[PATCH net] ipv6: fix device binding after failed datagram connect

__ip6_datagram_connect() can change sk_bound_dev_if when connecting to
an address that needs a scope ID. If ip6_datagram_dst_update() fails,
the old peer address and port are restored, but the new device binding
is left behind.

A UDP socket can then keep reporting its original peer while sends and
attempts to reconnect to that peer fail with ENETUNREACH. Replies to the
socket are also dropped by the device match. The same problem occurs
when a scoped multicast destination picks up np->mcast_oif.

Save the original binding before processing the scope and restore it
along with the peer information on failure. This completes the rollback
in commit 2f987a76a977 ("net: ipv6: keep sk status consistent after
datagram connect failure") and preserves the new binding on success.

Found while reviewing scope-ID handling in __ip6_datagram_connect().

Confirmed by exchanging UDP traffic over ::1, then attempting a link-local
connect with a nonexistent scope ifindex or a down interface. After that
connect fails, the original peer is still reported but communication and
reconnecting to it fail.

Tested before and after the fix in UML for three rounds, covering explicit
scopes, multicast interface fallback, initially unconnected sockets and
existing bindings. With the fix, the old binding and peer are retained,
and traffic and reconnection to the original peer work. Also built on
x86_64 with W=1 using adjusted allmodconfig and allyesconfig
configurations, with no new warnings.

Fixes: 2f987a76a977 ("net: ipv6: keep sk status consistent after datagram connect failure")
Cc: <redacted>
Signed-off-by: Xia Yan <redacted>
---
 net/ipv6/datagram.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/ipv6/datagram.c b/net/ipv6/datagram.c
index 191c9733ff9fa..ff7a7ffbf9833 100644
--- a/net/ipv6/datagram.c
+++ b/net/ipv6/datagram.c

@@ -149,6 +149,7 @@ int __ip6_datagram_connect(struct sock *sk, struct sockaddr_unsized *uaddr,
 	__be32			fl6_flowlabel = 0;
 	__be32			old_fl6_flowlabel;
 	__be16			old_dport;
+	int			old_bound_dev_if;
 	int			addr_type;
 	int			err;
 

@@ -219,6 +220,8 @@ ipv4_connected:
 		goto out;
 	}
 
+	old_bound_dev_if = sk->sk_bound_dev_if;
+
 	if (__ipv6_addr_needs_scope_id(addr_type)) {
 		if (addr_len >= sizeof(struct sockaddr_in6) &&
 		    usin->sin6_scope_id) {

@@ -261,6 +264,7 @@ ipv4_connected:
 		sk->sk_v6_daddr = old_daddr;
 		np->flow_label = old_fl6_flowlabel;
 		inet->inet_dport = old_dport;
+		WRITE_ONCE(sk->sk_bound_dev_if, old_bound_dev_if);
 		goto out;
 	}
 

base-commit: d7314b51341c037973ce022f12c6009fd07b021e
-- 
2.39.0.windows.2