Subject: locking issue in vhost_net_set_backend

From: Jiri Slaby <[email-masked]>
Date: Tue, 16 Mar 2010 14:58:23 +0100

Hi,

Stanse found a locking problem in the following function:

static long vhost_net_set_backend(struct vhost_net *n, unsigned index, 
int fd)
{
         struct socket *sock, *oldsock;
         struct vhost_virtqueue *vq;
         int r;

         mutex_lock(&n->dev.mutex);
         r = vhost_dev_check_owner(&n->dev);
         if (r)
                 goto err;

         if (index >= VHOST_NET_VQ_MAX) {
                 r = -ENOBUFS;
                 goto err;
         }
         vq = n->vqs + index;
         mutex_lock(&vq->mutex);        <---  locked

         /* Verify that ring has been setup correctly. */
         if (!vhost_vq_access_ok(vq)) {
                 r = -EFAULT;
                 goto err;              <--- not unlocked
         }
         sock = get_socket(fd);
         if (IS_ERR(sock)) {
                 r = PTR_ERR(sock);
                 goto err;              <--- not unlocked
         }

         /* start polling new socket */
         oldsock = vq->private_data;
         if (sock == oldsock)
                 goto done;             <--- not unlocked

         vhost_net_disable_vq(n, vq);
         rcu_assign_pointer(vq->private_data, sock);
         vhost_net_enable_vq(n, vq);
         mutex_unlock(&vq->mutex);
done:
         if (oldsock) {
                 vhost_net_flush_vq(n, index);
                 fput(oldsock->file);
         }
err:
         mutex_unlock(&n->dev.mutex);
         return r;
}

I don't see how the lock is unlocked on the error paths and as it is not 
on none of the them maybe I'm missing something?

thanks,
-- 
js