Re: [PATCH v5 1/2] arm64: Define Documentation/arm64/tagged-address-abi.txt

[Szabolcs Nagy]

On 13/06/2019 16:51, Vincenzo Frascino wrote:
> On arm64 the TCR_EL1.TBI0 bit has been always enabled hence
> the userspace (EL0) is allowed to set a non-zero value in the
> top byte but the resulting pointers are not allowed at the
> user-kernel syscall ABI boundary.
> 
> With the relaxed ABI proposed through this document, it is now possible
> to pass tagged pointers to the syscalls, when these pointers are in
> memory ranges obtained by an anonymous (MAP_ANONYMOUS) mmap().
> 
> This change in the ABI requires a mechanism to requires the userspace
> to opt-in to such an option.
> 
> Specify and document the way in which sysctl and prctl() can be used
> in combination to allow the userspace to opt-in this feature.
> 
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>
> CC: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>

Acked-by: Szabolcs Nagy <szabolcs.nagy@xxxxxxx>

> ---
>  Documentation/arm64/tagged-address-abi.txt | 134 +++++++++++++++++++++
>  1 file changed, 134 insertions(+)
>  create mode 100644 Documentation/arm64/tagged-address-abi.txt
> 
> diff --git a/Documentation/arm64/tagged-address-abi.txt b/Documentation/arm64/tagged-address-abi.txt
> new file mode 100644
> index 000000000000..0ae900d4bb2d
> --- /dev/null
> +++ b/Documentation/arm64/tagged-address-abi.txt
> @@ -0,0 +1,134 @@
> +ARM64 TAGGED ADDRESS ABI
> +========================
> +
> +This document describes the usage and semantics of the Tagged Address
> +ABI on arm64.
> +
> +1. Introduction
> +---------------
> +
> +On arm64 the TCR_EL1.TBI0 bit has been always enabled on the kernel, hence
> +the userspace (EL0) is entitled to perform a user memory access through a
> +64-bit pointer with a non-zero top byte but the resulting pointers are not
> +allowed at the user-kernel syscall ABI boundary.
> +
> +This document describes a relaxation of the ABI that makes it possible to
> +to pass tagged pointers to the syscalls, when these pointers are in memory
> +ranges obtained as described in section 2.
> +
> +Since it is not desirable to relax the ABI to allow tagged user addresses
> +into the kernel indiscriminately, arm64 provides a new sysctl interface
> +(/proc/sys/abi/tagged_addr) that is used to prevent the applications from
> +enabling the relaxed ABI and a new prctl() interface that can be used to
> +enable or disable the relaxed ABI.
> +A detailed description of the newly introduced mechanisms will be provided
> +in section 2.
> +
> +2. ARM64 Tagged Address ABI
> +---------------------------
> +
> +From the kernel syscall interface perspective, we define, for the purposes
> +of this document, a "valid tagged pointer" as a pointer that either has a
> +zero value set in the top byte or has a non-zero value, it is in memory
> +ranges privately owned by a userspace process and it is obtained in one of
> +the following ways:
> +  - mmap() done by the process itself, where either:
> +    * flags have MAP_PRIVATE and MAP_ANONYMOUS
> +    * flags have MAP_PRIVATE and the file descriptor refers to a regular
> +      file or "/dev/zero"
> +  - brk() system call done by the process itself (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 process's address space during
> +    creation and following the restrictions presented above (i.e. data, bss,
> +    stack).
> +
> +The ARM64 Tagged Address ABI is an opt-in feature, and an application can
> +control it using the following:
> + - /proc/sys/abi/tagged_addr: a new sysctl interface that can be used to
> +        prevent the applications from enabling the relaxed ABI.
> +        The sysctl is meant also for testing purposes in order to provide a
> +        simple way for the userspace to verify the return error checking of
> +        the prctl() commands without having to reconfigure the kernel.
> +        The sysctl supports the following configuration options:
> +         - 0: Disable ARM64 Tagged Address ABI for all the applications.
> +         - 1 (Default): Enable ARM64 Tagged Address ABI for all the
> +                        applications.
> +        If the ARM64 Tagged Address ABI is disabled at a certain point in
> +        time, all the applications that were using tagging before this event
> +        occurs, will continue to use tagging.
> +
> + - prctl()s:
> +  - PR_SET_TAGGED_ADDR_CTRL: can be used to enable or disable the Tagged
> +        Address ABI.
> +        The (unsigned int) arg2 argument is a bit mask describing the
> +        control mode used:
> +          - PR_TAGGED_ADDR_ENABLE: Enable ARM64 Tagged Address ABI.
> +        The arguments arg3, arg4, and arg5 are ignored.
> +
> +  - PR_GET_TAGGED_ADDR_CTRL: can be used to check the status of the Tagged
> +        Address ABI.
> +        The arguments arg2, arg3, arg4, and arg5 are ignored.
> +
> +The ABI properties set by the mechanisms described above are inherited by threads
> +of the same application and fork()'ed children but cleared by execve().
> +
> +As a consequence of invoking PR_SET_TAGGED_ADDR_CTRL prctl() by an applications,
> +the ABI guarantees the following behaviours:
> +
> +  - Every current or newly introduced syscall can accept any valid tagged
> +    pointers.
> +
> +  - If a non valid tagged pointer is passed to a syscall then the behaviour
> +    is undefined.
> +
> +  - Every valid tagged pointer is expected to work as an untagged one.
> +
> +  - The kernel preserves any valid tagged pointers and returns them to the
> +    userspace unchanged (i.e. on syscall return) in all the cases except the
> +    ones documented in the "Preserving tags" section of tagged-pointers.txt.
> +
> +A definition of the meaning of tagged pointers on arm64 can be found in:
> +Documentation/arm64/tagged-pointers.txt.
> +
> +3. ARM64 Tagged Address ABI Exceptions
> +--------------------------------------
> +
> +The behaviours described in section 2, with particular reference to the
> +acceptance by the syscalls of any valid tagged pointer are not applicable
> +to the following cases:
> +  - mmap() addr parameter.
> +  - mremap() new_address parameter.
> +  - prctl_set_mm() struct prctl_map fields.
> +  - prctl_set_mm_map() struct prctl_map fields.
> +
> +Any attempt to use non-zero tagged pointers will lead to undefined behaviour.
> +
> +4. Example of correct usage
> +---------------------------
> +
> +void main(void)
> +{
> +	static int tbi_enabled = 0;
> +	unsigned long tag = 0;
> +
> +	char *ptr = mmap(NULL, PAGE_SIZE, PROT_READ | PROT_WRITE,
> +			 MAP_ANONYMOUS, -1, 0);
> +
> +	if (prctl(PR_SET_TAGGED_ADDR_CTRL, PR_TAGGED_ADDR_ENABLE,
> +		  0, 0, 0) == 0)
> +		tbi_enabled = 1;
> +
> +	if (ptr == (void *)-1) /* MAP_FAILED */
> +		return -1;
> +
> +	if (tbi_enabled)
> +		tag = rand() & 0xff;
> +
> +	ptr = (char *)((unsigned long)ptr | (tag << TAG_SHIFT));
> +
> +	*ptr = 'a';
> +
> +	...
> +}
> +
>