On Tue, 28 May 2024 11:46:28 -0400, "Michael S. Tsirkin" <mst@xxxxxxxxxx> 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@xxxxxxxxxxxxxxxxx>
>
>
> 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
>
