linux-kernel - Re: [PATCH 3/3] mm: Care about shadow stack guard gap when getting an unmapped area

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Message-ID: <3ff7a6dc-a21a-4a53-b3d5-cc8b0b5333fb@sirena.org.uk>
Date: Tue, 3 Sep 2024 19:20:02 +0100
From: Mark Brown <broonie@...nel.org>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Richard Henderson <richard.henderson@...aro.org>,
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	Rick Edgecombe <rick.p.edgecombe@...el.com>
Subject: Re: [PATCH 3/3] mm: Care about shadow stack guard gap when getting
 an unmapped area

On Tue, Sep 03, 2024 at 06:49:46PM +0100, Lorenzo Stoakes wrote:
> On Mon, Sep 02, 2024 at 08:08:15PM GMT, Mark Brown wrote:

> > On x86 there is a custom arch_get_unmapped_area() which was updated by the
> > above commit to cover this case by specifying a start_gap for allocations
> > with VM_SHADOW_STACK. Both arm64 and RISC-V have equivalent features and
> > use the generic implementation of arch_get_unmapped_area() so let's make
> > the equivalent change there so they also don't get shadow stack pages
> > placed without guard pages.

> Don't you need to unwind that change in x86 now you're doing it in generic code?

No, x86 had a preexisting custom implementation for some other reason
(hence the "updated by the above commit" part above) - the shadow stack
support would most likely have been added in the core in the first place
were it not for that.

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