Revert "tcp: avoid the lookup process failing to get sk in ehash table"

This reverts commit 3f4ca5f.

Commit 3f4ca5f ("tcp: avoid the lookup process failing to get sk in
ehash table") reversed the order in how a socket is inserted into ehash
to fix an issue that ehash-lookup could fail when reqsk/full sk/twsk are
swapped.  However, it introduced another lookup failure.

The full socket in ehash is allocated from a slab with SLAB_TYPESAFE_BY_RCU
and does not have SOCK_RCU_FREE, so the socket could be reused even while
it is being referenced on another CPU doing RCU lookup.

Let's say a socket is reused and inserted into the same hash bucket during
lookup.  After the blamed commit, a new socket is inserted at the end of
the list.  If that happens, we will skip sockets placed after the previous
position of the reused socket, resulting in ehash lookup failure.

As described in Documentation/RCU/rculist_nulls.rst, we should insert a
new socket at the head of the list to avoid such an issue.

This issue, the swap-lookup-failure, and another variant reported in [0]
can all be handled properly by adding a locked ehash lookup suggested by
Eric Dumazet [1].

However, this issue could occur for every packet, thus more likely than
the other two races, so let's revert the change for now.

Link: https://lore.kernel.org/netdev/[email protected]/ [0]
Link: https://lore.kernel.org/netdev/CANn89iK8snOz8TYOhhwfimC7ykYA78GA3Nyv8x06SZYa1nKdyA@mail.gmail.com/ [1]
Fixes: 3f4ca5f ("tcp: avoid the lookup process failing to get sk in ehash table")
Signed-off-by: Kuniyuki Iwashima <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: q2ven

net/ipv4/inet_hashtables.c

@@ -650,20 +650,8 @@ bool inet_ehash_insert(struct sock *sk, struct sock *osk, bool *found_dup_sk)
 	spin_lock(lock);
 	if (osk) {
 		WARN_ON_ONCE(sk->sk_hash != osk->sk_hash);
-		ret = sk_hashed(osk);
-		if (ret) {
-			/* Before deleting the node, we insert a new one to make
-			 * sure that the look-up-sk process would not miss either
-			 * of them and that at least one node would exist in ehash
-			 * table all the time. Otherwise there's a tiny chance
-			 * that lookup process could find nothing in ehash table.
-			 */
-			__sk_nulls_add_node_tail_rcu(sk, list);
-			sk_nulls_del_node_init_rcu(osk);
-		}
-		goto unlock;
-	}
-	if (found_dup_sk) {
+		ret = sk_nulls_del_node_init_rcu(osk);
+	} else if (found_dup_sk) {
 		*found_dup_sk = inet_ehash_lookup_by_sk(sk, list);
 		if (*found_dup_sk)
 			ret = false;
@@ -672,7 +660,6 @@ bool inet_ehash_insert(struct sock *sk, struct sock *osk, bool *found_dup_sk)
 	if (ret)
 		__sk_nulls_add_node_rcu(sk, list);
 
-unlock:
 	spin_unlock(lock);
 
 	return ret;

net/ipv4/inet_timewait_sock.c

@@ -88,10 +88,10 @@ void inet_twsk_put(struct inet_timewait_sock *tw)
 }
 EXPORT_SYMBOL_GPL(inet_twsk_put);
 
-static void inet_twsk_add_node_tail_rcu(struct inet_timewait_sock *tw,
-					struct hlist_nulls_head *list)
+static void inet_twsk_add_node_rcu(struct inet_timewait_sock *tw,
+				   struct hlist_nulls_head *list)
 {
-	hlist_nulls_add_tail_rcu(&tw->tw_node, list);
+	hlist_nulls_add_head_rcu(&tw->tw_node, list);
 }
 
 static void inet_twsk_add_bind_node(struct inet_timewait_sock *tw,
@@ -144,7 +144,7 @@ void inet_twsk_hashdance(struct inet_timewait_sock *tw, struct sock *sk,
 
 	spin_lock(lock);
 
-	inet_twsk_add_node_tail_rcu(tw, &ehead->chain);
+	inet_twsk_add_node_rcu(tw, &ehead->chain);
 
 	/* Step 3: Remove SK from hash chain */
 	if (__sk_nulls_del_node_init_rcu(sk))