Re: [PATCH v10 24/40] arm64/signal: Expose GCS state in signal frames

Mark Brown
2024-08-15T15:46:14Z

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On Thu, Aug 15, 2024 at 04:33:25PM +0100, Dave Martin wrote:
> On Thu, Aug 15, 2024 at 04:05:32PM +0100, Mark Brown wrote:

> > The expectation (at least for arm64) is that the main program will only
> > have shadow stacks if everything says it can support them. If the
> > dynamic linker turns them on during startup prior to parsing the main
> > executables this means that it should turn them off before actually
> > starting the executable, taking care to consider any locking of features.

> Hmm, so we really do get a clear "enable shadow stack" call to the
> kernel, which we can reasonaly expect won't happen for ancient software?

Yes, userspace always has to explicitly enable the GCS.

> If so, I think dumping the GCS state in the sigframe could be made
> conditional on that without problems (?)

It is - we only allocate the sigframe if the task has GCS enabled.

> > > Related question: does shadow stack work with ucontext-based coroutines?
> > > Per-context stacks need to be allocated by the program for that.

> > Yes, ucontext based coroutines are the sort of thing I meant when I was
> > talking about returning to a different context?

> Ah, right. Doing this asynchronously on the back of a signal (instead
> of doing a sigreturn) is the bad thing. setcontext() officially
> doesn't work for this any more, and doing it by hacking or rebuilding
> the sigframe is extremely hairy and probably a terrible idea for the
> reasons I gave.

I see. I tend to view this as more adventurous than I personally would
be when writing userspace code but equally I don't see a need to
actively break things. There's no *requirement* to use libc...

> So, overall I think making ucontext coroutines with with GCS is purely
> a libc matter that is "interesting" here, but we don't need to worry
> about.

Yes, it's not our problem so long as we don't get in the way somehow.

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