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

[Andy Lutomirski]

On Tue, Sep 1, 2020 at 10:23 AM Yu, Yu-cheng <yu-cheng.yu@xxxxxxxxx> wrote:
>
> 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@xxxxxxxxx> 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?

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?

--Andy