linux-kernel - Re: [PATCH 5/6] x86/shstk: don't create the shadow stack for PF_USER

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Message-ID: <c66150c2-a745-48b1-8d80-1984bfa265fc@sirena.org.uk>
Date: Fri, 15 Aug 2025 18:08:10 +0100
From: Mark Brown <broonie@...nel.org>
To: Oleg Nesterov <oleg@...hat.com>
Cc: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
	"bp@...en8.de" <bp@...en8.de>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"hpa@...or.com" <hpa@...or.com>,
	"axboe@...nel.dk" <axboe@...nel.dk>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"Mehta, Sohil" <sohil.mehta@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	"debug@...osinc.com" <debug@...osinc.com>
Subject: Re: [PATCH 5/6] x86/shstk: don't create the shadow stack for
 PF_USER_WORKERs

On Fri, Aug 15, 2025 at 06:00:23PM +0200, Oleg Nesterov wrote:
> On 08/15, Mark Brown wrote:

> > OK, that's entirely x86 specific - there's no reason we'd want to do
> > that for arm64.

> Since I know nothing about arm64. Any reason we do want to have the unnecessary
> ARCH_SHSTK_SHSTK/shstk on arm64?

If you mean allocating the userspace shadow stack for threads that never
go to userspace then no, it's not needed.

> And... do you agree that shstk_alloc_thread_stack() without update_fpu_shstk()
> in copy_thread() path doesn't look right? Even if nothing really bad can happen.

Honestly the update_fpu_ stuff is sufficently x86 specific that it
doesn't really register as something I'd expect to see in that path.

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