arm64: Correctly align the SVE signal context

[andrew]

Summary

The SVE signal context needs to be correctly aligned. Fix this by
creating a new macro to calculate the needed size to provide this
alignment, and use it when setting and checking the saved SVE signal
context.

Reported by: cperciva
Sponsored by: Arm Ltd

[cperciva]

Thanks, this fixes the issue I was seeing.

[cperciva]

This code looks correct to me, but I'm not an expert in these bits so it would be great if @manu or someone else with arm64 experience can review.

Adding secteam as a reviewer for visibility since this should get an Errata Notice.

[markj]

In D54396#1245327, @cperciva wrote:

This code looks correct to me, but I'm not an expert in these bits so it would be great if @manu or someone else with arm64 experience can review.

Adding secteam as a reviewer for visibility since this should get an Errata Notice.

Thanks, I added a placeholder for this.

Which FreeBSD versions does this bug affect? Looks like 15.0 and probably 14.3?

sys/arm64/arm64/exec_machdep.c

754

Why is it necessary to update the recorded ctx_size here? Below we are still only copying out sizeof(ctx) + buf_size bytes.

[andrew]

In D54396#1245665, @markj wrote:

Which FreeBSD versions does this bug affect? Looks like 15.0 and probably 14.3?

It looks like that's correct. I didn't merge SVE support to 13.

sys/arm64/arm64/exec_machdep.c

754

It needs to be aligned correctly to point to the next entry. We could zero the padding, but userspace shouldn't care about it & the kernel doesn't read it back in on sigreturn.

[cperciva]

sys/arm64/arm64/exec_machdep.c

754

Just to confirm, nothing is being written into that padding from the kernel? So having non-zeroed padding doesn't risk leaking information, it's just staying as whatever was there in userland already?

[andrew]

sys/arm64/arm64/exec_machdep.c

754

Correct

[markj]

sys/arm64/arm64/exec_machdep.c

754

That is my reading of it, yes.