Re: [PATCH v2 22/39] mm: Don't allow write GUPs to shadow stack memory

Kees Cook
2022-10-04T04:22:15Z

On Mon, Oct 03, 2022 at 03:49:18PM -0700, Andy Lutomirski wrote:
> On 10/3/22 11:39, Kees Cook wrote:
> > On Thu, Sep 29, 2022 at 03:29:19PM -0700, Rick Edgecombe wrote:
> > > [...]
> > > Still allow FOLL_FORCE to write through shadow stack protections, as it
> > > does for read-only protections.
> >
> > As I asked in the cover letter: why do we need to add this for shstk? It
> > was a mistake for general memory. :P
>
> For debuggers, which use FOLL_FORCE, quite intentionally, to modify text.
> And once a debugger has ptrace write access to a target, shadow stacks
> provide exactly no protection -- ptrace can modify text and all registers.

i.e. via ptrace? Yeah, I grudgingly accept the ptrace need for
FOLL_FORCE.

> But /proc/.../mem may be a different story, and I'd be okay with having
> FOLL_PROC_MEM for legacy compatibility via /proc/.../mem and not allowing
> that to access shadow stacks. This does seem like it may not be very
> useful, though.

I *really* don't like the /mem use of FOLL_FORCE, though. I think the
rationale has been "using PTRACE_POKE is too slow". Again, I can live
with it, I was just hoping we could avoid expanding that questionable
behavior, especially since it's a bypass of WRSS.

--
Kees Cook