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

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Message-ID: <475f31e1-0f6f-44a9-b93a-540c1d43e1bb@sirena.org.uk>
Date:   Wed, 2 Aug 2023 17:27:44 +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 08:57:59PM +0000, Edgecombe, Rick P wrote:
> On Tue, 2023-08-01 at 18:57 +0100, Mark Brown wrote:

> > 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.

> To make sure we are on the same page: What I'm saying is say we do
> something like add another flag SHADOW_STACK_SET_MARKER that means add
> a marker at the end (making the token off by one frame). Then you can
> just reject any flags != (SHADOW_STACK_SET_MARKER |
> SHADOW_STACK_SET_TOKEN) value, and leave the rest of the code as is. So
> not really implementing anything new. 

> Then x86 could use the same flag meanings if/when it implements end
> markers. If it doesn't seem worth it, it's not a big deal on my end.
> Just seemed that they were needlessly diverging.

Yes, my understanding of the flags is the same.  I'll definitely
implement omitting the cap since there's an actual use case for that
(extending an existing stack, it's marginally safer to not have any
opportunity to pivot into the newly allocated region).

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