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

From: Lee Jones <[email-masked]>
Date: Wed, 2 Mar 2022 13:56:35 +0000

On Wed, 02 Mar 2022, Michael S. Tsirkin wrote:

> On Wed, Mar 02, 2022 at 07:54:21AM +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 is take the same lock
> > during virtqueue clean-up and we mitigate the reported issues.
> > 
> > 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 | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index 59edb5a1ffe28..bbaff6a5e21b8 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -693,6 +693,7 @@ void vhost_dev_cleanup(struct vhost_dev *dev)
> >  	int i;
> >  
> >  	for (i = 0; i < dev->nvqs; ++i) {
> > +		mutex_lock(&dev->vqs[i]->mutex);
> >  		if (dev->vqs[i]->error_ctx)
> >  			eventfd_ctx_put(dev->vqs[i]->error_ctx);
> >  		if (dev->vqs[i]->kick)
> > @@ -700,6 +701,7 @@ void vhost_dev_cleanup(struct vhost_dev *dev)
> >  		if (dev->vqs[i]->call_ctx.ctx)
> >  			eventfd_ctx_put(dev->vqs[i]->call_ctx.ctx);
> >  		vhost_vq_reset(dev, dev->vqs[i]);
> > +		mutex_unlock(&dev->vqs[i]->mutex);
> >  	}
> 
> So this is a mitigation plan but the bug is still there though
> we don't know exactly what it is.  I would prefer adding something like
> WARN_ON(mutex_is_locked(vqs[i]->mutex) here - does this make sense?

As a rework to this, or as a subsequent patch?

Just before the first lock I assume?

-- 
Lee Jones [李琼斯]
Principal Technical Lead - Developer Services
Linaro.org │ Open source software for Arm SoCs
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