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

[H.J. Lu]

On Wed, Aug 26, 2020 at 11:49 AM Yu, Yu-cheng <yu-cheng.yu@xxxxxxxxx> wrote:
>
> On 8/26/2020 10:04 AM, Andy Lutomirski wrote:
> > On Wed, Aug 26, 2020 at 9:52 AM Florian Weimer <fweimer@xxxxxxxxxx> 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?
>

arch/arm64/include/uapi/asm/mman.h:

#define PROT_BTI   0x10     /* BTI guarded page */

-- 
H.J.