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

[Oleg Nesterov <oleg@redhat.com>]

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.