tcp: tcp_child_process() related UAF

tcp_child_process( .. child ...) currently calls sock_put(child).

Unfortunately @child (named @nsk in callers) can be used after
this point to send a RST packet.

To fix this UAF, I remove the sock_put() from tcp_child_process()
and let the callers handle this after it is safe.

Remove @rsk variable in tcp_v4_do_rcv() and change tcp_v6_do_rcv()
so that both functions look the same.

Fixes: cfb6eeb4c860 ("[TCP]: MD5 Signature Option (RFC2385) support.")
Reported-by: Damiano Melotti <melotti@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260505153927.3435532-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/tcp_ipv4.c

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 8fc24c3..c0526cc 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1827,7 +1827,6 @@ INDIRECT_CALLABLE_DECLARE(struct dst_entry *ipv4_dst_check(struct dst_entry *,
 int tcp_v4_do_rcv(struct sock *sk, struct sk_buff *skb)
 {
 	enum skb_drop_reason reason;
-	struct sock *rsk;
 
 	reason = psp_sk_rx_policy_check(sk, skb);
 	if (reason)
@@ -1863,24 +1862,21 @@ int tcp_v4_do_rcv(struct sock *sk, struct sk_buff *skb)
 			return 0;
 		if (nsk != sk) {
 			reason = tcp_child_process(sk, nsk, skb);
-			if (reason) {
-				rsk = nsk;
+			sock_put(nsk);
+			if (reason)
 				goto reset;
-			}
 			return 0;
 		}
 	} else
 		sock_rps_save_rxhash(sk, skb);
 
 	reason = tcp_rcv_state_process(sk, skb);
-	if (reason) {
-		rsk = sk;
+	if (reason)
 		goto reset;
-	}
 	return 0;
 
 reset:
-	tcp_v4_send_reset(rsk, skb, sk_rst_convert_drop_reason(reason));
+	tcp_v4_send_reset(sk, skb, sk_rst_convert_drop_reason(reason));
 discard:
 	sk_skb_reason_drop(sk, skb, reason);
 	/* Be careful here. If this function gets more complicated and
@@ -2193,8 +2189,10 @@ int tcp_v4_rcv(struct sk_buff *skb)
 
 				rst_reason = sk_rst_convert_drop_reason(drop_reason);
 				tcp_v4_send_reset(nsk, skb, rst_reason);
+				sock_put(nsk);
 				goto discard_and_relse;
 			}
+			sock_put(nsk);
 			sock_put(sk);
 			return 0;
 		}

net/ipv4/tcp_minisocks.c

diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
index 199f0b5..e6092c3 100644
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c
@@ -1012,6 +1012,6 @@ enum skb_drop_reason tcp_child_process(struct sock *parent, struct sock *child,
 	}
 
 	bh_unlock_sock(child);
-	sock_put(child);
+
 	return reason;
 }

net/ipv6/tcp_ipv6.c

diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index 2c3f7a7..51583aef 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -1617,12 +1617,13 @@ int tcp_v6_do_rcv(struct sock *sk, struct sk_buff *skb)
 	if (sk->sk_state == TCP_LISTEN) {
 		struct sock *nsk = tcp_v6_cookie_check(sk, skb);
 
+		if (!nsk)
+			return 0;
 		if (nsk != sk) {
-			if (nsk) {
-				reason = tcp_child_process(sk, nsk, skb);
-				if (reason)
-					goto reset;
-			}
+			reason = tcp_child_process(sk, nsk, skb);
+			sock_put(nsk);
+			if (reason)
+				goto reset;
 			return 0;
 		}
 	} else
@@ -1827,8 +1828,10 @@ INDIRECT_CALLABLE_SCOPE int tcp_v6_rcv(struct sk_buff *skb)
 
 				rst_reason = sk_rst_convert_drop_reason(drop_reason);
 				tcp_v6_send_reset(nsk, skb, rst_reason);
+				sock_put(nsk);
 				goto discard_and_relse;
 			}
+			sock_put(nsk);
 			sock_put(sk);
 			return 0;
 		}