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

Yu, Yu-cheng
2020-08-27T19:34:00Z

On 8/27/2020 11:56 AM, Andy Lutomirski wrote:
>
>
>> On Aug 27, 2020, at 11:13 AM, Yu, Yu-cheng <yu-cheng.yu@intel.com> wrote:
>>
>> ﻿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 admit I don’t see a show stopping technical reason we can’t add arguments to an existing syscall, but I’m pretty sure it’s unprecedented, and it doesn’t seem like a good idea.
>

There are nine existing arch_prctl calls now. If the concern is the
extra new arguments getting misused, we can mask them out for the
existing calls. Otherwise, I have not seen anything that can break.

Yu-cheng