Re: [PATCH 00/35] Shadow stacks for userspace

[H.J. Lu]

On Tue, May 31, 2022 at 10:34 AM Edgecombe, Rick P
<rick.p.edgecombe@xxxxxxxxx> wrote:
>
> On Tue, 2022-05-31 at 19:36 +0300, Mike Rapoport wrote:
> > > WRSS is a feature where you would usually want to lock it as
> > > disabled,
> > > but WRSS cannot be enabled if shadow stack is not enabled. Locking
> > > shadow stack and WRSS off together doesn't have any security
> > > benefits
> > > in theory. so I'm thinking glibc doesn't need to do this. The
> > > kernel
> > > could even refuse to lock WRSS without shadow stack being enabled.
> > > Could we avoid the extra ptrace functionality then?
> >
> > What I see for is that a program can support shadow stack, glibc
> > enables
> > shadow stack, does not enable WRSS and than calls
> >
> >         arch_prctl(ARCH_X86_FEATURE_LOCK,
> >                    LINUX_X86_FEATURE_SHSTK | LINUX_X86_FEATURE_WRSS);
>
> I see the logic is glibc will lock SHSTK|IBT if either is enabled in
> the elf header. I guess that is why I didn't see the locking happening
> for me, because my manual enablement test doesn't have either set in
> the header.
>
> It can't see where that glibc knows about WRSS though...
>
> The glibc logic seems wrong to me also, because shadow stack or IBT
> could be force-disabled via glibc tunables. I don't see why the elf
> header bit should exclusively control the feature locking. Or why both
> should be locked if only one is in the header.

glibc locks SHSTK and IBT only if they are enabled at run-time. It doesn't
enable/disable/lock WRSS at the moment.  If WRSS can be enabled
via arch_prctl at any time, we can't lock it.  If WRSS should be locked early,
how should it be enabled in application?  Also can WRSS be enabled
from a dlopened object?

> >
> > so that WRSS cannot be re-enabled.
> >
> > For the programs that do not support shadow stack, both SHSTK and
> > WRSS are
> > disabled, but still there is the same call to
> > arch_prctl(ARCH_X86_FEATURE_LOCK, ...) and then neither shadow stack
> > nor
> > WRSS can be enabled.
> >
> > My original plan was to run CRIU with no shadow stack, enable shadow
> > stack
> > and WRSS in the restored tasks using arch_prct() and after the shadow
> > stack
> > contents is restored disable WRSS.
> >
> > Obviously, this didn't work with glibc I have :)
>
> Were you disabling shadow stack via glibc tunnable? Or was the elf
> header marked for IBT? If it was a plain old binary, the code looks to
> me like it should not lock any features.
>
> >
> > On the bright side, having a ptrace call to unlock shadow stack and
> > wrss
> > allows running CRIU itself with shadow stack.
> >
>
> Yea, having something working is really great. My only hesitancy is
> that, per a discussion on the LAM patchset, we are going to make this
> enabling API CET only (same semantics for though). I suppose the
> locking API arch_prctl() could still be support other arch features,
> but it might be a second CET only regset. It's not the end of the
> world.
>
> I guess the other consideration is tieing CRIU to glibc peculiarities.
> Like even if we fix glibc, then CRIU may not work with some other libc
> or app that force disables for some weird reason. Is it supposed to be
> libc-agnostic?
>


-- 
H.J.