2019-08-22 00:47:29 +08:00
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==========================
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AArch64 TAGGED ADDRESS ABI
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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: 21 August 2019
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This document describes the usage and semantics of the Tagged Address
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ABI on AArch64 Linux.
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1. Introduction
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---------------
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On AArch64 the ``TCR_EL1.TBI0`` bit is set by default, allowing
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userspace (EL0) to perform memory accesses through 64-bit pointers with
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a non-zero top byte. This document describes the relaxation of the
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syscall ABI that allows userspace to pass certain tagged pointers to
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kernel syscalls.
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2. AArch64 Tagged Address ABI
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-----------------------------
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From the kernel syscall interface perspective and for the purposes of
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this document, a "valid tagged pointer" is a pointer with a potentially
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non-zero top-byte that references an address in the user process address
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space obtained in one of the following ways:
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- ``mmap()`` syscall where either:
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- flags have the ``MAP_ANONYMOUS`` bit set or
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- the file descriptor refers to a regular file (including those
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returned by ``memfd_create()``) or ``/dev/zero``
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- ``brk()`` syscall (i.e. the heap area between the initial location of
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the program break at process creation and its current location).
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- any memory mapped by the kernel in the address space of the process
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during creation and with the same restrictions as for ``mmap()`` above
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(e.g. data, bss, stack).
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The AArch64 Tagged Address ABI has two stages of relaxation depending
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how the user addresses are used by the kernel:
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1. User addresses not accessed by the kernel but used for address space
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2020-02-19 20:31:56 +08:00
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management (e.g. ``mprotect()``, ``madvise()``). The use of valid
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tagged pointers in this context is allowed with the exception of
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``brk()``, ``mmap()`` and the ``new_address`` argument to
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``mremap()`` as these have the potential to alias with existing
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user addresses.
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NOTE: This behaviour changed in v5.6 and so some earlier kernels may
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incorrectly accept valid tagged pointers for the ``brk()``,
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``mmap()`` and ``mremap()`` system calls.
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2019-08-22 00:47:29 +08:00
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2. User addresses accessed by the kernel (e.g. ``write()``). This ABI
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relaxation is disabled by default and the application thread needs to
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explicitly enable it via ``prctl()`` as follows:
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- ``PR_SET_TAGGED_ADDR_CTRL``: enable or disable the AArch64 Tagged
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Address ABI for the calling thread.
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The ``(unsigned int) arg2`` argument is a bit mask describing the
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control mode used:
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- ``PR_TAGGED_ADDR_ENABLE``: enable AArch64 Tagged Address ABI.
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Default status is disabled.
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Arguments ``arg3``, ``arg4``, and ``arg5`` must be 0.
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- ``PR_GET_TAGGED_ADDR_CTRL``: get the status of the AArch64 Tagged
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Address ABI for the calling thread.
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Arguments ``arg2``, ``arg3``, ``arg4``, and ``arg5`` must be 0.
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The ABI properties described above are thread-scoped, inherited on
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clone() and fork() and cleared on exec().
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Calling ``prctl(PR_SET_TAGGED_ADDR_CTRL, PR_TAGGED_ADDR_ENABLE, 0, 0, 0)``
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returns ``-EINVAL`` if the AArch64 Tagged Address ABI is globally
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disabled by ``sysctl abi.tagged_addr_disabled=1``. The default
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``sysctl abi.tagged_addr_disabled`` configuration is 0.
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When the AArch64 Tagged Address ABI is enabled for a thread, the
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following behaviours are guaranteed:
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- All syscalls except the cases mentioned in section 3 can accept any
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valid tagged pointer.
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- The syscall behaviour is undefined for invalid tagged pointers: it may
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result in an error code being returned, a (fatal) signal being raised,
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or other modes of failure.
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- The syscall behaviour for a valid tagged pointer is the same as for
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the corresponding untagged pointer.
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A definition of the meaning of tagged pointers on AArch64 can be found
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in Documentation/arm64/tagged-pointers.rst.
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3. AArch64 Tagged Address ABI Exceptions
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-----------------------------------------
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The following system call parameters must be untagged regardless of the
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ABI relaxation:
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- ``prctl()`` other than pointers to user data either passed directly or
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indirectly as arguments to be accessed by the kernel.
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- ``ioctl()`` other than pointers to user data either passed directly or
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indirectly as arguments to be accessed by the kernel.
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- ``shmat()`` and ``shmdt()``.
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Any attempt to use non-zero tagged pointers may result in an error code
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being returned, a (fatal) signal being raised, or other modes of
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failure.
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4. Example of correct usage
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---------------------------
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.. code-block:: c
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#include <stdlib.h>
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#include <string.h>
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#include <unistd.h>
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#include <sys/mman.h>
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#include <sys/prctl.h>
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#define PR_SET_TAGGED_ADDR_CTRL 55
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#define PR_TAGGED_ADDR_ENABLE (1UL << 0)
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#define TAG_SHIFT 56
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int main(void)
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{
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int tbi_enabled = 0;
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unsigned long tag = 0;
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char *ptr;
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/* check/enable the tagged address ABI */
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if (!prctl(PR_SET_TAGGED_ADDR_CTRL, PR_TAGGED_ADDR_ENABLE, 0, 0, 0))
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tbi_enabled = 1;
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/* memory allocation */
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ptr = mmap(NULL, sysconf(_SC_PAGE_SIZE), PROT_READ | PROT_WRITE,
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MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
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if (ptr == MAP_FAILED)
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return 1;
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/* set a non-zero tag if the ABI is available */
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if (tbi_enabled)
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tag = rand() & 0xff;
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ptr = (char *)((unsigned long)ptr | (tag << TAG_SHIFT));
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/* memory access to a tagged address */
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strcpy(ptr, "tagged pointer\n");
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/* syscall with a tagged pointer */
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write(1, ptr, strlen(ptr));
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return 0;
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}
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