udp: Support gro_ipv4_max_size > 65536

Currently, gro_max_size and gro_ipv4_max_size can be set to values
bigger than 65536, and GRO will happily aggregate UDP to the configured
size (for example, with TCP traffic in VXLAN tunnels). However,
udp_gro_complete uses the 16-bit length field in the UDP header to store
the length of the aggregated packet. It leads to the packet truncation
later in udp_rcv.

Fix this by storing 0 to the UDP length field and by restoring the real
length from skb->len in udp_rcv. IP GRO already can store 0 to the IP
length field, and iph_totlen()/ipv6_payload_len() are capable of
restoring the real length, because the relevant packets (BIG TCP
tunneled in UDP tunnels) will have skb_is_gso_tcp == true.

Additionally, restrict handling uh->len=0 in udpv6_rcv to BIG TCP and
jumbograms only by using the udp_get_len helper.

Signed-off-by: Alice Mikityanska <alice@isovalent.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260710134242.216538-5-alice.kernel@fastmail.im
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

4 files changed, 7 insertions, 7 deletions

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index cb86124fd963..2b9b55a14758 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -2592,8 +2592,8 @@ int udp_rcv(struct sk_buff *skb)
 	struct rtable *rt = skb_rtable(skb);
 	struct net *net = dev_net(skb->dev);
 	struct sock *sk = NULL;
-	unsigned short ulen;
 	__be32 saddr, daddr;
+	unsigned int ulen;
 	struct udphdr *uh;
 	bool refcounted;
 	int drop_reason;
@@ -2607,7 +2607,7 @@ int udp_rcv(struct sk_buff *skb)
 		goto drop;		/* No space for header. */
 
 	uh   = udp_hdr(skb);
-	ulen = ntohs(uh->len);
+	ulen = udp_get_len(skb, uh, 0);
 	saddr = ip_hdr(skb)->saddr;
 	daddr = ip_hdr(skb)->daddr;
 
diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
index 493e2b9e16fb..4f9a3922937c 100644
--- a/net/ipv4/udp_offload.c
+++ b/net/ipv4/udp_offload.c
@@ -919,7 +919,7 @@ int udp_gro_complete(struct sk_buff *skb, int nhoff,
 	struct sock *sk;
 	int err;
 
-	udp_set_len_short(uh, newlen);
+	udp_set_len(uh, newlen);
 
 	sk = INDIRECT_CALL_INET(lookup, udp6_lib_lookup_skb,
 				udp4_lib_lookup_skb, skb, uh->source, uh->dest);
@@ -956,7 +956,7 @@ INDIRECT_CALLABLE_SCOPE int udp4_gro_complete(struct sk_buff *skb, int nhoff)
 
 	/* do fraglist only if there is no outer UDP encap (or we already processed it) */
 	if (NAPI_GRO_CB(skb)->is_flist && !NAPI_GRO_CB(skb)->encap_mark) {
-		udp_set_len_short(uh, skb->len - nhoff);
+		udp_set_len(uh, skb->len - nhoff);
 
 		skb_shinfo(skb)->gso_type |= (SKB_GSO_FRAGLIST|SKB_GSO_UDP_L4);
 		skb_shinfo(skb)->gso_segs = NAPI_GRO_CB(skb)->count;
diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index 8bbd55546b6e..02272466f4c2 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -1081,12 +1081,12 @@ INDIRECT_CALLABLE_SCOPE int udpv6_rcv(struct sk_buff *skb)
 	daddr = &ipv6_hdr(skb)->daddr;
 	uh = udp_hdr(skb);
 
-	ulen = ntohs(uh->len);
+	ulen = udp_get_len(skb, uh, 0);
 	if (ulen > skb->len)
 		goto short_packet;
 
 	/* Check for jumbo payload */
-	if (ulen == 0)
+	if (ulen == 0 && inet6_is_jumbogram(skb))
 		ulen = skb->len;
 
 	if (ulen < sizeof(*uh))
diff --git a/net/ipv6/udp_offload.c b/net/ipv6/udp_offload.c
index c92cf5ee3e6a..7370bcb80332 100644
--- a/net/ipv6/udp_offload.c
+++ b/net/ipv6/udp_offload.c
@@ -171,7 +171,7 @@ int udp6_gro_complete(struct sk_buff *skb, int nhoff)
 
 	/* do fraglist only if there is no outer UDP encap (or we already processed it) */
 	if (NAPI_GRO_CB(skb)->is_flist && !NAPI_GRO_CB(skb)->encap_mark) {
-		udp_set_len_short(uh, skb->len - nhoff);
+		udp_set_len(uh, skb->len - nhoff);
 
 		skb_shinfo(skb)->gso_type |= (SKB_GSO_FRAGLIST|SKB_GSO_UDP_L4);
 		skb_shinfo(skb)->gso_segs = NAPI_GRO_CB(skb)->count;