Re: [PATCH] virtio-net: check the existence of peer before accesing its config

[Jason Wang]

On 2020/7/27 下午6:17, Michael S. Tsirkin wrote:
> On Mon, Jul 27, 2020 at 05:49:47PM +0800, Jason Wang wrote:
> > On 2020/7/27 下午5:41, Michael S. Tsirkin wrote:
> > > On Mon, Jul 27, 2020 at 03:43:28PM +0800, Jason Wang wrote:
> > > > We try to get config from peer unconditionally which may lead NULL
> > > > pointer dereference. Add a check before trying to access the config.
> > > >
> > > > Fixes: 108a64818e69b ("vhost-vdpa: introduce vhost-vdpa backend")
> > > > Cc: Cindy Lu <lulu@redhat.com>
> > > > Tested-by: Cornelia Huck <cohuck@redhat.com>
> > > > Signed-off-by: Jason Wang <jasowang@redhat.com>
> > > I am a bit lost here. Isn't this invoked
> > > when guest attempts to read the config?
> > > With no peer, what do we return to guest?
> >
> > With no peer, it just work as in the past (read from the qemu's own emulated
> > config space). With a vDPA as its peer, it tries to read it from vhost-vDPA.
> Are these scenarios where guest would sometimes get one and
> sometimes another? E.g. does it happen on disconnect?
> If yes that might become a problem ...


For disconnecting yes, but I wonder if we need to care about that case 
anyway.

Thanks


> > > A code comment might be helpful here.
> >
> > Does something like above help?
> >
> > Thanks
> >
> >
> > > > ---
> > > >    hw/net/virtio-net.c | 22 +++++++++++-----------
> > > >    1 file changed, 11 insertions(+), 11 deletions(-)
> > > >
> > > > diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> > > > index 4895af1cbe..935b9ef5c7 100644
> > > > --- a/hw/net/virtio-net.c
> > > > +++ b/hw/net/virtio-net.c
> > > > @@ -125,6 +125,7 @@ static void virtio_net_get_config(VirtIODevice *vdev, 
> > > > uint8_t *config)
> > > >    {
> > > >        VirtIONet *n = VIRTIO_NET(vdev);
> > > >        struct virtio_net_config netcfg;
> > > > +    NetClientState *nc = qemu_get_queue(n->nic);
> > > >        int ret = 0;
> > > >        memset(&netcfg, 0 , sizeof(struct virtio_net_config));
> > > > @@ -142,13 +143,12 @@ static void virtio_net_get_config(VirtIODevice *vdev, 
> > > > uint8_t *config)
> > > >                     VIRTIO_NET_RSS_SUPPORTED_HASHES);
> > > >        memcpy(config, &netcfg, n->config_size);
> > > > -    NetClientState *nc = qemu_get_queue(n->nic);
> > > > -    if (nc->peer->info->type == NET_CLIENT_DRIVER_VHOST_VDPA) {
> > > > +    if (nc->peer && nc->peer->info->type == NET_CLIENT_DRIVER_VHOST_VDPA) {
> > > >            ret = vhost_net_get_config(get_vhost_net(nc->peer), (uint8_t 
> > > > *)&netcfg,
> > > > -                             n->config_size);
> > > > -    if (ret != -1) {
> > > > -        memcpy(config, &netcfg, n->config_size);
> > > > -    }
> > > > +                                   n->config_size);
> > > > +        if (ret != -1) {
> > > > +            memcpy(config, &netcfg, n->config_size);
> > > > +        }
> > > >        }
> > > >    }
> > > > @@ -156,6 +156,7 @@ static void virtio_net_set_config(VirtIODevice *vdev, const 
> > > > uint8_t *config)
> > > >    {
> > > >        VirtIONet *n = VIRTIO_NET(vdev);
> > > >        struct virtio_net_config netcfg = {};
> > > > +    NetClientState *nc = qemu_get_queue(n->nic);
> > > >        memcpy(&netcfg, config, n->config_size);
> > > > @@ -166,11 +167,10 @@ static void virtio_net_set_config(VirtIODevice *vdev, 
> > > > const uint8_t *config)
> > > >            qemu_format_nic_info_str(qemu_get_queue(n->nic), n->mac);
> > > >        }
> > > > -    NetClientState *nc = qemu_get_queue(n->nic);
> > > > -    if (nc->peer->info->type == NET_CLIENT_DRIVER_VHOST_VDPA) {
> > > > -        vhost_net_set_config(get_vhost_net(nc->peer), (uint8_t *)&netcfg,
> > > > -                               0, n->config_size,
> > > > -                        VHOST_SET_CONFIG_TYPE_MASTER);
> > > > +    if (nc->peer && nc->peer->info->type == NET_CLIENT_DRIVER_VHOST_VDPA) {
> > > > +        vhost_net_set_config(get_vhost_net(nc->peer),
> > > > +                             (uint8_t *)&netcfg, 0, n->config_size,
> > > > +                             VHOST_SET_CONFIG_TYPE_MASTER);
> > > >          }
> > > >    }
> > > > --
> > > > 2.20.1