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 contains any number of the following values in the PR_MTE_TCF_MASK bit-field:

  • PR_MTE_TCF_NONE  - Ignore tag check faults
    (ignored if combined with other options)
  • PR_MTE_TCF_SYNC - Synchronous tag check fault mode
  • PR_MTE_TCF_ASYNC - Asynchronous tag check fault mode

If no modes are specified, tag check faults are ignored. If a single mode is specified, the program will run in that mode. If multiple modes are specified, the mode is selected as described in the “Per-CPU preferred tag checking modes” section below.

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.

Per-CPU preferred tag checking mode

On some CPUs the performance of MTE in stricter tag checking modes is similar to that of less strict tag checking modes. This makes it worthwhile to enable stricter checks on those CPUs when a less strict checking mode is requested, in order to gain the error detection benefits of the stricter checks without the performance downsides. To support this scenario, a privileged user may configure a stricter tag checking mode as the CPU’s preferred tag checking mode.

The preferred tag checking mode for each CPU is controlled by /sys/devices/system/cpu/cpu<N>/mte_tcf_preferred, to which a privileged user may write the value async or sync. The default preferred mode for each CPU is async.

To allow a program to potentially run in the CPU’s preferred tag checking mode, the user program may set multiple tag check fault mode bits in the flags argument to the prctl(PR_SET_TAGGED_ADDR_CTRL, flags, 0, 0, 0) system call. If the CPU’s preferred tag checking mode is in the task’s set of provided tag checking modes (this will always be the case at present because the kernel only supports two tag checking modes, but future kernels may support more modes), that mode will be selected. Otherwise, one of the modes in the task’s mode set will be selected in a currently unspecified manner.

Initial process state

On execve(), the new process has the following configuration:

  • PR_TAGGED_ADDR_ENABLE set to 0 (disabled)
  • No tag checking modes are selected (tag check faults ignored)
  • 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 AArch64 TAGGED ADDRESS ABI and above. The corresponding regset is 1 element of 8 bytes (sizeof(long))).

Example of correct usage

MTE Example code

/*
 * 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 or asynchronous MTE
         * tag check faults (based on per-CPU preference) 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 | PR_MTE_TCF_ASYNC |
                  (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;
}