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

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.

Besides, Eric pointed out there's one more race condition where it
handles tw socket hashdance. Only by adding to the tail of the list
before deleting the old one can we avoid the race if the reader has
already begun the bucket traversal and it would possibly miss the head.

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

Fixes: 5e0724d027f0 ("tcp/dccp: fix hashdance race for passive sessions")
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jason Xing <kernelxing@tencent.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/lkml/20230112065336.41034-1-kerneljasonxing@gmail.com/
Link: https://lore.kernel.org/r/20230118015941.1313-1-kerneljasonxing@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
index 24a38b56fab9e9d7d893e23b30d26e275359ec70..f58d73888638b916904083af542a1e921fc0467a 100644 (file)
--- a/net/ipv4/inet_hashtables.c
+++ b/net/ipv4/inet_hashtables.c
@@ -650,8 +650,20 @@ 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_nulls_del_node_init_rcu(osk);
-       } else if (found_dup_sk) {
+               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) {
                *found_dup_sk = inet_ehash_lookup_by_sk(sk, list);
                if (*found_dup_sk)
                        ret = false;
@@ -660,6 +672,7 @@ 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;

diff --git a/net/ipv4/inet_timewait_sock.c b/net/ipv4/inet_timewait_sock.c
index 1d77d992e6e77f7d96bd061be6dbb802c2566b3f..beed32fff484183f21e9d20834d67474e4565076 100644 (file)
--- a/net/ipv4/inet_timewait_sock.c
+++ b/net/ipv4/inet_timewait_sock.c
@@ -91,10 +91,10 @@ void inet_twsk_put(struct inet_timewait_sock *tw)
 }
 EXPORT_SYMBOL_GPL(inet_twsk_put);
 
-static void inet_twsk_add_node_rcu(struct inet_timewait_sock *tw,
-                                  struct hlist_nulls_head *list)
+static void inet_twsk_add_node_tail_rcu(struct inet_timewait_sock *tw,
+                                       struct hlist_nulls_head *list)
 {
-       hlist_nulls_add_head_rcu(&tw->tw_node, list);
+       hlist_nulls_add_tail_rcu(&tw->tw_node, list);
 }
 
 static void inet_twsk_add_bind_node(struct inet_timewait_sock *tw,
@@ -147,7 +147,7 @@ void inet_twsk_hashdance(struct inet_timewait_sock *tw, struct sock *sk,
 
        spin_lock(lock);
 
-       inet_twsk_add_node_rcu(tw, &ehead->chain);
+       inet_twsk_add_node_tail_rcu(tw, &ehead->chain);
 
        /* Step 3: Remove SK from hash chain */
        if (__sk_nulls_del_node_init_rcu(sk))