linux-kernel - Re: [PATCH v3 21/36] arm64/mm: Implement map_shadow

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Message-ID: <55c629cc-0545-460b-91cb-2ebdb8ae9051@sirena.org.uk>
Date:   Tue, 1 Aug 2023 18:57:08 +0100
From:   Mark Brown <broonie@...nel.org>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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Subject: Re: [PATCH v3 21/36] arm64/mm: Implement map_shadow_stack()

On Tue, Aug 01, 2023 at 05:07:00PM +0000, Edgecombe, Rick P wrote:
> On Tue, 2023-08-01 at 15:01 +0100, Mark Brown wrote:

> > > It could be a different flag, like SHADOW_STACK_SET_TOKEN_MARKER,
> > > or it
> > > could be SHADOW_STACK_SET_MARKER, and callers could pass
> > > (SHADOW_STACK_SET_TOKEN | SHADOW_STACK_SET_MARKER) to get what you
> > > have
> > > implemented here. What do you think?

> > For arm64 code this would mean that it would be possible (and fairly
> > easy) to create stacks which don't have a termination record which
> > would
> > make life harder for unwinders to rely on.  I don't think this is
> > insurmountable, creating manually shouldn't be the standard and it'll
> > already be an issue on x86 anyway.

> If you are going to support optionally writing to shadow stacks (which
> x86 needed for CRIU, and also seems like a nice thing for several other
> reasons), you are already at that point. Can't you also do a bunch of
> gcspopm's to the top of the GCS stack, and have no marker to hit before
> the end of the stack? (maybe not in GCS, I don't know...)

It's definitely possible to use writes or pops to achive the same
effect, it's just that it's less likely to be something that happens
through simple oversight than missing a flag off the initial map call
would be.

> > The other minor issue is that the current arm64 marker is all bits 0
> > so by itself for arm64 _MARKER would have no perceptible impact, it
> > would only serve to push the token down a slot in the stack (I'm
> > guessing that's the intended meaning?).

> Pushing the token down a frame is what flags==0 does in this patch,
> right?

Yes, exactly - if we make the top of stack record optional then if that
flag is omitted we'd not do that.

> You don't have to support all the flags actually, you could just
> support the one mode you already have and reject all other
> combinations... Then it matches between arch's, and you still have the
> guaranteed-ish end marker.

Sure, though if we're going to the trouble of checking for the flag we
probably may as well implement it.  I guess x86 is locked in at this
point by existing userspace.  I guess I'll implement it assuming nobody
from userspace complains, it's trivial for a kernel.

> So the question is not what mode should arm support, but should we have
> the flags match between x86 and ARM?

The flags should definitely match, no disagreement there.

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