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

[ Upstream commit 3f4ca5fafc08881d7a57daa20449d171f2887043 ]

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: 95c4998b98f4 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
index 25334aa3da04e5cfc59a739a04d590e78b99fc7f..33292983b8cfbcc1dbbf394fefaccf5c963ae706 100644 (file)
--- a/net/ipv4/inet_hashtables.c
+++ b/net/ipv4/inet_hashtables.c
@@ -536,8 +536,20 @@ 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;
@@ -546,6 +558,7 @@ 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 c411c87ae865fba8361addd7f1339b909123a397..a00102d7c7fd4b62b9bacabbffea22cb2f3d28f7 100644 (file)
--- a/net/ipv4/inet_timewait_sock.c
+++ b/net/ipv4/inet_timewait_sock.c
@@ -81,10 +81,10 @@ 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,
@@ -120,7 +120,7 @@ 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))