linux-kernel - Re: [PATCH RFC 0/3] arm64/gcs: Allow reuse of user managed shadow stacks

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Message-ID: <5397025d-7528-4b9c-b38d-b843ab004f47@sirena.org.uk>
Date: Fri, 26 Sep 2025 17:09:08 +0100
From: Mark Brown <broonie@...nel.org>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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Subject: Re: [PATCH RFC 0/3] arm64/gcs: Allow reuse of user managed shadow
 stacks

On Fri, Sep 26, 2025 at 03:46:26PM +0000, Edgecombe, Rick P wrote:
> On Fri, 2025-09-26 at 01:44 +0100, Mark Brown wrote:

> > I agree it seems clearly better from a security point of view to have
> > writable shadow stacks than none at all, I don't think there's much
> > argument there other than the concerns about the memory consumption
> > and performance tradeoffs.

> IIRC the WRSS equivalent works the same for ARM where you need to use a
> special instruction, right? So we are not talking about full writable

Yes, it's GCSSTR for arm64.

> shadow stacks that could get attacked from any overflow, rather,
> limited spots that have the WRSS (or similar) instruction. In the
> presence of forward edge CFI, we might be able to worry less about
> attackers being able to actually reach it? Still not quite as locked
> down as having it disabled, but maybe not such a huge gap compared to
> the mmap/munmap() stuff that is the alternative we are weighing.

Agreed, as I said it's a definite win still - just not quite as strong.

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