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

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Message-ID: <e50065a9-d5e2-4e94-94b2-e34c5fac9720@sirena.org.uk>
Date: Thu, 14 Aug 2025 19:33:24 +0100
From: Mark Brown <broonie@...nel.org>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc: "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>,
	"oleg@...hat.com" <oleg@...hat.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 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 it seems better to have the feature enabled and hopefully give
us some protection against our mistake, and if something causes the
worker thread to start a normal thread then things should work as
expected.

> How about just adding the 'minimal' condition to:
> 	if (clone_flags & CLONE_VFORK) {
> 		shstk->base = 0;
> 		shstk->size = 0;
> 		return 0;
> 	}
> ...then update all the comments where vfork is called out as the only case that
> does this?

Perhaps we should factor the logic for deciding if we need to allocate a
userspace shadow stack out into the arch independent code and do
something like set a flag in kernel_clone_args that the arches can
check?  I think the logic is the same for all arches at the minute and
don't see a reason why it'd diverge.  That'd collide a bit with my
clone3() series, there's some overlap there with that creating another
reason why the decision would change.  Reducing the duplication would be
nice.

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