Re: [PATCH v9 23/42] Documentation/x86: Add CET shadow stack description

["H.J. Lu" <hjl.tools@gmail.com>]

On Wed, Jun 21, 2023 at 3:23 PM Edgecombe, Rick P
<rick.p.edgecombe@intel.com> wrote:
>
> On Wed, 2023-06-21 at 11:54 -0700, Rick Edgecombe wrote:
> > > > > > > > there is no magic, longjmp should be implemented as:
> > > > > > > >
> > > > > > > >         target_ssp = read from jmpbuf;
> > > > > > > >         current_ssp = read ssp;
> > > > > > > >         for (p = target_ssp; p != current_ssp; p--) {
> > > > > > > >                 if (*p == restore-token) {
> > > > > > > >                         // target_ssp is on a different
> > > > > > > > shstk.
> > > > > > > >                         switch_shstk_to(p);
> > > > > > > >                         break;
> > > > > > > >                 }
> > > > > > > >         }
> > > > > > > >         for (; p != target_ssp; p++)
> > > > > > > >                 // ssp is now on the same shstk as
> > > > > > > > target.
> > > > > > > >                 inc_ssp();
> > > > > > > >
> > > > > > > > this is what setcontext is doing and longjmp can do the
> > > > > > > > same:
> > > > > > > > for programs that always longjmp within the same shstk
> > > > > > > > the
> > > > > > > > first
> > > > > > > > loop is just p = current_ssp, but it also works when
> > > > > > > > longjmp
> > > > > > > > target is on a different shstk assuming nothing is
> > > > > > > > running
> > > > > > > > on
> > > > > > > > that shstk, which is only possible if there is a restore
> > > > > > > > token
> > > > > > > > on top.
> > > > > > > >
> > > > > > > > this implies if the kernel switches shstk on signal entry
> > > > > > > > it has
> > > > > > > > to add a restore-token on the switched away shstk.
>
> Wait a second, the claim is that the kernel should add a restore token
> on the current shadow stack before handling a signal, to allow to
> unwind from an alt shadow stack, right? But in this series there is not
> an alt shadow stack, so signal will be handled on the current shadow
> stack. If the user stays on the current shadow stack, the existing
> simple INCSSP based solution will work.
>
> If the user swapcontext()'s away while handling a signal (which *is*
> currently supported) they will leave their own restore token on the old
> stack. Hypothetically glibc could unwind back through a series of
> ucontext stacks by pivoting, if it kept some metadata somewhere about
> where to restore to. So there are actually already enough tokens to
> make it back in this case, glibc just doesn't do this.
>
> But how does the proposed token placed by the kernel on the original
> stack help this problem? The longjmp() would have to be able to find
> the location of the restore tokens somehow, which would not necessarily
> be near the setjmp() point. The signal token could even be on a
> different shadow stack.
>
> So I think the above is short of a design for a universally compatible
> longjmp().
>
> Which makes me think if we did want to make a more compatible longjmp()
> a better the way to do it might be an arch_prctl that emits a token at
> the current SSP. This would be loosening up the security somewhat (have
> to be an opt-in), but less so then enabling WRSS. But it would also be
> way simpler, work for all cases (I think), and be faster (maybe?) than
> INCSSPing through a bunch of stacks.

Since longjmp isn't required to be called after setjmp, leaving a restore
token doesn't work when longjmp isn't called.

> I'm also not sure leaving a token on signal doesn't weaken the security
> it it's own way as well. Any thread could then swap to that token.
> Where as the shadow stack signal frame ssp pointer can only be used
> from the shadow stack the signal was handled on.
>
> So I think, in addition to blocking the shadow stack overflow use case
> in the future, leaving a token behind on signal will not really help
> longjmp(). (or at least I'm not following)
>


-- 
H.J.