linux-kernel - Re: [PATCH v2 0/3] vhost_task: Fix a bug where KVM wakes an exited task

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Message-ID: <aK-f45qszH2VEzV7@google.com>
Date: Wed, 27 Aug 2025 17:16:35 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Paolo Bonzini <pbonzini@...hat.com>, "Michael S. Tsirkin" <mst@...hat.com>, 
	Jason Wang <jasowang@...hat.com>, kvm@...r.kernel.org, virtualization@...ts.linux.dev, 
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/3] vhost_task: Fix a bug where KVM wakes an exited task

On Wed, Aug 27, 2025, Sebastian Andrzej Siewior wrote:
> On 2025-08-27 12:41:04 [-0700], Sean Christopherson wrote:
> > Michael,
> 
> Sean,
> 
> would the bellow work by chance? It is a quick shot but it looks
> symmetrical…

Gah, sorry, I flagged your earlier mail and then forgot to circle back to it
(for whatever reason, I didn't entirely grok what you were suggesting).

> diff --git a/kernel/vhost_task.c b/kernel/vhost_task.c
> index bc738fa90c1d6..27107dcc1cbfe 100644
> --- a/kernel/vhost_task.c
> +++ b/kernel/vhost_task.c
> @@ -100,6 +100,7 @@ void vhost_task_stop(struct vhost_task *vtsk)
>  	 * freeing it below.
>  	 */
>  	wait_for_completion(&vtsk->exited);
> +	put_task_struct(vtsk->task);
>  	kfree(vtsk);
>  }
>  EXPORT_SYMBOL_GPL(vhost_task_stop);
> @@ -148,7 +149,7 @@ struct vhost_task *vhost_task_create(bool (*fn)(void *),
>  		return ERR_CAST(tsk);
>  	}
>  
> -	vtsk->task = tsk;
> +	vtsk->task = get_task_struct(tsk);
>  	return vtsk;
>  }
>  EXPORT_SYMBOL_GPL(vhost_task_create);

Nice!  This fixes things too.  Either solution works for me.  Or maybe do both?
Attempting to wake a task that vhost_task knows has exited (is exiting?) is a
bit gross, but even with that hardening, guarding against UAF is very nice to
have too.

Tested-by: Sean Christopherson <seanjc@...gle.com>