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