linux-kernel - Re: [PATCH v9 16/42] mm: Add guard pages around a shadow stack.

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Message-ID: <ad6df14b-1fbd-4136-abcd-314425c28306@sirena.org.uk>
Date:   Mon, 26 Jun 2023 13:45:32 +0100
From:   Mark Brown <broonie@...nel.org>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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Subject: Re: [PATCH v9 16/42] mm: Add guard pages around a shadow stack.

On Sun, Jun 25, 2023 at 04:44:32PM +0000, Edgecombe, Rick P wrote:
> On Fri, 2023-06-23 at 13:17 +0100, Mark Brown wrote:

> > This isn't an x86 specific concept, arm64 has a very similar
> > extension
> > called Guarded Control Stack (which I should be publishing changes
> > for
> > in the not too distant future) and riscv also has something.  For
> > arm64
> > I'm using the generic mm changes wholesale, we have a similar need
> > for
> > guard pages around the GCS and while the mechanics of accessing are
> > different the requirement ends up being the same.  Perhaps we could
> > just
> > rewrite the comment to say that guard pages prevent over/underflow of
> > the stack by userspace and that a single page is sufficient for all
> > current architectures, with the details of the working for x86 put in
> > some x86 specific place?

> Something sort of similar came up in regards to the riscv series, about
> adding something like an is_shadow_stack_vma() helper. The plan was to
> not make too many assumptions about the final details of the other
> shadow stack features and leave that for refactoring. I think some kind
> of generic comment like you suggest makes sense, but I don't want to
> try to assert any arch specifics for features that are not upstream. It
> should be very easy to tweak the comment when the time comes.

> The points about x86 details not belonging in non-arch headers and
> having some arch generic explanation in the file are well taken though.

I think a statement to the effect that "this works for currently
supported architectures" is fine, if something comes along with
additional requirements then the comment can be adjusted as part of
merging the new thing.

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