[PATCH net-next v9 06/14] vsock/loopback: add netns support

[Bobby Eshleman <bobbyeshleman@gmail.com>]

From: Bobby Eshleman <bobbyeshleman@meta.com>

Add NS support to vsock loopback. Sockets in a global mode netns
communicate with each other, regardless of namespace. Sockets in a local
mode netns may only communicate with other sockets within the same
namespace.

Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
---
Changes in v9:
- remove per-netns vsock_loopback and workqueues, just re-using
  the net and net_mode in skb->cb achieved the same result in a simpler
  way. Also removed need for pernet_subsys.
- properly track net references

Changes in v7:
- drop for_each_net() init/exit, drop net_rwsem, the pernet registration
  handles this automatically and race-free
- flush workqueue before destruction, purge pkt list
- remember net_mode instead of current net mode
- keep space after INIT_WORK()
- change vsock_loopback in netns_vsock to ->priv void ptr
- rename `orig_net_mode` to `net_mode`
- remove useless comment
- protect `register_pernet_subsys()` with `net_rwsem`
- do cleanup before releasing `net_rwsem` when failure happens
- call `unregister_pernet_subsys()` in `vsock_loopback_exit()`
- call `vsock_loopback_deinit_vsock()` in `vsock_loopback_exit()`

Changes in v6:
- init pernet ops for vsock_loopback module
- vsock_loopback: add space in struct to clarify lock protection
- do proper cleanup/unregister on vsock_loopback_exit()
- vsock_loopback: use virtio_vsock_skb_net()

Changes in v5:
- add callbacks code to avoid reverse dependency
- add logic for handling vsock_loopback setup for already existing
  namespaces
---
 net/vmw_vsock/vsock_loopback.c | 41 ++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 40 insertions(+), 1 deletion(-)

diff --git a/net/vmw_vsock/vsock_loopback.c b/net/vmw_vsock/vsock_loopback.c
index d3ac056663ea..e62f6c516992 100644
--- a/net/vmw_vsock/vsock_loopback.c
+++ b/net/vmw_vsock/vsock_loopback.c
@@ -32,6 +32,9 @@ static int vsock_loopback_send_pkt(struct sk_buff *skb, struct net *net,
 	struct vsock_loopback *vsock = &the_vsock_loopback;
 	int len = skb->len;
 
+	virtio_vsock_skb_set_net(skb, net);
+	virtio_vsock_skb_set_net_mode(skb, net_mode);
+
 	virtio_vsock_skb_queue_tail(&vsock->pkt_queue, skb);
 	queue_work(vsock->workqueue, &vsock->pkt_work);
 
@@ -116,8 +119,10 @@ static void vsock_loopback_work(struct work_struct *work)
 {
 	struct vsock_loopback *vsock =
 		container_of(work, struct vsock_loopback, pkt_work);
+	enum vsock_net_mode net_mode;
 	struct sk_buff_head pkts;
 	struct sk_buff *skb;
+	struct net *net;
 
 	skb_queue_head_init(&pkts);
 
@@ -131,7 +136,41 @@ static void vsock_loopback_work(struct work_struct *work)
 		 */
 		virtio_transport_consume_skb_sent(skb, false);
 		virtio_transport_deliver_tap_pkt(skb);
-		virtio_transport_recv_pkt(&loopback_transport, skb, NULL, 0);
+
+		/* In the case of virtio_transport_reset_no_sock(), the skb
+		 * does not hold a reference on the socket, and so does not
+		 * transitively hold a reference on the net.
+		 *
+		 * There is an ABA race condition in this sequence:
+		 * 1. the sender sends a packet
+		 * 2. worker calls virtio_transport_recv_pkt(), using the
+		 *    sender's net
+		 * 3. virtio_transport_recv_pkt() uses t->send_pkt() passing the
+		 *    sender's net
+		 * 4. virtio_transport_recv_pkt() free's the skb, dropping the
+		 *    reference to the socket
+		 * 5. the socket closes, frees its reference to the net
+		 * 6. Finally, the worker for the second t->send_pkt() call
+		 *    processes the skb, and uses the now stale net pointer for
+		 *    socket lookups.
+		 *
+		 * To prevent this, we acquire a net reference in vsock_loopback_send_pkt()
+		 * and hold it until virtio_transport_recv_pkt() completes.
+		 *
+		 * Additionally, we must grab a reference on the skb before
+		 * calling virtio_transport_recv_pkt() to prevent it from
+		 * freeing the skb before we have a chance to release the net.
+		 */
+		net_mode = virtio_vsock_skb_net_mode(skb);
+		net = virtio_vsock_skb_net(skb);
+
+		skb_get(skb);
+
+		virtio_transport_recv_pkt(&loopback_transport, skb, net,
+					  net_mode);
+
+		virtio_vsock_skb_clear_net(skb);
+		kfree_skb(skb);
 	}
 }
 

-- 
2.47.3