[PATCH net] tcp: avoid the lookup process failing to get sk in ehash table

[Jason Xing <kerneljasonxing@gmail.com>]

From: Jason Xing <kernelxing@tencent.com>

While one cpu is working on looking up the right socket from ehash
table, another cpu is done deleting the request socket and is about
to add (or is adding) the big socket from the table. It means that
we could miss both of them, even though it has little chance.

Let me draw a call trace map of the server side.
   CPU 0                           CPU 1
   -----                           -----
tcp_v4_rcv()                  syn_recv_sock()
                            inet_ehash_insert()
                            -> sk_nulls_del_node_init_rcu(osk)
__inet_lookup_established()
                            -> __sk_nulls_add_node_rcu(sk, list)

Notice that the CPU 0 is receiving the data after the final ack
during 3-way shakehands and CPU 1 is still handling the final ack.

Why could this be a real problem?
This case is happening only when the final ack and the first data
receiving by different CPUs. Then the server receiving data with
ACK flag tries to search one proper established socket from ehash
table, but apparently it fails as my map shows above. After that,
the server fetches a listener socket and then sends a RST because
it finds a ACK flag in the skb (data), which obeys RST definition
in RFC 793.

Many thanks to Eric for great help from beginning to end.

Fixes: 5e0724d027f0 ("tcp/dccp: fix hashdance race for passive sessions")
Signed-off-by: Jason Xing <kernelxing@tencent.com>
---
 net/ipv4/inet_hashtables.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
index 24a38b56fab9..18f88cb4efcb 100644
--- a/net/ipv4/inet_hashtables.c
+++ b/net/ipv4/inet_hashtables.c
@@ -650,7 +650,16 @@ 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);
+		if (sk_hashed(osk))
+			/* 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_rcu(sk, list);
 		ret = sk_nulls_del_node_init_rcu(osk);
+		goto unlock;
 	} else if (found_dup_sk) {
 		*found_dup_sk = inet_ehash_lookup_by_sk(sk, list);
 		if (*found_dup_sk)
@@ -660,6 +669,7 @@ 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;
-- 
2.37.3