tcp/dccp: fix hashdance race for passive sessions

Multiple cpus can process duplicates of incoming ACK messages
matching a SYN_RECV request socket. This is a rare event under
normal operations, but definitely can happen.

Only one must win the race, otherwise corruption would occur.

To fix this without adding new atomic ops, we use logic in
inet_ehash_nolisten() to detect the request was present in the same
ehash bucket where we try to insert the new child.

If request socket was not found, we have to undo the child creation.

This actually removes a spin_lock()/spin_unlock() pair in
reqsk_queue_unlink() for the fast path.

Fixes: e994b2f0fb92 ("tcp: do not lock listener to process SYN packets")
Fixes: 079096f103fa ("tcp/dccp: install syn_recv requests into ehash table")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 4 insertions, 2 deletions

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 30dd45c1f568..1c2648bbac4b 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1247,7 +1247,9 @@ EXPORT_SYMBOL(tcp_v4_conn_request);
  */
 struct sock *tcp_v4_syn_recv_sock(const struct sock *sk, struct sk_buff *skb,
                                   struct request_sock *req,
-                                  struct dst_entry *dst)
+                                  struct dst_entry *dst,
+                                  struct request_sock *req_unhash,
+                                  bool *own_req)
 {
         struct inet_request_sock *ireq;
         struct inet_sock *newinet;
@@ -1323,7 +1325,7 @@ struct sock *tcp_v4_syn_recv_sock(const struct sock *sk, struct sk_buff *skb,
 
         if (__inet_inherit_port(sk, newsk) < 0)
                 goto put_and_exit;
-        __inet_hash_nolisten(newsk, NULL);
+        *own_req = inet_ehash_nolisten(newsk, req_to_sk(req_unhash));
 
         return newsk;