Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for shadow stack

Yu, Yu-cheng
2020-08-27T18:13:55Z

On 8/27/2020 6:36 AM, Florian Weimer wrote:
> * H. J. Lu:
>
>> On Thu, Aug 27, 2020 at 6:19 AM Florian Weimer <fweimer@redhat.com> wrote:
>>>
>>> * Dave Martin:
>>>
>>>> You're right that this has implications: for i386, libc probably pulls
>>>> more arguments off the stack than are really there in some situations.
>>>> This isn't a new problem though. There are already generic prctls with
>>>> fewer than 4 args that are used on x86.
>>>
>>> As originally posted, glibc prctl would have to know that it has to pull
>>> an u64 argument off the argument list for ARCH_X86_CET_DISABLE. But
>>> then the u64 argument is a problem for arch_prctl as well.
>>>
>>
>> Argument of ARCH_X86_CET_DISABLE is int and passed in register.
>
> The commit message and the C source say otherwise, I think (not sure
> about the C source, not a kernel hacker).
>

H.J. Lu suggested that we fix x86 arch_prctl() to take four arguments,
and then keep MMAP_SHSTK as an arch_prctl(). Because now the map flags
and size are all in registers, this also solves problems being pointed
out earlier. Without a wrapper, the shadow stack mmap call (from user
space) will be:

syscall(_NR_arch_prctl, ARCH_X86_CET_MMAP_SHSTK, size, MAP_32BIT).

I think this would be a nice alternative to another new syscall.

If this looks good to everyone, I can send out new patches as response
to my current version, and then after all issues fixed, send v12.

Thanks,
Yu-cheng