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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20250815121702.GB11549@redhat.com>
Date: Fri, 15 Aug 2025 14:17:03 +0200
From: Oleg Nesterov <oleg@...hat.com>
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>,
	"broonie@...nel.org" <broonie@...nel.org>,
	"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 08/14, Edgecombe, Rick P wrote:
>
> +Mark and Deepak
>
> 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.
> >
> > Plus the current code doesn't even look correct, in this case fpu_clone()
> > won't call update_fpu_shstk().

...

> I agree we don't need to allocate a shadow stack in this case,

Great,

> but I'm not sure
> it is right to fully disable shadow stack in thread.features.

Why?

> First of all,
> disabling it from shstk_alloc_thread_stack() seems weird. It just handles
> allocating shadow stacks.

I agree in advance with any other change.

> Lastly, it doesn't seem there is any way to clone from IO uring today,

Not sure I understand... create_io_thread() ?

> 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?

but create_io_thread() and vhost_task_create() do not use CLONE_VFORK?

Oleg.