Re: [PATCH v7 01/41] Documentation/x86: Add CET shadow stack description

Edgecombe, Rick P
2023-03-06T18:11:25Z

On Mon, 2023-03-06 at 17:31 +0100, Florian Weimer wrote:
> Ideally, we would implement the backtrace function (in glibc) as just
> a
> shadow stack copy. But this needs to follow the chain of alternate
> stacks, and it may also need some form of markup for signal handler
> frames (which need program counter adjustment to reflect that a
> *non-signal* frame is conceptually nested within the previous
> instruction, and not the function the return address points to).

In the alt shadow stack case, the shadow stack sigframe will have a
special shadow stack frame with a pointer to the shadow stack stack it
came from. This may be a thread stack, or some other stack. This
writeup in the v2 of the series has more details and analysis on the
signal piece:

https://lore.kernel.org/lkml/20220929222936.14584-1-rick.p.edgecombe@intel.com/

So in that design, you should be able to backtrace out of a chain of
alt stacks.

> But I
> think we can add support for this incrementally.

Yea, I think so too.

>
> I assume there is no desire at all on the kernel side that
> sigaltstack
> transparently allocates the shadow stack?

It could have some nice benefit for some apps, so I did look into it.

> Because there is no
> deallocation function today for sigaltstack?

Yea, this is why we can't do it transparently. There was some
discussion up the thread on this.