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

[Oleg Nesterov <oleg@redhat.com>]

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.