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

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Message-ID: <20250915182232-mutt-send-email-mst@kernel.org>
Date: Mon, 15 Sep 2025 18:23:41 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Paolo Bonzini <pbonzini@...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 at 05:16:35PM -0700, Sean Christopherson wrote:
> 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>

Sure let's do both.

-- 
MST