[PATCH net] tcp: initialize mapped children before route lookup

[Eric Dumazet <edumazet@kernel.org>]

From: Kyle Zeng <kylebot@openai.com>

IPv4 passive opens on a dual-stack listener go through
tcp_v4_syn_recv_sock() with tcp_v6_mapped_child_init() as the optional
initializer. The callback currently runs after the output route lookup.
If that lookup fails, tcp_done() destroys a clone whose pinet6 and
ipv6_fl_list still refer to the listener's state.

With the flowlabel list head in inet_sock, fl6_free_socklist() walks
the clone's list and RCU-frees its nodes without unlinking them from the
live listener. Subsequent flowlabel operations can use or free the nodes
again. Commit 858d2a4f67ff ("tcp: fix potential race in
tcp_v6_syn_recv_sock()") moved the initializer before ehash insertion,
but did not cover this error path.

Invoke the existing initializer after the IPv4 fields it consumes have
been set, but before route lookup can fail. It needs neither the
destination nor sk_setup_caps(), and all later disposal paths remain
covered.

Fixes: 1c17f4373d4d ("ipv6: Move ipv6_fl_list from ipv6_pinfo to inet_sock.")
Signed-off-by: Kyle Zeng <kylebot@openai.com>
Signed-off-by: Eric Dumazet <edumazet@kernel.org>
---
 net/ipv4/tcp_ipv4.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 3f8563a36db1e9ea853699ecb547af7ee9491b11..c08de825fa448cfcd42c9a8b407200b0dc5b7b1d 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1723,6 +1723,12 @@ struct sock *tcp_v4_syn_recv_sock(const struct sock *sk, struct sk_buff *skb,
 	if (READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_reflect_tos))
 		newinet->tos = tcp_rsk(req)->syn_tos & ~INET_ECN_MASK;
 
+#if IS_ENABLED(CONFIG_IPV6)
+	/* Detach IPv6 state before a route lookup can fail. */
+	if (opt_child_init)
+		opt_child_init(newsk, sk);
+#endif
+
 	if (!dst) {
 		dst = inet_csk_route_child_sock(sk, newsk, req);
 		if (!dst)
@@ -1732,10 +1738,6 @@ struct sock *tcp_v4_syn_recv_sock(const struct sock *sk, struct sk_buff *skb,
 	}
 	sk_setup_caps(newsk, dst);
 
-#if IS_ENABLED(CONFIG_IPV6)
-	if (opt_child_init)
-		opt_child_init(newsk, sk);
-#endif
 	tcp_ca_openreq_child(newsk, dst);
 
 	tcp_sync_mss(newsk, dst4_mtu(dst));
-- 
2.53.0