Re: [PATCH v9 2/3] arm64: Define Documentation/arm64/tagged-address-abi.rst

[Andrey Konovalov <andreyknvl@google.com>]

On Wed, Aug 21, 2019 at 6:47 PM Catalin Marinas <catalin.marinas@arm.com> wrote:
>
> From: Vincenzo Frascino <vincenzo.frascino@arm.com>
>
> On AArch64 the TCR_EL1.TBI0 bit is set by default, allowing userspace
> (EL0) to perform memory accesses through 64-bit pointers with a non-zero
> top byte. Introduce the document describing the relaxation of the
> syscall ABI that allows userspace to pass certain tagged pointers to
> kernel syscalls.
>
> Cc: Will Deacon <will.deacon@arm.com>
> Cc: Andrey Konovalov <andreyknvl@google.com>
> Cc: Szabolcs Nagy <szabolcs.nagy@arm.com>
> Cc: Kevin Brodsky <kevin.brodsky@arm.com>
> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
> Co-developed-by: Catalin Marinas <catalin.marinas@arm.com>
> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Acked-by: Andrey Konovalov <andreyknvl@google.com>


> ---
>  Documentation/arm64/tagged-address-abi.rst | 156 +++++++++++++++++++++
>  1 file changed, 156 insertions(+)
>  create mode 100644 Documentation/arm64/tagged-address-abi.rst
>
> diff --git a/Documentation/arm64/tagged-address-abi.rst b/Documentation/arm64/tagged-address-abi.rst
> new file mode 100644
> index 000000000000..d4a85d535bf9
> --- /dev/null
> +++ b/Documentation/arm64/tagged-address-abi.rst
> @@ -0,0 +1,156 @@
> +==========================
> +AArch64 TAGGED ADDRESS ABI
> +==========================
> +
> +Authors: Vincenzo Frascino <vincenzo.frascino@arm.com>
> +         Catalin Marinas <catalin.marinas@arm.com>
> +
> +Date: 21 August 2019
> +
> +This document describes the usage and semantics of the Tagged Address
> +ABI on AArch64 Linux.
> +
> +1. Introduction
> +---------------
> +
> +On AArch64 the ``TCR_EL1.TBI0`` bit is set by default, allowing
> +userspace (EL0) to perform memory accesses through 64-bit pointers with
> +a non-zero top byte. This document describes the relaxation of the
> +syscall ABI that allows userspace to pass certain tagged pointers to
> +kernel syscalls.
> +
> +2. AArch64 Tagged Address ABI
> +-----------------------------
> +
> +From the kernel syscall interface perspective and for the purposes of
> +this document, a "valid tagged pointer" is a pointer with a potentially
> +non-zero top-byte that references an address in the user process address
> +space obtained in one of the following ways:
> +
> +- ``mmap()`` syscall where either:
> +
> +  - flags have the ``MAP_ANONYMOUS`` bit set or
> +  - the file descriptor refers to a regular file (including those
> +    returned by ``memfd_create()``) or ``/dev/zero``
> +
> +- ``brk()`` syscall (i.e. the heap area between the initial location of
> +  the program break at process creation and its current location).
> +
> +- any memory mapped by the kernel in the address space of the process
> +  during creation and with the same restrictions as for ``mmap()`` above
> +  (e.g. data, bss, stack).
> +
> +The AArch64 Tagged Address ABI has two stages of relaxation depending
> +how the user addresses are used by the kernel:
> +
> +1. User addresses not accessed by the kernel but used for address space
> +   management (e.g. ``mmap()``, ``mprotect()``, ``madvise()``). The use
> +   of valid tagged pointers in this context is always allowed.
> +
> +2. User addresses accessed by the kernel (e.g. ``write()``). This ABI
> +   relaxation is disabled by default and the application thread needs to
> +   explicitly enable it via ``prctl()`` as follows:
> +
> +   - ``PR_SET_TAGGED_ADDR_CTRL``: enable or disable the AArch64 Tagged
> +     Address ABI for the calling thread.
> +
> +     The ``(unsigned int) arg2`` argument is a bit mask describing the
> +     control mode used:
> +
> +     - ``PR_TAGGED_ADDR_ENABLE``: enable AArch64 Tagged Address ABI.
> +       Default status is disabled.
> +
> +     Arguments ``arg3``, ``arg4``, and ``arg5`` must be 0.
> +
> +   - ``PR_GET_TAGGED_ADDR_CTRL``: get the status of the AArch64 Tagged
> +     Address ABI for the calling thread.
> +
> +     Arguments ``arg2``, ``arg3``, ``arg4``, and ``arg5`` must be 0.
> +
> +   The ABI properties described above are thread-scoped, inherited on
> +   clone() and fork() and cleared on exec().
> +
> +   Calling ``prctl(PR_SET_TAGGED_ADDR_CTRL, PR_TAGGED_ADDR_ENABLE, 0, 0, 0)``
> +   returns ``-EINVAL`` if the AArch64 Tagged Address ABI is globally
> +   disabled by ``sysctl abi.tagged_addr_disabled=1``. The default
> +   ``sysctl abi.tagged_addr_disabled`` configuration is 0.
> +
> +When the AArch64 Tagged Address ABI is enabled for a thread, the
> +following behaviours are guaranteed:
> +
> +- All syscalls except the cases mentioned in section 3 can accept any
> +  valid tagged pointer.
> +
> +- The syscall behaviour is undefined for invalid tagged pointers: it may
> +  result in an error code being returned, a (fatal) signal being raised,
> +  or other modes of failure.
> +
> +- The syscall behaviour for a valid tagged pointer is the same as for
> +  the corresponding untagged pointer.
> +
> +
> +A definition of the meaning of tagged pointers on AArch64 can be found
> +in Documentation/arm64/tagged-pointers.rst.
> +
> +3. AArch64 Tagged Address ABI Exceptions
> +-----------------------------------------
> +
> +The following system call parameters must be untagged regardless of the
> +ABI relaxation:
> +
> +- ``prctl()`` other than pointers to user data either passed directly or
> +  indirectly as arguments to be accessed by the kernel.
> +
> +- ``ioctl()`` other than pointers to user data either passed directly or
> +  indirectly as arguments to be accessed by the kernel.
> +
> +- ``shmat()`` and ``shmdt()``.
> +
> +Any attempt to use non-zero tagged pointers may result in an error code
> +being returned, a (fatal) signal being raised, or other modes of
> +failure.
> +
> +4. Example of correct usage
> +---------------------------
> +.. code-block:: c
> +
> +   #include <stdlib.h>
> +   #include <string.h>
> +   #include <unistd.h>
> +   #include <sys/mman.h>
> +   #include <sys/prctl.h>
> +
> +   #define PR_SET_TAGGED_ADDR_CTRL     55
> +   #define PR_TAGGED_ADDR_ENABLE       (1UL << 0)
> +
> +   #define TAG_SHIFT           56
> +
> +   int main(void)
> +   {
> +       int tbi_enabled = 0;
> +       unsigned long tag = 0;
> +       char *ptr;
> +
> +       /* check/enable the tagged address ABI */
> +       if (!prctl(PR_SET_TAGGED_ADDR_CTRL, PR_TAGGED_ADDR_ENABLE, 0, 0, 0))
> +               tbi_enabled = 1;
> +
> +       /* memory allocation */
> +       ptr = mmap(NULL, sysconf(_SC_PAGE_SIZE), PROT_READ | PROT_WRITE,
> +                  MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> +       if (ptr == MAP_FAILED)
> +               return 1;
> +
> +       /* set a non-zero tag if the ABI is available */
> +       if (tbi_enabled)
> +               tag = rand() & 0xff;
> +       ptr = (char *)((unsigned long)ptr | (tag << TAG_SHIFT));
> +
> +       /* memory access to a tagged address */
> +       strcpy(ptr, "tagged pointer\n");
> +
> +       /* syscall with a tagged pointer */
> +       write(1, ptr, strlen(ptr));
> +
> +       return 0;
> +   }

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