[PATCH 6.19 126/342] virtio-net: correct hdr_len handling for tunnel gso

Greg Kroah-Hartman
2026-03-31T16:37:27Z

From: Xuan Zhuo <xuanzhuo@linux.alibaba.com>

[ Upstream commit 6c860dc02a8e60b438e26940227dfa641fcdb66a ]

The commit a2fb4bc4e2a6a03 ("net: implement virtio helpers to handle UDP
GSO tunneling.") introduces support for the UDP GSO tunnel feature in
virtio-net.

The virtio spec says:

If the \field{gso_type} has the VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV4 bit or
VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV6 bit set, \field{hdr_len} accounts for
all the headers up to and including the inner transport.

The commit did not update the hdr_len to include the inner transport.

I observed that the "hdr_len" is 116 for this packet:

17:36:18.241105 52:55:00:d1:27:0a > 2e:2c:df:46:a9:e1, ethertype IPv4 (0x0800), length 2912: (tos 0x0, ttl 64, id 45197, offset 0, flags [none], proto UDP (17), length 2898)
192.168.122.100.50613 > 192.168.122.1.4789: [bad udp cksum 0x8106 -> 0x26a0!] VXLAN, flags [I] (0x08), vni 1
fa:c3:ba:82:05:ee > ce:85:0c:31:77:e5, ethertype IPv4 (0x0800), length 2862: (tos 0x0, ttl 64, id 14678, offset 0, flags [DF], proto TCP (6), length 2848)
192.168.3.1.49880 > 192.168.3.2.9898: Flags [P.], cksum 0x9266 (incorrect -> 0xaa20), seq 515667:518463, ack 1, win 64, options [nop,nop,TS val 2990048824 ecr 2798801412], length 2796

116 = 14(mac) + 20(ip) + 8(udp) + 8(vxlan) + 14(inner mac) + 20(inner ip) + 32(innner tcp)

Fixes: a2fb4bc4e2a6a03 ("net: implement virtio helpers to handle UDP GSO tunneling.")
Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Link: https://patch.msgid.link/20260320021818.111741-3-xuanzhuo@linux.alibaba.com
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
include/linux/virtio_net.h | 19 +++++++++++++++++++
1 file changed, 19 insertions(+)

diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
index 361b60c8be680..f36d21b5bc19e 100644
--- a/include/linux/virtio_net.h
+++ b/include/linux/virtio_net.h
@@ -224,6 +224,22 @@ static inline void __virtio_net_set_hdrlen(const struct sk_buff *skb,
hdr->hdr_len = __cpu_to_virtio16(little_endian, hdr_len);
}

+/* This function must be called after virtio_net_hdr_from_skb(). */
+static inline void __virtio_net_set_tnl_hdrlen(const struct sk_buff *skb,
+ struct virtio_net_hdr *hdr)
+{
+ u16 hdr_len;
+
+ hdr_len = skb_inner_transport_offset(skb);
+
+ if (hdr->gso_type == VIRTIO_NET_HDR_GSO_UDP_L4)
+ hdr_len += sizeof(struct udphdr);
+ else
+ hdr_len += inner_tcp_hdrlen(skb);
+
+ hdr->hdr_len = __cpu_to_virtio16(true, hdr_len);
+}
+
static inline int virtio_net_hdr_from_skb(const struct sk_buff *skb,
struct virtio_net_hdr *hdr,
bool little_endian,
@@ -440,6 +456,9 @@ virtio_net_hdr_tnl_from_skb(const struct sk_buff *skb,
if (ret)
return ret;

+ if (feature_hdrlen && hdr->hdr_len)
+ __virtio_net_set_tnl_hdrlen(skb, hdr);
+
if (skb->protocol == htons(ETH_P_IPV6))
hdr->gso_type |= VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV6;
else
--
2.51.0