[PATCH v2 4/4] arm64/syscalls: Specific usage of verify_pre_usermode_state

[Thomas Garnier]

On Thu, Mar 9, 2017 at 4:23 AM, Mark Rutland <mark.rutland at arm.com> wrote:
> We generally stick to lower case for the arm64 assembly macros. If we
> need this, we should stick to the existing convention.
>
>> +/* Similar to set_fs(USER_DS) in verify_pre_usermode_state without a warning. */
>> +.macro VERIFY_PRE_USERMODE_STATE
>> +     mov     x1, #TASK_SIZE_64
>> +     str     x1, [tsk, #TSK_TI_ADDR_LIMIT]
>> +.endm
>
> We need arm64's set_fs() to configure UAO, too, so this is much weaker
> than set_fs(), and will leave __{get,put}_user and
> __copy_{to,from}_user() able to access kernel memory.
>
> We don't currently have an asm helper to clear UAO, and unconditionally
> poking that on exception return is liable to be somewhat expensive.
>
> Also, given we're only trying to catch this in syscalls, I'm afraid I
> don't see what we gain by doing this in the entry assembly.
>

I optimized all architectures from the arm (32-bit) discussion. I will
come back to a simple bl to the verify function. Thanks!
-- 
Thomas