vsock/virtio: use GFP_ATOMIC under RCU read lock

virtio_transport_send_pkt in now called on transport fast path,
under RCU read lock. In that case, we have a bug: virtio_add_sgs
is called with GFP_KERNEL, and might sleep.

Pass the gfp flags as an argument, and use GFP_ATOMIC on
the fast path.

Link: https://lore.kernel.org/all/hfcr2aget2zojmqpr4uhlzvnep4vgskblx5b6xf2ddosbsrke7@nt34bxgp7j2x
Fixes: efcd71af ("vsock/virtio: avoid queuing packets when intermediate queue is empty")
Reported-by: Christian Brauner <brauner@kernel.org>
Cc: Stefano Garzarella <sgarzare@redhat.com>
Cc: Luigi Leonardi <luigi.leonardi@outlook.com>
Message-ID: <3fbfb6e871f625f89eb578c7228e127437b1975a.1727876449.git.mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Reviewed-by: Luigi Leonardi <luigi.leonardi@outlook.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>

net/vmw_vsock/virtio_transport.c

@@ -96,7 +96,7 @@ static u32 virtio_transport_get_local_cid(void)
 
 /* Caller need to hold vsock->tx_lock on vq */
 static int virtio_transport_send_skb(struct sk_buff *skb, struct virtqueue *vq,
-				     struct virtio_vsock *vsock)
+				     struct virtio_vsock *vsock, gfp_t gfp)
 {
 	int ret, in_sg = 0, out_sg = 0;
 	struct scatterlist **sgs;
@@ -140,7 +140,7 @@ static int virtio_transport_send_skb(struct sk_buff *skb, struct virtqueue *vq,
 		}
 	}
 
-	ret = virtqueue_add_sgs(vq, sgs, out_sg, in_sg, skb, GFP_KERNEL);
+	ret = virtqueue_add_sgs(vq, sgs, out_sg, in_sg, skb, gfp);
 	/* Usually this means that there is no more space available in
 	 * the vq
 	 */
@@ -178,7 +178,7 @@ virtio_transport_send_pkt_work(struct work_struct *work)
 
 		reply = virtio_vsock_skb_reply(skb);
 
-		ret = virtio_transport_send_skb(skb, vq, vsock);
+		ret = virtio_transport_send_skb(skb, vq, vsock, GFP_KERNEL);
 		if (ret < 0) {
 			virtio_vsock_skb_queue_head(&vsock->send_pkt_queue, skb);
 			break;
@@ -221,7 +221,7 @@ static int virtio_transport_send_skb_fast_path(struct virtio_vsock *vsock, struc
 	if (unlikely(ret == 0))
 		return -EBUSY;
 
-	ret = virtio_transport_send_skb(skb, vq, vsock);
+	ret = virtio_transport_send_skb(skb, vq, vsock, GFP_ATOMIC);
 	if (ret == 0)
 		virtqueue_kick(vq);