Subject: [PATCH net v2] net: iptunnel: fix stale transport header after GRE/TEB decap

From: Jiayuan Chen <[email-masked]>
Date: Sun, 19 Apr 2026 17:08:15 +0800

syzbot reported a BUG.

I found that after GRE decapsulation in gretap/ip6gretap paths, the
transport_header becomes stale with a negative offset. The sequence is:

1. Before decap, transport_header points to the outer L4 (GRE) header.
2. __iptunnel_pull_header() calls skb_pull_rcsum() to advance skb->data
   past the GRE header, but does not update transport_header.
3. For TEB (gretap/ip6gretap), eth_type_trans() in ip_tunnel_rcv() /
   __ip6_tnl_rcv() further pulls ETH_HLEN (14 bytes) from skb->data.

After these two pulls, skb->data has moved forward while transport_header
still points to the old (now behind skb->data) position, resulting in a
negative skb_transport_offset(): typically -4 after GRE pull alone, or
-18 after GRE + inner Ethernet pull.

In the normal case where the inner frame is a recognizable protocol
(e.g., IPv4/TCP), the transport_header is subsequently overwritten by
ip_rcv_core() (or inet_gro_receive() on the GRO path) via
skb_set_transport_header(), and the stale value never reaches downstream
consumers. However, if the inner frame cannot be parsed (e.g.,
eth_type_trans() classifies it as ETH_P_802_2 due to a zero/invalid
inner Ethernet header), neither rescue runs, and the stale offset
persists into __netif_receive_skb_core().

When this stale offset is combined with contradictory GSO metadata (e.g.,
SKB_GSO_TCPV4 injected via virtio_net_hdr from a tun device),
qdisc_pkt_len_segs_init() trusts the negative offset: the unsigned
wraparound makes pskb_may_pull() effectively a no-op, and __tcp_hdrlen()
then reads from an invalid memory location, causing a use-after-free.

The UAF only triggers on the GSO path, where qdisc_pkt_len_segs_init()
dereferences the transport header to compute per-segment length. Fix
this by introducing iptunnel_rebuild_transport_header(), which is a
no-op for non-GSO packets and otherwise re-probes the transport header
via the flow dissector. If re-probing fails, the contradictory GSO
metadata is cleared via skb_gso_reset() so downstream consumers cannot
trust stale offsets. Restricting the rebuild to GSO packets keeps the
flow-dissector cost off the common rx fast path.

reproducer: https://gist.github.com/mrpre/5ba943fd86367af748b70de99263da4b

Link: https://syzkaller.appspot.com/bug?extid=83181a31faf9455499c5
Fixes: c54419321455 ("GRE: Refactor GRE tunneling code.")
Fixes: 0d3c703a9d17 ("ipv6: Cleanup IPv6 tunnel receive path")
Reported-by: [email-masked]
Closes: https://lore.kernel.org/all/[email-masked]/T/
Signed-off-by: Jiayuan Chen <[email-masked]>
---

As a follow-up for production reliability, I am wondering whether we
can extend the existing safety net in __netif_receive_skb_core() to
also handle set-but-negative transport_header:

        if (!skb_transport_header_was_set(skb) ||
            skb_transport_offset(skb) < 0)
                skb_reset_transport_header(skb);
---
 include/net/ip_tunnels.h | 12 ++++++++++++
 net/ipv4/ip_tunnel.c     |  2 ++
 net/ipv6/ip6_tunnel.c    |  2 ++
 3 files changed, 16 insertions(+)

diff --git a/include/net/ip_tunnels.h b/include/net/ip_tunnels.h
index d708b66e55cd..9b4e662833a1 100644
--- a/include/net/ip_tunnels.h
+++ b/include/net/ip_tunnels.h
@@ -662,6 +662,18 @@ static inline int iptunnel_pull_offloads(struct sk_buff *skb)
 	return 0;
 }
 
+static inline void iptunnel_rebuild_transport_header(struct sk_buff *skb)
+{
+	if (!skb_is_gso(skb))
+		return;
+
+	skb->transport_header = (typeof(skb->transport_header))~0U;
+	skb_probe_transport_header(skb);
+
+	if (!skb_transport_header_was_set(skb))
+		skb_gso_reset(skb);
+}
+
 static inline void iptunnel_xmit_stats(struct net_device *dev, int pkt_len)
 {
 	if (pkt_len > 0) {
diff --git a/net/ipv4/ip_tunnel.c b/net/ipv4/ip_tunnel.c
index 50d0f5fe4e4c..c46be68cfafa 100644
--- a/net/ipv4/ip_tunnel.c
+++ b/net/ipv4/ip_tunnel.c
@@ -445,6 +445,8 @@ int ip_tunnel_rcv(struct ip_tunnel *tunnel, struct sk_buff *skb,
 	if (tun_dst)
 		skb_dst_set(skb, (struct dst_entry *)tun_dst);
 
+	iptunnel_rebuild_transport_header(skb);
+
 	gro_cells_receive(&tunnel->gro_cells, skb);
 	return 0;
 
diff --git a/net/ipv6/ip6_tunnel.c b/net/ipv6/ip6_tunnel.c
index 46bc06506470..f95348cf3c77 100644
--- a/net/ipv6/ip6_tunnel.c
+++ b/net/ipv6/ip6_tunnel.c
@@ -879,6 +879,8 @@ static int __ip6_tnl_rcv(struct ip6_tnl *tunnel, struct sk_buff *skb,
 	if (tun_dst)
 		skb_dst_set(skb, (struct dst_entry *)tun_dst);
 
+	iptunnel_rebuild_transport_header(skb);
+
 	gro_cells_receive(&tunnel->gro_cells, skb);
 	return 0;
 
-- 
2.43.0