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

[Thomas Garnier <thgarnie@google.com>]

On Thu, Mar 9, 2017 at 8:05 AM, Mark Rutland <mark.rutland@arm.com> wrote:
> On Thu, Mar 09, 2017 at 07:56:49AM -0800, Thomas Garnier wrote:
>> On Thu, Mar 9, 2017 at 4:23 AM, Mark Rutland <mark.rutland@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!
>
> What I was trying to ask was do we need to touch the assembly at all
> here?

You don't but he generic solution add code to every single syscall.

> Are we trying to protect the non-syscall cases by doing this in
> assembly? If so, it'd be worth calling out in the commit message.

It is an added benefit but not required.

> If so, we could add the necessary helper to clear UAO.

I can look at set_fs and fix it on the next iteraiton.

> If not, doing this in the entry assembly only saves the small overhead
> of reading and comparing the addr_limit for in-kernel use of the
> syscalls (e.g. in the compat wrappers), and we may as well rely on the
> common !ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE implementation.

You also don't have the code added for each syscall and a call.

>
> Thanks,
> Mark.



-- 
Thomas