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

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Message-ID: <20250815130851.GE11549@redhat.com>
Date: Fri, 15 Aug 2025 15:08:52 +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, Oleg Nesterov wrote:
>
> On 08/14, Mark Brown wrote:
> >
> > On Thu, Aug 14, 2025 at 05:03:36PM +0000, Edgecombe, Rick P wrote:
> > > On Thu, 2025-08-14 at 12:14 +0200, Oleg Nesterov wrote:
> >
> > > > If a features_enabled(ARCH_SHSTK_SHSTK) userspace thread creates a
> > > > PF_USER_WORKER thread, shstk_alloc_thread_stack() allocates the shadow
> > > > stack for no reason, the new (kernel) thread will never return to usermode.
> >
> > > I agree we don't need to allocate a shadow stack in this case, but I'm not sure
> > > it is right to fully disable shadow stack in thread.features. First of all,
> > > disabling it from shstk_alloc_thread_stack() seems weird. It just handles
> > > allocating shadow stacks.
> >
> > 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.

Oleg.