Re: [PATCH v7 33/41] x86/shstk: Introduce map_shadow_stack syscall

Deepak Gupta
2023-03-16T19:31:00Z

On Tue, Mar 14, 2023 at 12:19 AM Mike Rapoport <rppt@kernel.org> wrote:
>
> Hi,
>
> On Thu, Mar 09, 2023 at 10:55:11AM -0800, Deepak Gupta wrote:
> > On Thu, Mar 02, 2023 at 05:22:07PM +0000, Szabolcs Nagy wrote:
> > > The 02/27/2023 14:29, Rick Edgecombe wrote:
> > > > Previously, a new PROT_SHADOW_STACK was attempted,
> > > ...
> > > > So rather than repurpose two existing syscalls (mmap, madvise) that don't
> > > > quite fit, just implement a new map_shadow_stack syscall to allow
> > > > userspace to map and setup new shadow stacks in one step. While ucontext
> > > > is the primary motivator, userspace may have other unforeseen reasons to
> > > > setup it's own shadow stacks using the WRSS instruction. Towards this
> > > > provide a flag so that stacks can be optionally setup securely for the
> > > > common case of ucontext without enabling WRSS. Or potentially have the
> > > > kernel set up the shadow stack in some new way.
> > > ...
> > > > The following example demonstrates how to create a new shadow stack with
> > > > map_shadow_stack:
> > > > void *shstk = map_shadow_stack(addr, stack_size, SHADOW_STACK_SET_TOKEN);
> > >
> > > i think
> > >
> > > mmap(addr, size, PROT_READ, MAP_ANON|MAP_SHADOW_STACK, -1, 0);
> > >
> > > could do the same with less disruption to users (new syscalls
> > > are harder to deal with than new flags). it would do the
> > > guard page and initial token setup too (there is no flag for
> > > it but could be squeezed in).
> >
> > Discussion on this topic in v6
> > https://lore.kernel.org/all/20230223000340.GB945966@debug.ba.rivosinc.com/
> >
> > Again I know earlier CET patches had protection flag and somehow due to pushback
> > on mailing list, it was adopted to go for special syscall because no one else
> > had shadow stack.
> >
> > Seeing a response from Szabolcs, I am assuming arm4 would also want to follow
> > using mmap to manufacture shadow stack. For reference RFC patches for risc-v shadow stack,
> > use a new protection flag = PROT_SHADOWSTACK.
> > https://lore.kernel.org/lkml/20230213045351.3945824-1-debug@rivosinc.com/
> >
> > I know earlier discussion had been that we let this go and do a re-factor later as other
> > arch support trickle in. But as I thought more on this and I think it may just be
> > messy from user mode point of view as well to have cognition of two different ways of
> > creating shadow stack. One would be special syscall (in current libc) and another `mmap`
> > (whenever future re-factor happens)
> >
> > If it's not too late, it would be more wise to take `mmap`
> > approach rather than special `syscall` approach.
>
> I disagree.
>
> Having shadow stack flags for mmap() adds unnecessary complexity to the
> core-mm, while having a dedicated syscall hides all the details in the
> architecture specific code.

Again reiterating it would've made sense if only x86 had a shadow stack.
aarch64 announced support for guarded stack. risc-v spec is in
development to support shadow stack.

So there will be shadow stack related flow in these arches.

>
> Another reason to use a dedicated system call allows for better
> extensibility if/when we'd need to update the way shadow stack VMA is
> created.

I see two valid points here
- Shadow stack doesn't need conversion into different memory types
(which is usually the case for address ranges created by mmap)
So there is a static page permissions on shadow stack which is
not mutable.

- Future feature addition (if there is one needed) at the time of
shadow stack creation
It would avoid future tax on mmap

I'll think more about this.

>
> As for the userspace convenience, it is anyway required to add special
> code for creating the shadow stack and it wouldn't matter if that code
> would use mmap(NEW_FLAG) or map_shadow_stack().

Yes *strictly* from userspace convenience, it doesn't matter which option.

>
> > > most of the mmap features need not be available (EINVAL) when
> > > MAP_SHADOW_STACK is specified.
> > >
> > > the main drawback is running out of mmap flags so extension
> > > is limited. (but the new syscall has limitations too).
>
> --
> Sincerely yours,
> Mike.