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

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Message-ID: <20250815154311.GG11549@redhat.com>
Date: Fri, 15 Aug 2025 17:43:11 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Mark Brown <broonie@...nel.org>
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 08/15, Mark Brown wrote:
>
> On Fri, Aug 15, 2025 at 03:08:52PM +0200, Oleg Nesterov wrote:
> > On 08/15, Oleg Nesterov wrote:
> > > On 08/14, Mark Brown wrote:
>
> > > > I agree that it's better to leave userspace shadow stacks enabled, given
> > > > that the reason we're not allocating the shadow stack is that we don't
> > > > expect to ever return to userspace then it should be fine to leave the
> > > > feature turned on for userspace.  If we mess up and do somehow return to
> > > > userspace
>
> > > But a PF_USER_WORKER task can never return to userspace. It doesn't differ
> > > from PF_KTHREAD in this respect.
>
> > ... of course unless it does exec.
>
> Sure, but OTOH at least for arm64 there's no cost to leaving the feature
> enabled unless you actually execute userspace code so if we never return
> to userspace writing the code to disable isn't really buying us anything.

The fact that a kernel thread can have the pointless ARCH_SHSTK_SHSTK is
the only reason I know why x86_task_fpu(PF_USER_WORKER) has to work.

I'd like to make this logic consistent with PF_KTHREAD, and in the longer
term change the x86 FPU code so that the kernel threads can run without
without "struct fpu" attached to task_struct.

Again, please see
https://lore.kernel.org/all/20250813191441.GA26754@redhat.com/

Oleg.