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

[H.J. Lu]

On Wed, Jun 21, 2023 at 11:54 AM Edgecombe, Rick P
<rick.p.edgecombe@xxxxxxxxx> wrote:
>
> On Wed, 2023-06-21 at 12:36 +0100, szabolcs.nagy@xxxxxxx wrote:
> > > The 06/20/2023 19:34, Edgecombe, Rick P wrote:
> > > > > On Tue, 2023-06-20 at 10:17 +0100, szabolcs.nagy@xxxxxxx wrote:
> > > > > > > if there is a fix that's good, i haven't seen it.
> > > > > > >
> > > > > > > my point was that the current unwinder works with current
> > > > > > > kernel
> > > > > > > patches, but does not allow future extensions which
> > > > > > > prevents
> > > > > > > sigaltshstk to work. the unwinder is not versioned so this
> > > > > > > cannot
> > > > > > > be fixed later. it only works if distros ensure shstk is
> > > > > > > disabled
> > > > > > > until the unwinder is fixed. (however there is no way to
> > > > > > > detect
> > > > > > > old unwinder if somebody builds gcc from source.)
> > > > >
> > > > > This is a problem the kernel is having to deal with, not
> > > > > causing. > > The
> > > > > userspace changes were upstreamed before the kernel. Userspace
> > > > > > > folks
> > > > > are adamantly against moving to a new elf bit, to start over
> > > > > with a
> > > > > clean slate. I tried everything to influence this and was not
> > > > > successful. So I'm still not sure what the proposal here is for
> > > > > the
> > > > > kernel.
> > >
> > > i agree, the glibc and libgcc patches should not have been accepted
> > > before a linux abi.
> > >
> > > but the other direction also holds: the linux patches should not be
> > > pushed before the userspace design is discussed. (the current code
> > > upstream is wrong, and new code for the proposed linux abi is not
> > > posted yet. this is not your fault, i'm saying it here, because the
> > > discussion is here.)
>
> This series has been discussed with glibc/gcc developers regularly
> throughout the enabling effort. In fact there have been ongoing
> discussions about future shadow stack functionality.
>
> It's not like this feature has been a fast or hidden effort. You are
> just walking into the tail end of it. (much of it predates my
> involvement BTW, including the initial glibc support)
>
> AFAIK HJ presented the enabling changes at some glibc meeting. The
> signal side of glibc is unchanged from what is already upstream. So I'm
> not sure characterizing it that way is fair. It seems you were not part
> of those old discussions, but that might be because your interest is
> new. In any case we are constrained by some of these earlier outcomes.
> More on that below.
>
> > >
> > > > > I am guessing that the fnon-call-exceptions/expanded frame size
> > > > > incompatibilities could end up causing something to grow an
> > > > > opt-in > > at
> > > > > some point.
> > >
> > > there are independent userspace components and not every component
> > > has a chance to opt-in.
> > >
> > > > > > > how does "fixed shadow stack signal frame size" relates to
> > > > > > > "-fnon-call-exceptions"?
> > > > > > >
> > > > > > > if there were instruction boundaries within a function
> > > > > > > where the
> > > > > > > ret addr is not yet pushed or already poped from the shstk
> > > > > > > then
> > > > > > > the flag would be relevant, but since push/pop happens
> > > > > > > atomically
> > > > > > > at function entry/return -fnon-call-exceptions makes no
> > > > > > > difference as far as shstk unwinding is concerned.
> > > > >
> > > > > As I said, the existing unwinding code for fnon-call-
> > > > > excecptions
> > > > > assumes a fixed shadow stack signal frame size of 8 bytes.
> > > > > Since > > the
> > > > > exception is thrown out of a signal, it needs to know how to
> > > > > unwind
> > > > > through the shadow stack signal frame.
> > >
> > > sorry but there is some misunderstanding about -fnon-call-
> > > exceptions.
> > >
> > > it is for emitting cleanup and exception handler data for a
> > > function
> > > such that throwing from certain instructions within that function
> > > works, while normally only throwing from calls work.
> > >
> > > it is not about *unwinding* from an async signal handler, which is
> > > -fasynchronous-unwind-tables and should always work on linux, nor
> > > for
> > > dealing with cleanup/exception handlers above the interrupted frame
> > > (likewise it works on linux without special cflags).
> > >
> > > as far as i can tell the current unwinder handles shstk unwinding
> > > correctly across signal handlers (sync or async and >
> > > cleanup/exceptions
> > > handlers too), i see no issue with "fixed shadow stack signal frame
> > > size of 8 bytes" other than future extensions and discontinous
> > > shstk.
>
> HJ, can you link your patch that makes it extensible and we can clear
> this up? Maybe the issue extends beyond fnon-call-exceptions, but that
> is where I reproduced it.

Here is the patch:

https://gitlab.com/x86-gcc/gcc/-/commit/aab4c24b67b5f05b72e52a3eaae005c2277710b9

> > >
> > > > > > > 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.
> > > > >
> > > > > I actually did a POC for this, but rejected it. The problem is,
> > > > > if
> > > > > there is a shadow stack overflow at that point then the kernel
> > > > > > > can't
> > > > > push the shadow stack token to the old stack. And shadow stack
> > > > > > > overflow
> > > > > is exactly the alt shadow stack use case. So it doesn't really
> > > > > > > solve
> > > > > the problem.
> > >
> > > the restore token in the alt shstk case does not regress anything
> > > but
> > > makes some use-cases work.
> > >
> > > alt shadow stack is important if code tries to jump in and out of
> > > signal handlers (dosemu does this with swapcontext) and for that a
> > > restore token is needed.
> > >
> > > alt shadow stack is important if the original shstk did not
> > > overflow
> > > but the signal handler would overflow it (small thread stack, huge
> > > sigaltstack case).
> > >
> > > alt shadow stack is also important for crash reporting on shstk
> > > overflow even if longjmp does not work then. longjmp to a
> > > makecontext
> > > stack would still work and longjmp back to the original stack can
> > > be
> > > made to mostly work by an altshstk option to overwrite the top
> > > entry
> > > with a restore token on overflow (this can break unwinding though).
> > >
>
> There was previously a request to create an alt shadow stack for the
> purpose of handling shadow stack overflow. So you are now suggesting to
> to exclude that and instead target a different use case for alt shadow
> stack?
>
> But I'm not sure how much we should change the ABI at this point since
> we are constrained by existing userspace. If you read the history, we
> may end up needing to deprecate the whole elf bit for this and other
> reasons.
>
> So should we struggle to find a way to grow the existing ABI without
> disturbing the existing userspace? Or should we start with something,
> finally, and see where we need to grow and maybe get a chance at a
> fresh start to grow it?
>
> Like, maybe 3 people will show up saying "hey, I *really* need to use
> shadow stack and longjmp from a ucontext stack", and no one says
> anything about shadow stack overflow. Then we know what to do. And
> maybe dosemu decides it doesn't need to implement shadow stack (highly
> likely I would think). Now that I think about it, AFAIU SS_AUTODISARM
> was created for dosemu, and the alt shadow stack patch adopted this
> behavior. So it's speculation that there is even a problem in that
> scenario.
>
> Or maybe people just enable WRSS for longjmp() and directly jump back
> to the setjmp() point. Do most people want fast setjmp/longjmp() at the
> cost of a little security?
>
> Even if, with enough discussion, we could optimize for all
> hypotheticals without real user feedback, I don't see how it helps
> users to hold shadow stack. So I think we should move forward with the
> current ABI.
>
>


-- 
H.J.