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: <20250815130125.GD11549@redhat.com>
Date: Fri, 15 Aug 2025 15:01:26 +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 08/14, 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.

Oleg.