Re: [RFC 1/2] virtio_net: timeout control virtqueue commands

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

On Tue, Oct 14, 2025 at 11:14:40AM +0200, Maxime Coquelin wrote:
> On Tue, Oct 14, 2025 at 10:29 AM Michael S. Tsirkin <mst@redhat.com> wrote:
> >
> > On Tue, Oct 07, 2025 at 03:06:21PM +0200, Eugenio Pérez wrote:
> > > An userland device implemented through VDUSE could take rtnl forever if
> > > the virtio-net driver is running on top of virtio_vdpa.  Let's break the
> > > device if it does not return the buffer in a longer-than-assumible
> > > timeout.
> >
> > So now I can't debug qemu with gdb because guest dies :(
> > Let's not break valid use-cases please.
> >
> >
> > Instead, solve it in vduse, probably by handling cvq within
> > kernel.
> 
> Would a shadow control virtqueue implementation in the VDUSE driver work?
> It would ack systematically messages sent by the Virtio-net driver,
> and so assume the userspace application will Ack them.
> 
> When the userspace application handles the message, if the handling fails,
> it somehow marks the device as broken?
> 
> Thanks,
> Maxime

Yes but it's a bit more convoluted  than just acking them.
Once you use the buffer you can get another one and so on
with no limit.
One fix is to actually maintain device state in the
kernel, update it, and then notify userspace.



> >
> > > A less agressive path can be taken to recover the device, like only
> > > resetting the control virtqueue.  However, the state of the device after
> > > this action is taken races, as the vq could be reset after the device
> > > writes the OK.  Leaving TODO anyway.
> > >
> > > Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> > > ---
> > >  drivers/net/virtio_net.c | 10 ++++++++++
> > >  1 file changed, 10 insertions(+)
> > >
> > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > index 31bd32bdecaf..ed68ad69a019 100644
> > > --- a/drivers/net/virtio_net.c
> > > +++ b/drivers/net/virtio_net.c
> > > @@ -3576,6 +3576,7 @@ 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;
> > > +     unsigned long end_time;
> > >       bool ok;
> > >       int ret;
> > >
> > > @@ -3614,11 +3615,20 @@ static bool virtnet_send_command_reply(struct virtnet_info *vi, u8 class, u8 cmd
> > >
> > >       /* Spin for a response, the kick causes an ioport write, trapping
> > >        * into the hypervisor, so the request should be handled immediately.
> > > +      *
> > > +      * Long timeout so a malicious device is not able to lock rtnl forever.
> > >        */
> > > +     end_time = jiffies + 30 * HZ;
> > >       while (!virtqueue_get_buf(vi->cvq, &tmp) &&
> > >              !virtqueue_is_broken(vi->cvq)) {
> > >               cond_resched();
> > >               cpu_relax();
> > > +
> > > +             if (time_after(end_time, jiffies)) {
> > > +                     /* TODO Reset vq if possible? */
> > > +                     virtio_break_device(vi->vdev);
> > > +                     break;
> > > +             }
> > >       }
> > >
> > >  unlock:
> > > --
> > > 2.51.0
> >