xsk: do not call synchronize_net() under RCU read lock

The XSKMAP update and delete functions called synchronize_net(), which
can sleep. It is not allowed to sleep during an RCU read section.

Instead we need to make sure that the sock sk_destruct (xsk_destruct)
function is asynchronously called after an RCU grace period. Setting
the SOCK_RCU_FREE flag for XDP sockets takes care of this.

Fixes: fbfc504a24 ("bpf: introduce new bpf AF_XDP map type BPF_MAP_TYPE_XSKMAP")
Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

kernel/bpf/xskmap.c

@@ -192,11 +192,8 @@ static int xsk_map_update_elem(struct bpf_map *map, void *key, void *value,
 	sock_hold(sock->sk);
 
 	old_xs = xchg(&m->xsk_map[i], xs);
-	if (old_xs) {
-		/* Make sure we've flushed everything. */
-		synchronize_net();
+	if (old_xs)
 		sock_put((struct sock *)old_xs);
-	}
 
 	sockfd_put(sock);
 	return 0;
@@ -212,11 +209,8 @@ static int xsk_map_delete_elem(struct bpf_map *map, void *key)
 		return -EINVAL;
 
 	old_xs = xchg(&m->xsk_map[k], NULL);
-	if (old_xs) {
-		/* Make sure we've flushed everything. */
-		synchronize_net();
+	if (old_xs)
 		sock_put((struct sock *)old_xs);
-	}
 
 	return 0;
 }

net/xdp/xsk.c

@@ -744,6 +744,8 @@ static int xsk_create(struct net *net, struct socket *sock, int protocol,
 	sk->sk_destruct = xsk_destruct;
 	sk_refcnt_debug_inc(sk);
 
+	sock_set_flag(sk, SOCK_RCU_FREE);
+
 	xs = xdp_sk(sk);
 	mutex_init(&xs->mutex);
 	spin_lock_init(&xs->tx_completion_lock);