[PATCH v10 23/40] arm64/signal: Set up and restore the GCS context for signal handlers

[Mark Brown]

On Wed, Aug 14, 2024 at 03:51:42PM +0100, Dave Martin wrote:
> On Thu, Aug 01, 2024 at 01:06:50PM +0100, Mark Brown wrote:

> > +	put_user_gcs((unsigned long)sigtramp, gcspr_el0 - 2, &ret);
> > +	put_user_gcs(GCS_SIGNAL_CAP(gcspr_el0 - 1), gcspr_el0 - 1, &ret);
> > +	if (ret != 0)
> > +		return ret;

> What happens if we went wrong here, or if the signal we are delivering
> was caused by a GCS overrun or bad GCSPR_EL0 in the first place?

> It feels like a program has no way to rescue itself from excessive
> recursion in some thread.  Is there something equivalent to
> sigaltstack()?

> Or is the shadow stack always supposed to be big enough to cope with
> recursion that exhausts the main stack and alternate signal stack (and
> if so, how is this ensured)?

There's no sigaltstack() for GCS, this is also the ABI with the existing
shadow stack on x86 and should be addressed in a cross architecture
fashion.  There have been some discussions about providing a shadow alt
stack but they've generally been circular and inconclusive, there were a
bunch of tradeoffs for corner cases and nobody had a clear sense as to
what a good solution should be.  It was a bit unclear that actively
doing anything was worthwhile.  The issues were IIRC around unwinders
and disjoint shadow stacks, compatibility with non-shadow stacks and
behaviour when we overflow the shadow stack.  I think there were also
some applications trying to be very clever with alt stacks that needed
to be interacted with and complicated everything but I could be
misremembering there.

Practically speaking since we're only storing return addresses the
default GCS should be extremely large so it's unlikely to come up
without first encountering and handling issues on the normal stack.
Users allocating their own shadow stacks should be careful.  This isn't
really satisfying but is probably fine in practice, there's certainly
not been any pressure yet from the existing x86 deployments (though at
present nobody can explicitly select their own shadow stack size,
perhaps it'll become more of an issue when the clone3() stuff is in).
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