If backlog handler is running during a tear down operation we may enqueue
data on the ingress msg queue while tear down is trying to free it.

 sk_psock_backlog()
   sk_psock_handle_skb()
     skb_psock_skb_ingress()
       sk_psock_skb_ingress_enqueue()
         sk_psock_queue_msg(psock,msg)
                                           spin_lock(ingress_lock)
                                            sk_psock_zap_ingress()
                                             _sk_psock_purge_ingerss_msg()
                                              _sk_psock_purge_ingress_msg()
                                            -- free ingress_msg list --
                                           spin_unlock(ingress_lock)
           spin_lock(ingress_lock)
           list_add_tail(msg,ingress_msg) <- entry on list with no on
                                             left to free it.
           spin_unlock(ingress_lock)

To fix we only enqueue from backlog if the ENABLED bit is set. The tear
down logic clears the bit with ingress_lock set so we wont enqueue the
msg in the last step.

Fixes: 799aa7f98d53 ("skmsg: Avoid lock_sock() in sk_psock_backlog()")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
---
 include/linux/skmsg.h | 54 ++++++++++++++++++++++++++++---------------
 net/core/skmsg.c      |  6 -----
 2 files changed, 35 insertions(+), 25 deletions(-)

diff --git a/include/linux/skmsg.h b/include/linux/skmsg.h
index 96f319099744..883638888f93 100644
--- a/include/linux/skmsg.h
+++ b/include/linux/skmsg.h

@@ -285,11 +285,45 @@ static inline struct sk_psock *sk_psock(const struct sock *sk)
 	return rcu_dereference_sk_user_data(sk);
 }
 
+static inline void sk_psock_set_state(struct sk_psock *psock,
+				      enum sk_psock_state_bits bit)
+{
+	set_bit(bit, &psock->state);
+}
+
+static inline void sk_psock_clear_state(struct sk_psock *psock,
+					enum sk_psock_state_bits bit)
+{
+	clear_bit(bit, &psock->state);
+}
+
+static inline bool sk_psock_test_state(const struct sk_psock *psock,
+				       enum sk_psock_state_bits bit)
+{
+	return test_bit(bit, &psock->state);
+}
+
+static void sock_drop(struct sock *sk, struct sk_buff *skb)
+{
+	sk_drops_add(sk, skb);
+	kfree_skb(skb);
+}
+
+static inline void drop_sk_msg(struct sk_psock *psock, struct sk_msg *msg)
+{
+	if (msg->skb)
+		sock_drop(psock->sk, msg->skb);
+	kfree(msg);
+}
+
 static inline void sk_psock_queue_msg(struct sk_psock *psock,
 				      struct sk_msg *msg)
 {
 	spin_lock_bh(&psock->ingress_lock);
-	list_add_tail(&msg->list, &psock->ingress_msg);
+        if (sk_psock_test_state(psock, SK_PSOCK_TX_ENABLED))