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

[Edgecombe, Rick P]

On Mon, 2023-06-26 at 15:08 +0100, szabolcs.nagy@xxxxxxx wrote:
> The 06/22/2023 16:46, Edgecombe, Rick P wrote:
> > You previously said:
> > 
> > On Wed, 2023-06-21 at 12:36 +0100, szabolcs.nagy@xxxxxxx wrote:
> > > 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.
> > 
> > I took that to mean that you didn't see how the the existing
> > unwinder
> > prevented alt shadow stacks. Hopefully we're all on the same page
> > now. 
> 
> well alt shstk is discontinous.
> 
> there were two separate confusions:
> 
> - your mention of fnon-call-exception threw me off since that is a
> very specific corner case.
> 
> - i was talking about an unwind design that can deal with altshstk
> which requires ssp switch. (current uwinder does not support this,
> but i assumed we can add that now and ignore old broken unwinders).
> you are saying that alt shstk support needs additional shstk tokens
> in the signal frame to maintain alt shstk state for the kernel.
> 
> i think now we are on the same page.

I don't think I fully understand your point still. It would really help
if you could do a levelset summary, like I asked on other branches of
this thread.

> 
> > BTW, when alt shadow stack's were POCed, I hadn't encountered this
> > GCC
> > behavior yet. So it was assumed it could be bolted on later without
> > disturbing anything. If Linus or someone wants to say we're ok with
> > breaking these old GCCs in this way, the first thing I would do
> > would
> > be to pad the shadow stack signal frame with room for alt shadow
> > stack
> > and more. I actually have a patch to do this, but alas we are
> > already
> > pushing it regression wise.
> 
> sounds like it will be hard to add alt shstk later.

Adding alt shadow stack without moving the elf bit runs the risk of
regressions because of the old GCCs. But moving the elf bit is easy
(from the technical side at least). There were several threads about
this in the past.

So I don't think any harder than it is now.

> 
> (i think maintaining alt shstk state on the stack instead of
> shstk should work too. but if that does not work, then alt
> shstk will require another abi opt-in.)

The x86 sigframe is pretty full AFAIK. There are already features that
require an opt in. It might be true that the alt shadow stack base and
size don't need to be protected like the previous SSP. I'd have to
think about it. It could be nice to optionally put some IBT stuff on
the shadow stack as well, which would require growing the frame, and at
that point the alt shadow stack stuff might as well go there too.