[PATCH v2 net 2/2] ipv4: reserve one IP ID per segment for UDP GSO packets

[Eric Dumazet <edumazet@kernel.org>]

__ip_make_skb() reserves a single IP ID for UDP GSO packets, but GSO
assigns one IP ID per segment. Following packets then reuse these IDs,
either from inet->inet_id or from the shared generator.

Unless IP_PMTUDISC_DO/PROBE is used, DF is not set on UDP GSO packets,
so segments can be fragmented on the path and IP ID reuse can lead to
incorrect reassembly.

Reserve one IP ID per segment, using the same test as udp_send_skb().

Fixes: bec1f6f69736 ("udp: generate gso with UDP_SEGMENT")
Signed-off-by: Eric Dumazet <edumazet@kernel.org>
Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Cc: Willem de Bruijn <willemb@google.com>
---
 net/ipv4/ip_output.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index a24cc8ee11d3ea3069bcc0d4d12e6867c2c475f7..47a78f297a7f5f50633c2157b3bcaea3df5fe0b4 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -1410,6 +1410,7 @@ struct sk_buff *__ip_make_skb(struct sock *sk,
 	struct iphdr *iph;
 	u8 pmtudisc, ttl;
 	__be16 df = 0;
+	int segs;
 
 	skb = __skb_dequeue(queue);
 	if (!skb)
@@ -1464,7 +1465,19 @@ struct sk_buff *__ip_make_skb(struct sock *sk,
 	iph->ttl = ttl;
 	iph->protocol = sk->sk_protocol;
 	ip_copy_addrs(iph, fl4);
-	ip_select_ident(net, skb, sk);
+
+	/* UDP GSO packets are segmented later (see udp_send_skb()):
+	 * reserve one IP ID per segment.
+	 */
+	segs = 1;
+	if (cork->gso_size && sk_is_udp(sk)) {
+		int datalen = skb->len - skb_transport_offset(skb) -
+			      sizeof(struct udphdr);
+
+		if (datalen > cork->gso_size)
+			segs = DIV_ROUND_UP(datalen, cork->gso_size);
+	}
+	ip_select_ident_segs(net, skb, sk, segs);
 
 	if (opt) {
 		iph->ihl += opt->optlen >> 2;
-- 
2.56.0.rc1.315.gc6ed9934b7-goog