Re: [PATCH net 2/2] virtio_net: fix missing lock protection on control_buf access

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, May 29, 2024 at 12:01:45AM +0800, Heng Qi wrote:
> On Tue, 28 May 2024 11:46:28 -0400, "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > On Tue, May 28, 2024 at 03:52:26PM +0800, Heng Qi wrote:
> > > Refactored the handling of control_buf to be within the cvq_lock
> > > critical section, mitigating race conditions between reading device
> > > responses and new command submissions.
> > > 
> > > Fixes: 6f45ab3e0409 ("virtio_net: Add a lock for the command VQ.")
> > > Signed-off-by: Heng Qi <hengqi@linux.alibaba.com>
> > 
> > 
> > I don't get what does this change. status can change immediately
> > after you drop the mutex, can it not? what exactly is the
> > race conditions you are worried about?
> 
> See the following case:
> 
> 1. Command A is acknowledged and successfully executed by the device.
> 2. After releasing the mutex (mutex_unlock), process P1 gets preempted before
>    it can read vi->ctrl->status, *which should be VIRTIO_NET_OK*.
> 3. A new command B (like the DIM command) is issued.
> 4. Post vi->ctrl->status being set to VIRTIO_NET_ERR by
>    virtnet_send_command_reply(), process P2 gets preempted.
> 5. Process P1 resumes, reads *vi->ctrl->status as VIRTIO_NET_ERR*, and reports
>    this error back for Command A. <-- Race causes incorrect results to be read.
> 
> Thanks.


Why is it important that P1 gets VIRTIO_NET_OK?
After all it is no longer the state.

> > 
> > > ---
> > >  drivers/net/virtio_net.c | 4 +++-
> > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > index 6b0512a628e0..3d8407d9e3d2 100644
> > > --- a/drivers/net/virtio_net.c
> > > +++ b/drivers/net/virtio_net.c
> > > @@ -2686,6 +2686,7 @@ static bool virtnet_send_command_reply(struct virtnet_info *vi, u8 class, u8 cmd
> > >  {
> > >  	struct scatterlist *sgs[5], hdr, stat;
> > >  	u32 out_num = 0, tmp, in_num = 0;
> > > +	bool ret;
> > >  	int err;
> > >  
> > >  	/* Caller should know better */
> > > @@ -2731,8 +2732,9 @@ static bool virtnet_send_command_reply(struct virtnet_info *vi, u8 class, u8 cmd
> > >  	}
> > >  
> > >  unlock:
> > > +	ret = vi->ctrl->status == VIRTIO_NET_OK;
> > >  	mutex_unlock(&vi->cvq_lock);
> > > -	return vi->ctrl->status == VIRTIO_NET_OK;
> > > +	return ret;
> > >  }
> > >  
> > >  static bool virtnet_send_command(struct virtnet_info *vi, u8 class, u8 cmd,
> > > -- 
> > > 2.32.0.3.g01195cf9f
> >