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

Yu, Yu-cheng
2020-09-01T17:23:41Z

On 9/1/2020 3:28 AM, Dave Martin wrote:
> On Thu, Aug 27, 2020 at 06:26:11AM -0700, H.J. Lu wrote:
>> On Wed, Aug 26, 2020 at 12:57 PM Dave Hansen <dave.hansen@intel.com> wrote:
>>>
>>> On 8/26/20 11:49 AM, Yu, Yu-cheng wrote:
>>>>> 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?
>>>
>>> No idea.
>>>
>>> But, mmap_special() is *basically* mmap2() with extra-big flags space.
>>> I suspect it will grow some more uses on top of shadow stacks. It could
>>> have, for instance, been used to allocate MPX bounds tables.
>>
>> There is no reason we can't use
>>
>> long arch_prctl (int, unsigned long, unsigned long, unsigned long, ..);
>>
>> for ARCH_X86_CET_MMAP_SHSTK. We just need to use
>>
>> syscall (SYS_arch_prctl, ARCH_X86_CET_MMAP_SHSTK, ...);
>
>
> For arm64 (and sparc etc.) we continue to use the regular mmap/mprotect
> family of calls. One or two additional arch-specific mmap flags are
> sufficient for now.
>
> Is x86 definitely not going to fit within those calls?

That can work for x86. Andy, what if we create PROT_SHSTK, which can
been seen only from the user. Once in kernel, it is translated to
VM_SHSTK. One question for mremap/mprotect is, do we allow a normal
data area to become shadow stack?

>
> For now, I can't see what arg[2] is used for (and hence the type
> argument of mmap_special()), but I haven't dug through the whole series.

If we use the approach above, then we don't need arch_prctl changes.

Thanks,
Yu-cheng