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 14:57:26 +0000

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

> On Wed, Mar 02, 2022 at 01:56:35PM +0000, Lee Jones wrote:
> > 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?
> 
> Can be a separate patch.
> 
> > Just before the first lock I assume?
> 
> I guess so, yes.

No problem.  Patch to follow.

I'm also going to attempt to debug the root cause, but I'm new to this
subsystem to it might take a while for me to get my head around.

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