netdev - Re: [PATCH net 2/2] virtio_net: fix missing lock protection on control

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Message-ID: <1716912105.4028382-1-hengqi@linux.alibaba.com>
Date: Wed, 29 May 2024 00:01:45 +0800
From: Heng Qi <hengqi@...ux.alibaba.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: netdev@...r.kernel.org,
 virtualization@...ts.linux.dev,
 Jason Wang <jasowang@...hat.com>,
 Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
 Eugenio Pérez <eperezma@...hat.com>,
 "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>,
 Jiri Pirko <jiri@...nulli.us>,
 Daniel Jurgens <danielj@...dia.com>
Subject: Re: [PATCH net 2/2] virtio_net: fix missing lock protection on control_buf access

On Tue, 28 May 2024 11:46:28 -0400, "Michael S. Tsirkin" <mst@...hat.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@...ux.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.

> 
> > ---
> >  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
>