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

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Message-ID: <20250815165445.GJ11549@redhat.com>
Date: Fri, 15 Aug 2025 18:54:46 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc: "debug@...osinc.com" <debug@...osinc.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>,
	"broonie@...nel.org" <broonie@...nel.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"axboe@...nel.dk" <axboe@...nel.dk>,
	"Mehta, Sohil" <sohil.mehta@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH 5/6] x86/shstk: don't create the shadow stack for
 PF_USER_WORKERs

On 08/15, Edgecombe, Rick P wrote:
>
> The bit in thread.features is like a sticky bit that is inherrited whenver a
> thread is cloned.

...

> You don't want to allow a protected app to spawn a new thread that
> escapes the enforcement.

Ah, this is clear. But again, PF_USER_WORKER is the kernel thread cloned
by the kernel. Yes, it shares the same thread-group, but this is only to
make SIGKILL/exit_group/etc work. It is not that userspace app can create
it via something like pthread_create().

> So what are we trying to do for PF_USER_WORKER? Prevent them from wasting a VMA
> with an unused shadow stack? Or set PF_USER_WORKER's aside from the logic that
> is about more than protecting the individual thread in the process?

Let me quote my answer to Mark:

	The fact that a kernel thread can have the pointless ARCH_SHSTK_SHSTK is
	the only reason I know why x86_task_fpu(PF_USER_WORKER) has to work.

	I'd like to make this logic consistent with PF_KTHREAD, and in the longer
	term change the x86 FPU code so that the kernel threads can run without
	without "struct fpu" attached to task_struct.

And again, please see

	Warning from x86_task_fpu()
	https://lore.kernel.org/all/aJVuZZgYjEMxiUYq@ly-workstation/

	PF_USER_WORKERs and shadow stack
	https://lore.kernel.org/all/20250813162824.GA15234@redhat.com/

and 6/6 in this series.

> No, to make it have the same logic as the vfork case (which doesn't allocate a
> new shadow stack).
>
> Like:
>  	if ((clone_flags & CLONE_VFORK) || minimal) {
>  		shstk->base = 0;
>  		shstk->size = 0;
>  		return 0;
>  	}

Aha, got it. Agreed, but I think we also need to clear ARCH_SHSTK_SHSTK
copied by arch_dup_task_struct() ?

Oleg.