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

[H.J. Lu]

On Wed, Jun 21, 2023 at 6:07 PM Edgecombe, Rick P
<rick.p.edgecombe@xxxxxxxxx> wrote:
>
> On Wed, 2023-06-21 at 16:15 -0700, Rick Edgecombe wrote:
> > On Wed, 2023-06-21 at 16:05 -0700, H.J. Lu wrote:
> > > > 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.
> >
> > Oh good point. Hmm.
>
> Just had a quick chat with HJ on this. It seems like it *might* be able
> to made to work. How it would go is setjmp() could act as a wrapper by
> calling it's own return address (the function that called setjmp()).
> This would mean in the case of longjmp() not being called, control flow
> would return through setjmp() before returning from the calling method.

It may not work since we can't tell if RAX (return value) is set by longjmp
or function return.

> This would allow libc to do a RSTORSSP when returning though setjmp()
> in the non-shadow stack case, and essentially skip over the kernel
> placed restore token, and then return from setjmp() like normal. In the
> case of longjmp() being called, it could RSTORSSP directly to the
> token, and then return from setjmp().
>
> Another option could be getting the compilers help to do the RSTORSSP
> in the case of longjmp() not being called. Apparently compilers are
> aware of setjmp() and already do special things around it (makes sense
> I guess, but news to me).
>
> And also, this all would actually work with IBT, because the compiler
> knows already to add an endbr at that point right after setjmp().
>
> I think neither of us were ready to bet on it, but thought maybe it
> could work. And even if it works it's much more complicated than I
> first thought, so I don't like it as much. It's also unclear what a
> change like that would mean for security.
>
> As for unwinding through the existing swapcontext() placed restore
> tokens, the problem was as assumed - that it's difficult to find them.
> Even considering brute force options like doing manual searches for a
> nearby token to use turned up edge cases pretty quick. So I think that
> kind of leaves us where we were originally, with no known solutions
> that would require breaking kernel ABI changes.
>
>
> Are you interested in helping get longjmp() from a ucontext stack
> working for shadow stack? One other thing that came up in the
> conversation was that while it is known that some apps are doing this,
> there are no tests for mixing longjmp and ucontext in glibc. So we may
> not know which combinations of mixing them together even work in the
> non-shadow stack case.
>
> It could be useful to add some tests for this to glibc and we could get
> some clarity on what behaviors shadow stack would actually need to
> support.



-- 
H.J.