Subject: [PATCH net 1/2] udp4: Rescan udp hash chains if cross-linked.

From: David Laight <[email-masked]>
Date: Mon, 27 Jan 2025 19:40:23 +0000

udp_lib_rehash() can get called at any time and will move a
socket to a different hash2 chain.
This can cause udp4_lib_lookup2() (processing incoming UDP) to
fail to find a socket and an ICMP port unreachable be sent.

Prior to ca065d0cf80fa the lookup used 'hlist_nulls' and checked
that the 'end if list' marker was on the correct list.

Signed-off-by: David Laight <[email-masked]>
---
 net/ipv4/udp.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 86d282618515..a8e2b431d348 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -425,16 +425,21 @@ static struct sock *udp4_lib_lookup2(const struct net *net,
 				     __be32 saddr, __be16 sport,
 				     __be32 daddr, unsigned int hnum,
 				     int dif, int sdif,
+				     unsigned int hash2, unsigned int mask,
 				     struct udp_hslot *hslot2,
 				     struct sk_buff *skb)
 {
+	unsigned int hash2_rescan;
 	struct sock *sk, *result;
 	int score, badness;
 	bool need_rescore;
 
+rescan:
+	hash2_rescan = hash2;
 	result = NULL;
 	badness = 0;
 	udp_portaddr_for_each_entry_rcu(sk, &hslot2->head) {
+		hash2_rescan = udp_sk(sk)->udp_portaddr_hash;
 		need_rescore = false;
 rescore:
 		score = compute_score(need_rescore ? result : sk, net, saddr,
@@ -475,6 +480,16 @@ static struct sock *udp4_lib_lookup2(const struct net *net,
 			goto rescore;
 		}
 	}
+
+	/* udp sockets can get moved to a different hash chain.
+	 * If the chains have got crossed then rescan.
+	 */                       
+	if ((hash2_rescan ^ hash2) & mask) {
+		/* Ensure hslot2->head is reread */
+		barrier();
+		goto rescan;
+	}
+
 	return result;
 }
 
@@ -654,7 +669,7 @@ struct sock *__udp4_lib_lookup(const struct net *net, __be32 saddr,
 	/* Lookup connected or non-wildcard socket */
 	result = udp4_lib_lookup2(net, saddr, sport,
 				  daddr, hnum, dif, sdif,
-				  hslot2, skb);
+				  hash2, udptable->mask, hslot2, skb);
 	if (!IS_ERR_OR_NULL(result) && result->sk_state == TCP_ESTABLISHED)
 		goto done;
 
@@ -680,7 +695,7 @@ struct sock *__udp4_lib_lookup(const struct net *net, __be32 saddr,
 
 	result = udp4_lib_lookup2(net, saddr, sport,
 				  htonl(INADDR_ANY), hnum, dif, sdif,
-				  hslot2, skb);
+				  hash2, udptable->mask, hslot2, skb);
 done:
 	if (IS_ERR(result))
 		return NULL;
-- 
2.39.5