From bc53aab4881adc134f495092034e9e7c0791d84c Mon Sep 17 00:00:00 2001 From: Xia Yan Date: Thu, 29 Oct 2026 21:58:22 +0800 Subject: [PATCH] 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: stable@vger.kernel.org Signed-off-by: Xia Yan Link: https://patch.msgid.link/20261029135822.448299-1-yanxia@cmss.chinamobile.com Signed-off-by: Paolo Abeni --- net/ipv6/datagram.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/net/ipv6/datagram.c b/net/ipv6/datagram.c index 191c9733ff9f..ff7a7ffbf983 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 @@ int __ip6_datagram_connect(struct sock *sk, struct sockaddr_unsized *uaddr, 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 @@ int __ip6_datagram_connect(struct sock *sk, struct sockaddr_unsized *uaddr, 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; }