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

Yu, Yu-cheng
2020-08-26T18:49:28Z

On 8/26/2020 10:04 AM, Andy Lutomirski wrote:
> On Wed, Aug 26, 2020 at 9:52 AM Florian Weimer <fweimer@redhat.com> wrote:
>>
>> * Dave Martin:
>>
>>> On Tue, Aug 25, 2020 at 04:34:27PM -0700, Yu, Yu-cheng wrote:
>>>> On 8/25/2020 4:20 PM, Dave Hansen wrote:
>>>>> On 8/25/20 2:04 PM, Yu, Yu-cheng wrote:
>>>>>>>> I think this is more arch-specific. Even if it becomes a new syscall,
>>>>>>>> we still need to pass the same parameters.
>>>>>>>
>>>>>>> Right, but without the copying in and out of memory.
>>>>>>>
>>>>>> Linux-api is already on the Cc list. Do we need to add more people to
>>>>>> get some agreements for the syscall?
>>>>> What kind of agreement are you looking for? I'd suggest just coding it
>>>>> up and posting the patches. Adding syscalls really is really pretty
>>>>> straightforward and isn't much code at all.
>>>>>
>>>>
>>>> Sure, I will do that.
>>>
>>> Alternatively, would a regular prctl() work here?
>>
>> Is this something appliation code has to call, or just the dynamic
>> loader?
>>
>> prctl in glibc is a variadic function, so if there's a mismatch between
>> the kernel/userspace syscall convention and the userspace calling
>> convention (for variadic functions) for specific types, it can't be made
>> to work in a generic way.
>>
>> The loader can use inline assembly for system calls and does not have
>> this issue, but applications would be implcated by it.
>>
>
> I would expect things like Go and various JITs to call it directly.
>
> If we wanted to be fancy and add a potentially more widely useful
> syscall, how about:
>
> mmap_special(void *addr, size_t length, int prot, int flags, int type);
>
> Where type is something like MMAP_SPECIAL_X86_SHSTK. Fundamentally,
> this is really just mmap() except that we want to map something a bit
> magical, and we don't want to require opening a device node to do it.
>

One benefit of MMAP_SPECIAL_* is there are more free bits than MAP_*.
Does ARM have similar needs for memory mapping, Dave?

Yu-cheng