Documentation/arm64/tagged-address-abi.rst

   1 ==========================
   2 AArch64 TAGGED ADDRESS ABI
   3 ==========================
   4
   5 Authors: Vincenzo Frascino <vincenzo.frascino@arm.com>
   6          Catalin Marinas <catalin.marinas@arm.com>
   7
   8 Date: 21 August 2019
   9
  10 This document describes the usage and semantics of the Tagged Address
  11 ABI on AArch64 Linux.
  12
  13 1. Introduction
  14 ---------------
  15
  16 On AArch64 the ``TCR_EL1.TBI0`` bit is set by default, allowing
  17 userspace (EL0) to perform memory accesses through 64-bit pointers with
  18 a non-zero top byte. This document describes the relaxation of the
  19 syscall ABI that allows userspace to pass certain tagged pointers to
  20 kernel syscalls.
  21
  22 2. AArch64 Tagged Address ABI
  23 -----------------------------
  24
  25 From the kernel syscall interface perspective and for the purposes of
  26 this document, a "valid tagged pointer" is a pointer with a potentially
  27 non-zero top-byte that references an address in the user process address
  28 space obtained in one of the following ways:
  29
  30 - ``mmap()`` syscall where either:
  31
  32   - flags have the ``MAP_ANONYMOUS`` bit set or
  33   - the file descriptor refers to a regular file (including those
  34     returned by ``memfd_create()``) or ``/dev/zero``
  35
  36 - ``brk()`` syscall (i.e. the heap area between the initial location of
  37   the program break at process creation and its current location).
  38
  39 - any memory mapped by the kernel in the address space of the process
  40   during creation and with the same restrictions as for ``mmap()`` above
  41   (e.g. data, bss, stack).
  42
  43 The AArch64 Tagged Address ABI has two stages of relaxation depending on
  44 how the user addresses are used by the kernel:
  45
  46 1. User addresses not accessed by the kernel but used for address space
  47    management (e.g. ``mprotect()``, ``madvise()``). The use of valid
  48    tagged pointers in this context is allowed with the exception of
  49    ``brk()``, ``mmap()`` and the ``new_address`` argument to
  50    ``mremap()`` as these have the potential to alias with existing
  51    user addresses.
  52
  53    NOTE: This behaviour changed in v5.6 and so some earlier kernels may
  54    incorrectly accept valid tagged pointers for the ``brk()``,
  55    ``mmap()`` and ``mremap()`` system calls.
  56
  57 2. User addresses accessed by the kernel (e.g. ``write()``). This ABI
  58    relaxation is disabled by default and the application thread needs to
  59    explicitly enable it via ``prctl()`` as follows:
  60
  61    - ``PR_SET_TAGGED_ADDR_CTRL``: enable or disable the AArch64 Tagged
  62      Address ABI for the calling thread.
  63
  64      The ``(unsigned int) arg2`` argument is a bit mask describing the
  65      control mode used:
  66
  67      - ``PR_TAGGED_ADDR_ENABLE``: enable AArch64 Tagged Address ABI.
  68        Default status is disabled.
  69
  70      Arguments ``arg3``, ``arg4``, and ``arg5`` must be 0.
  71
  72    - ``PR_GET_TAGGED_ADDR_CTRL``: get the status of the AArch64 Tagged
  73      Address ABI for the calling thread.
  74
  75      Arguments ``arg2``, ``arg3``, ``arg4``, and ``arg5`` must be 0.
  76
  77    The ABI properties described above are thread-scoped, inherited on
  78    clone() and fork() and cleared on exec().
  79
  80    Calling ``prctl(PR_SET_TAGGED_ADDR_CTRL, PR_TAGGED_ADDR_ENABLE, 0, 0, 0)``
  81    returns ``-EINVAL`` if the AArch64 Tagged Address ABI is globally
  82    disabled by ``sysctl abi.tagged_addr_disabled=1``. The default
  83    ``sysctl abi.tagged_addr_disabled`` configuration is 0.
  84
  85 When the AArch64 Tagged Address ABI is enabled for a thread, the
  86 following behaviours are guaranteed:
  87
  88 - All syscalls except the cases mentioned in section 3 can accept any
  89   valid tagged pointer.
  90
  91 - The syscall behaviour is undefined for invalid tagged pointers: it may
  92   result in an error code being returned, a (fatal) signal being raised,
  93   or other modes of failure.
  94
  95 - The syscall behaviour for a valid tagged pointer is the same as for
  96   the corresponding untagged pointer.
  97
  98
  99 A definition of the meaning of tagged pointers on AArch64 can be found
 100 in Documentation/arm64/tagged-pointers.rst.
 101
 102 3. AArch64 Tagged Address ABI Exceptions
 103 -----------------------------------------
 104
 105 The following system call parameters must be untagged regardless of the
 106 ABI relaxation:
 107
 108 - ``prctl()`` other than pointers to user data either passed directly or
 109   indirectly as arguments to be accessed by the kernel.
 110
 111 - ``ioctl()`` other than pointers to user data either passed directly or
 112   indirectly as arguments to be accessed by the kernel.
 113
 114 - ``shmat()`` and ``shmdt()``.
 115
 116 - ``brk()`` (since kernel v5.6).
 117
 118 - ``mmap()`` (since kernel v5.6).
 119
 120 - ``mremap()``, the ``new_address`` argument (since kernel v5.6).
 121
 122 Any attempt to use non-zero tagged pointers may result in an error code
 123 being returned, a (fatal) signal being raised, or other modes of
 124 failure.
 125
 126 4. Example of correct usage
 127 ---------------------------
 128 .. code-block:: c
 129
 130    #include <stdlib.h>
 131    #include <string.h>
 132    #include <unistd.h>
 133    #include <sys/mman.h>
 134    #include <sys/prctl.h>
 135
 136    #define PR_SET_TAGGED_ADDR_CTRL      55
 137    #define PR_TAGGED_ADDR_ENABLE        (1UL << 0)
 138
 139    #define TAG_SHIFT            56
 140
 141    int main(void)
 142    {
 143         int tbi_enabled = 0;
 144         unsigned long tag = 0;
 145         char *ptr;
 146
 147         /* check/enable the tagged address ABI */
 148         if (!prctl(PR_SET_TAGGED_ADDR_CTRL, PR_TAGGED_ADDR_ENABLE, 0, 0, 0))
 149                 tbi_enabled = 1;
 150
 151         /* memory allocation */
 152         ptr = mmap(NULL, sysconf(_SC_PAGE_SIZE), PROT_READ | PROT_WRITE,
 153                    MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
 154         if (ptr == MAP_FAILED)
 155                 return 1;
 156
 157         /* set a non-zero tag if the ABI is available */
 158         if (tbi_enabled)
 159                 tag = rand() & 0xff;
 160         ptr = (char *)((unsigned long)ptr | (tag << TAG_SHIFT));
 161
 162         /* memory access to a tagged address */
 163         strcpy(ptr, "tagged pointer\n");
 164
 165         /* syscall with a tagged pointer */
 166         write(1, ptr, strlen(ptr));
 167
 168         return 0;
 169    }