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

[Andy Lutomirski]

> On Aug 27, 2020, at 11:13 AM, Yu, Yu-cheng <yu-cheng.yu@xxxxxxxxx> 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@xxxxxxxxxx> 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.