Re: [PATCH v4 1/2] arm64: Define Documentation/arm64/tagged-address-abi.txt

[Szabolcs Nagy]

On 12/06/2019 15:21, 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>
> ---
>  Documentation/arm64/tagged-address-abi.txt | 111 +++++++++++++++++++++
>  1 file changed, 111 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..96e149e2c55c
> --- /dev/null
> +++ b/Documentation/arm64/tagged-address-abi.txt
> @@ -0,0 +1,111 @@
> +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 arm64 kernel,
> +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.
> +
> +This document describes a relaxation of the ABI with which it is possible
> +to pass tagged tagged pointers to the syscalls, when these pointers are in
           ^^^^^^^^^^^^^
typo.

> +memory ranges obtained as described in paragraph 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.
> +
> +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()
> +command without having to reconfigure the kernel.
> +
> +The ABI properties are inherited by threads of the same application and
> +fork()'ed children but cleared when a new process is spawn (execve()).

OK.

> +
> +2. ARM64 Tagged Address ABI
> +---------------------------
> +
> +From the kernel syscall interface prospective, we define, for the purposes
                                     ^^^^^^^^^^^
perspective

> +of this document, a "valid tagged pointer" as a pointer that either it has
> +a zero value set in the top byte or it 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 = MAP_PRIVATE | MAP_ANONYMOUS
> +    * flags = MAP_PRIVATE and the file descriptor refers to a regular
> +      file or "/dev/zero"

this does not make it clear if MAP_FIXED or other
flags are valid (there are many map flags i don't
know, but at least fixed should work and stack/growsdown.
i'd expect anything that's not incompatible with
private|anon to work).

> +  - a mapping below sbrk(0) done by the process itself

doesn't the mmap rule cover this?

> +  - 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).

OK.

Can a null pointer have a tag?
(in case NULL is valid to pass to a syscall)

> +
> +The ARM64 Tagged Address ABI is an opt-in feature, and an application can
> +control it using the following prctl()s:
> +  - PR_SET_TAGGED_ADDR_CTRL: can be used to enable the Tagged Address ABI.
> +  - PR_GET_TAGGED_ADDR_CTRL: can be used to check the status of the Tagged
> +                             Address ABI.
> +
> +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 in all the cases except the ones documented in the
> +    "Preserving tags" paragraph of tagged-pointers.txt.

OK.

i guess pointers of another process are not "valid tagged
pointers" for the current one, so e.g. in ptrace the
ptracer has to clear the tags before PEEK etc.

> +
> +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 paragraph 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.

i don't understand the exception: does it mean
that passing a tagged address to these syscalls
is undefined?

> +
> +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)
> +		return -1;

mmap returns MAP_FAILED on failure.

> +
> +	if (tbi_enabled)
> +		tag = rand() & 0xff;
> +
> +	ptr = (char *)((unsigned long)ptr | (tag << TAG_SHIFT));
> +
> +	*ptr = 'a';
> +
> +	...
> +}
> +
>