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

[Dave Hansen]

On 9/1/20 10:45 AM, Andy Lutomirski wrote:
>>> 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?
> I'm unconvinced that we want to use a somewhat precious PROT_ or VM_
> bit for this.  Using a flag bit makes sense if we expect anyone to
> ever map an fd or similar as a shadow stack, but that seems a bit odd
> in the first place.  To me, it seems more logical for a shadow stack
> to be a special sort of mapping with a special vm_ops, not a normal
> mapping with a special flag set.  Although I realize that we want
> shadow stacks to work like anonymous memory with respect to fork().
> Dave?

I actually don't like the idea of *creating* mappings much.

I think the pkey model has worked out pretty well where we separate
creating the mapping from doing something *to* it, like changing
protections.  For instance, it would be nice if we could preserve things
like using hugetlbfs or heck even doing KSM for shadow stacks.

If we're *creating* mappings, we've pretty much ruled out things like
hugetlbfs.

Something like mprotect_shstk() would allow an implementation today that
only works on anonymous memory *and* sets up a special vm_ops.  But, the
same exact ABI could do wonky stuff in the future if we decided we
wanted to do shadow stacks on DAX or hugetlbfs or whatever.

I don't really like the idea of PROT_SHSTK those are plumbed into a
bunch of interfaces.  But, I also can't deny that it seems to be working
fine for the arm64 folks.