From: Eric Dumazet <edumazet@kernel.org>

A successful connect() of an IPv6 socket to a v4-mapped address switches
icsk_af_ops, sk_backlog_rcv and af_specific to their IPv4-mapped variants.
These are only restored if the connect() fails.

After connect(AF_UNSPEC), the socket can be reused for listen() or for a
native IPv6 connect while still using the mapped handlers. IPv6 packets
processed from the socket backlog are then handled by tcp_v4_do_rcv().
tcp_v4_send_reset() and tcp_v4_send_ack() interpret
TCP_SKB_CB(skb)->header.h6 as IPv4 options, and __ip_options_echo() can
overflow its 40-byte destination buffer.

Commit 8cc3aef0cb19 ("tcp: Do not allow buggy transitions between ehash
and lhash2.") prevents listen() after connect(AF_UNSPEC), but a native IPv6
connect is still allowed.

Add tcp_v6_disconnect() to restore the IPv6 handlers when a v4-mapped
socket is disconnected, and use the same helper in the connect error path.

MPTCP subflows override sk_prot, and two teardown paths call
tcp_disconnect() directly. Route those calls through the protocol-specific
disconnect operation so IPv6 MPTCP subflows receive the same restoration.

Tested with TCP and MPTCP by reconnecting an AF_INET6 socket from a
v4-mapped peer to a native IPv6 peer after connect(AF_UNSPEC).

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: Tong Liu <lyutoon@gmail.com>
Reported-by: Qi Tang <tpluszz77@gmail.com>
Closes: https://lore.kernel.org/netdev/CALf2hKvp0NPVLwr=a3+jUHtPohxbra8gox+aYAfWathJnn-J+Q@mail.gmail.com/
Link: https://lore.kernel.org/netdev/CAL4WiiqFJpmcV4=TWRpukCq6q=-729658FwGvieNcPXe97kLvQ@mail.gmail.com/
Signed-off-by: Eric Dumazet <edumazet@kernel.org>
Co-developed-by: Zhiyu Zhang <zhiyuzhang999@gmail.com>
Signed-off-by: Zhiyu Zhang <zhiyuzhang999@gmail.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
---
Changes in v2:
- Drop redundant Reported-by tag.
- Drop the unrelated watch_queue patch link.
- Add Kuniyuki's Reviewed-by tag.
- No code changes.

v1: https://lore.kernel.org/r/20261030032314.1978575-1-zhiyuzhang999@gmail.com/

 net/ipv6/tcp_ipv6.c  | 37 ++++++++++++++++++++++++++++---------
 net/mptcp/protocol.c |  4 ++--
 2 files changed, 30 insertions(+), 11 deletions(-)

diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index e006a30360ea..a93410aab66e 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -130,6 +130,19 @@ static int tcp_v6_pre_connect(struct sock *sk, struct sockaddr_unsized *uaddr,
 	return BPF_CGROUP_RUN_PROG_INET6_CONNECT(sk, uaddr, &addr_len);
 }
 
+/* Undo the v4-mapped setup done by tcp_v6_connect(). */
+static void tcp_v6_restore_af_ops(struct sock *sk)
+{
+	/* Paired with READ_ONCE() in tcp_(get|set)sockopt() */
+	WRITE_ONCE(inet_csk(sk)->icsk_af_ops, &ipv6_specific);
+	if (sk_is_mptcp(sk))
+		mptcpv6_handle_mapped(sk, false);
+	sk->sk_backlog_rcv = tcp_v6_do_rcv;
+#if defined(CONFIG_TCP_MD5SIG) || defined(CONFIG_TCP_AO)
+	tcp_sk(sk)->af_specific = &tcp_sock_ipv6_specific;
+#endif
+}
+
 static int tcp_v6_connect(struct sock *sk, struct sockaddr_unsized *uaddr,
 			  int addr_len)
 {
@@ -243,14 +256,7 @@ static int tcp_v6_connect(struct sock *sk, struct sockaddr_unsized *uaddr,
 
 		if (err) {
 			icsk->icsk_ext_hdr_len = exthdrlen;
-			/* Paired with READ_ONCE() in tcp_(get|set)sockopt() */
-			WRITE_ONCE(icsk->icsk_af_ops, &ipv6_specific);
-			if (sk_is_mptcp(sk))
-				mptcpv6_handle_mapped(sk, false);
-			sk->sk_backlog_rcv = tcp_v6_do_rcv;
-#if defined(CONFIG_TCP_MD5SIG) || defined(CONFIG_TCP_AO)
-			tp->af_specific = &tcp_sock_ipv6_specific;
-#endif
+			tcp_v6_restore_af_ops(sk);
 			goto failure;
 		}
 		np->saddr = sk->sk_v6_rcv_saddr;
@@ -2093,6 +2099,19 @@ static int tcp_v6_init_sock(struct sock *sk)
 	return 0;
 }
 
+static int tcp_v6_disconnect(struct sock *sk, int flags)
+{
+	int ret = tcp_disconnect(sk, flags);
+
+	/* A socket used for a v4-mapped connection must not keep IPv4
+	 * handlers when it is reused for listen() or a native IPv6 connect().
+	 */
+	if (sk->sk_backlog_rcv == tcp_v4_do_rcv)
+		tcp_v6_restore_af_ops(sk);
+
+	return ret;
+}
+
 #ifdef CONFIG_PROC_FS
 /* Proc filesystem TCPv6 sock list dumping. */
 static void get_openreq6(struct seq_file *seq,
@@ -2283,7 +2302,7 @@ struct proto tcpv6_prot = {
 	.close			= tcp_close,
 	.pre_connect		= tcp_v6_pre_connect,
 	.connect		= tcp_v6_connect,
-	.disconnect		= tcp_disconnect,
+	.disconnect		= tcp_v6_disconnect,
 	.accept			= inet_csk_accept,
 	.ioctl			= tcp_ioctl,
 	.init			= tcp_v6_init_sock,
diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 7a22e9388e0e..fdfbc7de2ec0 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -2608,7 +2608,7 @@ static void __mptcp_subflow_disconnect(struct sock *ssk,
 		/* The MPTCP code never wait on the subflow sockets, TCP-level
 		 * disconnect should never fail
 		 */
-		WARN_ON_ONCE(tcp_disconnect(ssk, 0));
+		WARN_ON_ONCE(ssk->sk_prot->disconnect(ssk, 0));
 		mptcp_subflow_ctx_reset(subflow);
 	} else {
 		tcp_shutdown(ssk, SEND_SHUTDOWN);
@@ -3287,7 +3287,7 @@ void mptcp_subflow_shutdown(struct sock *sk, struct sock *ssk, int how)
 			break;
 		fallthrough;
 	case TCP_SYN_SENT:
-		WARN_ON_ONCE(tcp_disconnect(ssk, O_NONBLOCK));
+		WARN_ON_ONCE(ssk->sk_prot->disconnect(ssk, O_NONBLOCK));
 		break;
 	default:
 		if (__mptcp_check_fallback(mptcp_sk(sk))) {

base-commit: 5838f10563d58afca6711edb5e4eda968ca181d9
-- 
2.34.1

Hi Eric,

Thank you for your modifications, that's great!

Our CI did some validations and here is its report:

- KVM Validation: normal (except selftest_mptcp_join): Success! ✅
- KVM Validation: normal (only selftest_mptcp_join): Success! ✅
- KVM Validation: debug (except selftest_mptcp_join): Success! ✅
- KVM Validation: debug (only selftest_mptcp_join): Success! ✅
- KVM Validation: btf-normal (only bpftest_all): Unstable: 3 failed test(s): bpftest_test_progs-cpuv4_mptcp bpftest_test_progs-no_alu32_mptcp bpftest_test_progs_mptcp ⚠️ 
- KVM Validation: btf-debug (only bpftest_all): Unstable: 3 failed test(s): bpftest_test_progs-cpuv4_mptcp bpftest_test_progs-no_alu32_mptcp bpftest_test_progs_mptcp ⚠️ 
- Perf: Unstable: 1 failed test(s): validation:sched-weak-secondary ⚠️ 
- Task: https://github.com/multipath-tcp/mptcp_net-next/actions/runs/41338757537

Initiator: Patchew Applier
Commits: https://github.com/multipath-tcp/mptcp_net-next/commits/f81decb25049
Patchwork: https://patchwork.kernel.org/project/mptcp/list/?series=1198226


If there are some issues, you can reproduce them using the same environment as
the one used by the CI thanks to a docker image, e.g.:

    $ cd [kernel source code]
    $ docker run -v "${PWD}:${PWD}:rw" -w "${PWD}" --privileged --rm -it \
        --pull always mptcp/mptcp-upstream-virtme-docker:latest \
        auto-normal

For more details:

    https://github.com/multipath-tcp/mptcp-upstream-virtme-docker


Please note that despite all the efforts that have been already done to have a
stable tests suite when executed on a public CI like here, it is possible some
reported issues are not due to your modifications. Still, do not hesitate to
help us improve that ;-)

Cheers,
MPTCP GH Action bot
Bot operated by Matthieu Baerts (NGI0 Core)