Linux.
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Introduction
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============
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ARMv8.5 based processors introduce the Memory Tagging Extension (MTE)
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feature. MTE is built on top of the ARMv8.0 virtual address tagging TBI
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(Top Byte Ignore) feature and allows software to access a 4-bit
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allocation tag for each 16-byte granule in the physical address space.
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Such memory range must be mapped with the Normal-Tagged memory
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attribute. A logical tag is derived from bits 59-56 of the virtual
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address used for the memory access. A CPU with MTE enabled will compare
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the logical tag against the allocation tag and potentially raise an
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exception on mismatch, subject to system registers configuration.
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Userspace Support
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=================
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When ``CONFIG_ARM64_MTE`` is selected and Memory Tagging Extension is
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supported by the hardware, the kernel advertises the feature to
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userspace via ``HWCAP2_MTE``.
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PROT_MTE
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--------
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To access the allocation tags, a user process must enable the Tagged
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memory attribute on an address range using a new ``prot`` flag for
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``mmap()`` and ``mprotect()``:
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``PROT_MTE`` - Pages allow access to the MTE allocation tags.
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The allocation tag is set to 0 when such pages are first mapped in the
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user address space and preserved on copy-on-write. ``MAP_SHARED`` is
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supported and the allocation tags can be shared between processes.
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**Note**: ``PROT_MTE`` is only supported on ``MAP_ANONYMOUS`` and
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RAM-based file mappings (``tmpfs``, ``memfd``). Passing it to other
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types of mapping will result in ``-EINVAL`` returned by these system
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calls.
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**Note**: The ``PROT_MTE`` flag (and corresponding memory type) cannot
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be cleared by ``mprotect()``.
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**Note**: ``madvise()`` memory ranges with ``MADV_DONTNEED`` and
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``MADV_FREE`` may have the allocation tags cleared (set to 0) at any
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point after the system call.
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Tag Check Faults
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----------------
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When ``PROT_MTE`` is enabled on an address range and a mismatch between
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the logical and allocation tags occurs on access, there are three
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configurable behaviours:
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- *Ignore* - This is the default mode. The CPU (and kernel) ignores the
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tag check fault.
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- *Synchronous* - The kernel raises a ``SIGSEGV`` synchronously, with
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``.si_code = SEGV_MTESERR`` and ``.si_addr = <fault-address>``. The
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memory access is not performed. If ``SIGSEGV`` is ignored or blocked
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by the offending thread, the containing process is terminated with a
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``coredump``.
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- *Asynchronous* - The kernel raises a ``SIGSEGV``, in the offending
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thread, asynchronously following one or multiple tag check faults,
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with ``.si_code = SEGV_MTEAERR`` and ``.si_addr = 0`` (the faulting
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address is unknown).
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The user can select the above modes, per thread, using the
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``prctl(PR_SET_TAGGED_ADDR_CTRL, flags, 0, 0, 0)`` system call where
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``flags`` contain one of the following values in the ``PR_MTE_TCF_MASK``
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bit-field:
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- ``PR_MTE_TCF_NONE`` - *Ignore* tag check faults
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- ``PR_MTE_TCF_SYNC`` - *Synchronous* tag check fault mode
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- ``PR_MTE_TCF_ASYNC`` - *Asynchronous* tag check fault mode
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The current tag check fault mode can be read using the
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``prctl(PR_GET_TAGGED_ADDR_CTRL, 0, 0, 0, 0)`` system call.
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Tag checking can also be disabled for a user thread by setting the
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``PSTATE.TCO`` bit with ``MSR TCO, #1``.
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**Note**: Signal handlers are always invoked with ``PSTATE.TCO = 0``,
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irrespective of the interrupted context. ``PSTATE.TCO`` is restored on
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``sigreturn()``.
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**Note**: There are no *match-all* logical tags available for user
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applications.
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**Note**: Kernel accesses to the user address space (e.g. ``read()``
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system call) are not checked if the user thread tag checking mode is
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``PR_MTE_TCF_NONE`` or ``PR_MTE_TCF_ASYNC``. If the tag checking mode is
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``PR_MTE_TCF_SYNC``, the kernel makes a best effort to check its user
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address accesses, however it cannot always guarantee it. Kernel accesses
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to user addresses are always performed with an effective ``PSTATE.TCO``
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value of zero, regardless of the user configuration.
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Excluding Tags in the ``IRG``, ``ADDG`` and ``SUBG`` instructions
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-----------------------------------------------------------------
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The architecture allows excluding certain tags to be randomly generated
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via the ``GCR_EL1.Exclude`` register bit-field. By default, Linux
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excludes all tags other than 0. A user thread can enable specific tags
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in the randomly generated set using the ``prctl(PR_SET_TAGGED_ADDR_CTRL,
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flags, 0, 0, 0)`` system call where ``flags`` contains the tags bitmap
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in the ``PR_MTE_TAG_MASK`` bit-field.
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**Note**: The hardware uses an exclude mask but the ``prctl()``
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interface provides an include mask. An include mask of ``0`` (exclusion
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mask ``0xffff``) results in the CPU always generating tag ``0``.
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Initial process state
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---------------------
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On ``execve()``, the new process has the following configuration:
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- ``PR_TAGGED_ADDR_ENABLE`` set to 0 (disabled)
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- Tag checking mode set to ``PR_MTE_TCF_NONE``
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- ``PR_MTE_TAG_MASK`` set to 0 (all tags excluded)
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- ``PSTATE.TCO`` set to 0
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- ``PROT_MTE`` not set on any of the initial memory maps
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On ``fork()``, the new process inherits the parent's configuration and
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memory map attributes with the exception of the ``madvise()`` ranges
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with ``MADV_WIPEONFORK`` which will have the data and tags cleared (set
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to 0).
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The ``ptrace()`` interface
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--------------------------
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``PTRACE_PEEKMTETAGS`` and ``PTRACE_POKEMTETAGS`` allow a tracer to read
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the tags from or set the tags to a tracee's address space. The
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``ptrace()`` system call is invoked as ``ptrace(request, pid, addr,
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data)`` where:
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- ``request`` - one of ``PTRACE_PEEKMTETAGS`` or ``PTRACE_POKEMTETAGS``.
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- ``pid`` - the tracee's PID.
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- ``addr`` - address in the tracee's address space.
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- ``data`` - pointer to a ``struct iovec`` where ``iov_base`` points to
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a buffer of ``iov_len`` length in the tracer's address space.
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The tags in the tracer's ``iov_base`` buffer are represented as one
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4-bit tag per byte and correspond to a 16-byte MTE tag granule in the
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tracee's address space.
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**Note**: If ``addr`` is not aligned to a 16-byte granule, the kernel
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will use the corresponding aligned address.
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``ptrace()`` return value:
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- 0 - tags were copied, the tracer's ``iov_len`` was updated to the
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number of tags transferred. This may be smaller than the requested
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``iov_len`` if the requested address range in the tracee's or the
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tracer's space cannot be accessed or does not have valid tags.
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- ``-EPERM`` - the specified process cannot be traced.
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- ``-EIO`` - the tracee's address range cannot be accessed (e.g. invalid
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address) and no tags copied. ``iov_len`` not updated.
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- ``-EFAULT`` - fault on accessing the tracer's memory (``struct iovec``
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or ``iov_base`` buffer) and no tags copied. ``iov_len`` not updated.
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- ``-EOPNOTSUPP`` - the tracee's address does not have valid tags (never
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mapped with the ``PROT_MTE`` flag). ``iov_len`` not updated.
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**Note**: There are no transient errors for the requests above, so user
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programs should not retry in case of a non-zero system call return.
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``PTRACE_GETREGSET`` and ``PTRACE_SETREGSET`` with ``addr ==
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``NT_ARM_TAGGED_ADDR_CTRL`` allow ``ptrace()`` access to the tagged
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address ABI control and MTE configuration of a process as per the
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``prctl()`` options described in
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Documentation/arm64/tagged-address-abi.rst and above. The corresponding
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``regset`` is 1 element of 8 bytes (``sizeof(long))``).
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Example of correct usage
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========================
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*MTE Example code*
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.. code-block:: c
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/*
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* To be compiled with -march=armv8.5-a+memtag
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*/
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#include <errno.h>
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#include <stdint.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <unistd.h>
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#include <sys/auxv.h>
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#include <sys/mman.h>
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#include <sys/prctl.h>
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/*
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* From arch/arm64/include/uapi/asm/hwcap.h
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*/
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#define HWCAP2_MTE (1 << 18)
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/*
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* From arch/arm64/include/uapi/asm/mman.h
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*/
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#define PROT_MTE 0x20
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/*
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* From include/uapi/linux/prctl.h
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*/
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#define PR_SET_TAGGED_ADDR_CTRL 55
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#define PR_GET_TAGGED_ADDR_CTRL 56
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# define PR_TAGGED_ADDR_ENABLE (1UL << 0)
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# define PR_MTE_TCF_SHIFT 1
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# define PR_MTE_TCF_NONE (0UL << PR_MTE_TCF_SHIFT)
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# define PR_MTE_TCF_SYNC (1UL << PR_MTE_TCF_SHIFT)
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# define PR_MTE_TCF_ASYNC (2UL << PR_MTE_TCF_SHIFT)
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# define PR_MTE_TCF_MASK (3UL << PR_MTE_TCF_SHIFT)
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# define PR_MTE_TAG_SHIFT 3
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# define PR_MTE_TAG_MASK (0xffffUL << PR_MTE_TAG_SHIFT)
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/*
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* Insert a random logical tag into the given pointer.
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*/
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#define insert_random_tag(ptr) ({ \
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uint64_t __val; \
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asm("irg %0, %1" : "=r" (__val) : "r" (ptr)); \
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__val; \
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})
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/*
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* Set the allocation tag on the destination address.
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*/
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#define set_tag(tagged_addr) do { \
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asm volatile("stg %0, [%0]" : : "r" (tagged_addr) : "memory"); \
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} while (0)
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int main()
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{
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unsigned char *a;
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unsigned long page_sz = sysconf(_SC_PAGESIZE);
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unsigned long hwcap2 = getauxval(AT_HWCAP2);
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/* check if MTE is present */
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if (!(hwcap2 & HWCAP2_MTE))
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return EXIT_FAILURE;
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/*
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* Enable the tagged address ABI, synchronous MTE tag check faults and
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* allow all non-zero tags in the randomly generated set.
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*/
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if (prctl(PR_SET_TAGGED_ADDR_CTRL,
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PR_TAGGED_ADDR_ENABLE | PR_MTE_TCF_SYNC | (0xfffe << PR_MTE_TAG_SHIFT),
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0, 0, 0)) {
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perror("prctl() failed");
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return EXIT_FAILURE;
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}
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a = mmap(0, page_sz, PROT_READ | PROT_WRITE,
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MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
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if (a == MAP_FAILED) {
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perror("mmap() failed");
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return EXIT_FAILURE;
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}
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/*
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* Enable MTE on the above anonymous mmap. The flag could be passed
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* directly to mmap() and skip this step.
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*/
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if (mprotect(a, page_sz, PROT_READ | PROT_WRITE | PROT_MTE)) {
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perror("mprotect() failed");
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return EXIT_FAILURE;
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}
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/* access with the default tag (0) */
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a[0] = 1;
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a[1] = 2;
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printf("a[0] = %hhu a[1] = %hhu\n", a[0], a[1]);
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/* set the logical and allocation tags */
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a = (unsigned char *)insert_random_tag(a);
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set_tag(a);
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printf("%p\n", a);
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/* non-zero tag access */
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a[0] = 3;
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printf("a[0] = %hhu a[1] = %hhu\n", a[0], a[1]);
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/*
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* If MTE is enabled correctly the next instruction will generate an
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* exception.
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*/
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printf("Expecting SIGSEGV...\n");
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a[16] = 0xdd;
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/* this should not be printed in the PR_MTE_TCF_SYNC mode */
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printf("...haven't got one\n");
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return EXIT_FAILURE;
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}
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