Re: [PATCH 5/6] x86/shstk: don't create the shadow stack for PF_USER_WORKERs

[Oleg Nesterov]

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.