Re: [PATCH v2 00/39] Shadowstacks for userspace

Kees Cook
2022-10-04T05:01:17Z

On Mon, Oct 03, 2022 at 07:25:03PM +0200, Jann Horn wrote:
> On Mon, Oct 3, 2022 at 7:04 PM Kees Cook <keescook@chromium.org> wrote:
> > On Thu, Sep 29, 2022 at 03:28:57PM -0700, Rick Edgecombe wrote:
> > > This is an overdue followup to the “Shadow stacks for userspace” CET series.
> > > Thanks for all the comments on the first version [0]. They drove a decent
> > > amount of changes for v2. Since it has been awhile, I’ll try to summarize the
> > > areas that got major changes since last time. Smaller changes are listed in
> > > each patch.
> >
> > Thanks for the write-up!
> >
> > > [...]
> > > GUP
> > > ---
> > > Shadow stack memory is generally treated as writable by the kernel, but
> > > it behaves differently then other writable memory with respect to GUP.
> > > FOLL_WRITE will not GUP shadow stack memory unless FOLL_FORCE is also
> > > set. Shadow stack memory is writable from the perspective of being
> > > changeable by userspace, but it is also protected memory from
> > > userspace’s perspective. So preventing it from being writable via
> > > FOLL_WRITE help’s make it harder for userspace to arbitrarily write to
> > > it. However, like read-only memory, FOLL_FORCE can still write through
> > > it. This means shadow stacks can be written to via things like
> > > “/proc/self/mem”. Apps that want extra security will have to prevent
> > > access to kernel features that can write with FOLL_FORCE.
> >
> > This seems like a problem to me -- the point of SS is that there cannot be
> > a way to write to them without specific instruction sequences. The fact
> > that /proc/self/mem bypasses memory protections was an old design mistake
> > that keeps leading to surprising behaviors. It would be much nicer to
> > draw the line somewhere and just say that FOLL_FORCE doesn't work on
> > VM_SHADOW_STACK. Why must FOLL_FORCE be allowed to write to SS?
>
> But once you have FOLL_FORCE, you can also just write over stuff like
> executable code instead of writing over the stack. I don't think
> allowing FOLL_FORCE writes over shadow stacks from /proc/$pid/mem is
> making things worse in any way, and it's probably helpful for stuff
> like debuggers.
>
> If you don't want /proc/$pid/mem to be able to do stuff like that,
> then IMO the way to go is to change when /proc/$pid/mem uses
> FOLL_FORCE, or to limit overall write access to /proc/$pid/mem.

Yeah, all reasonable. I just wish we could ditch FOLL_FORCE; it continues
to weird me out how powerful that fd's side-effects are.

--
Kees Cook