Subject: Re: [PATCH 1/1] vhost: Protect the virtqueue from being cleared whilst still in use

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 7 Mar 2022 17:39:47 -0500

On Mon, Mar 07, 2022 at 08:33:27PM +0100, Greg KH wrote:
> On Mon, Mar 07, 2022 at 07:17:57PM +0000, Lee Jones wrote:
> > vhost_vsock_handle_tx_kick() already holds the mutex during its call
> > to vhost_get_vq_desc().  All we have to do here is take the same lock
> > during virtqueue clean-up and we mitigate the reported issues.
> > 
> > Also WARN() as a precautionary measure.  The purpose of this is to
> > capture possible future race conditions which may pop up over time.
> > 
> > Link: https://syzkaller.appspot.com/bug?extid=279432d30d825e63ba00
> > 
> > Cc: <[email-masked]>
> > Reported-by: [email-masked]
> > Signed-off-by: Lee Jones <[email-masked]>
> > ---
> >  drivers/vhost/vhost.c | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> > 
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index 59edb5a1ffe28..ef7e371e3e649 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -693,6 +693,15 @@ void vhost_dev_cleanup(struct vhost_dev *dev)
> >  	int i;
> >  
> >  	for (i = 0; i < dev->nvqs; ++i) {
> > +		/* No workers should run here by design. However, races have
> > +		 * previously occurred where drivers have been unable to flush
> > +		 * all work properly prior to clean-up.  Without a successful
> > +		 * flush the guest will malfunction, but avoiding host memory
> > +		 * corruption in those cases does seem preferable.
> > +		 */
> > +		WARN_ON(mutex_is_locked(&dev->vqs[i]->mutex));
> 
> So you are trading one syzbot triggered issue for another one in the
> future?  :)
> 
> If this ever can happen, handle it, but don't log it with a WARN_ON() as
> that will trigger the panic-on-warn boxes, as well as syzbot.  Unless
> you want that to happen?
> 
> And what happens if the mutex is locked _RIGHT_ after you checked it?
> You still have a race...
> 
> thanks,
> 
> greg k-h

Well it's a symptom of a kernel bug. I guess people with panic on
warn are not worried about DOS and more worried about integrity
and security ... am I right?

-- 
MST