Re: [PATCH v20 21/25] x86/cet/shstk: Handle signals for shadow stack

Kees Cook
2021-02-11T02:07:41Z

On Wed, Feb 10, 2021 at 01:38:10PM -0800, Yu, Yu-cheng wrote:
> On 2/10/2021 11:58 AM, Kees Cook wrote:
> > On Wed, Feb 10, 2021 at 09:56:59AM -0800, Yu-cheng Yu wrote:
> > > To deliver a signal, create a shadow stack restore token and put the token
> > > and the signal restorer address on the shadow stack. For sigreturn, verify
> > > the token and restore from it the shadow stack pointer.
> > >
> > > A shadow stack restore token marks a restore point of the shadow stack.
> > > The token is distinctively different from any shadow stack address.
> >
> > How is it different? It seems like it just has the last 2 bits
> > masked/set?
> >
>
> For example, for 64-bit apps,
>
> A shadow stack pointer value (*ssp) has to be in some code area, but for a
> token, (*ptr_of_token) = (ptr_of_token + 8), which has to be within the same
> shadow stack area. In cet_verify_rstor_token(), this is checked.
>
> > > In sigreturn, restoring from a token ensures the target address is the
> > > location pointed by the token.
> >
> > As in, a token (real stack address with 2-bit mask) is checked against
> > the real stack address? I don't see a comparison -- it only checks that
> > it is < TASK_SIZE.
> >
> > How does cet_restore_signal() figure into this? (As in, the MSR writes?)
> >
>
> The kernel takes the restore address from the token. It will not mistakenly
> take a wrong address from the shadow stack. I will put this in my commit
> logs.

Ah-ha, okay, got it now. Thank you!

Reviewed-by: Kees Cook <keescook@chromium.org>

--
Kees Cook