[PATCH net] net: clear transport header during tunnel decapsulation

[Eric Dumazet <edumazet@google.com>]

Syzbot triggered a DEBUG_NET_WARN_ON_ONCE(len > INT_MAX) assertion in
pskb_may_pull_reason() called from qdisc_pkt_len_segs_init().

The root cause is a stale, negative transport header offset carried over
during tunnel decapsulation. When a tunnel receiver (e.g., VXLAN or Geneve)
decapsulates a packet, it pulls the outer headers but leaves the transport
header pointing to the outer UDP header. This offset becomes negative
relative to the new skb->data (inner IP header).

If the packet bypasses GRO (e.g., an untrusted GSO packet flagged as
"unexpected GSO" by udp_unexpected_gso() due to missing tunnel GSO bits),
it is flushed directly to the stack as GRO_NORMAL. On ingress, Layer 2 Qdisc
processing (sch_handle_ingress) happens before Layer 3 IP reception
(ip_rcv_core) can run and reset the transport header. Consequently,
qdisc_pkt_len_segs_init() attempts to validate the transport header using
pskb_may_pull(skb, hdr_len + sizeof(tcphdr)). The negative hdr_len overflows
the unsigned cast in pskb_may_pull(), triggering the assertion.

Fix this by clearing the transport header to the ~0U sentinel value during
decapsulation. This ensures that:
1) The ingress Qdisc safely skips validation via !skb_transport_header_was_set()
   and returns early without warning.
2) The IP layer (ip_rcv_core) later correctly resets the transport header
   to the inner L4 header offset.

Introduce skb_unset_transport_header() helper and apply it in the main
decapsulation paths:
1) __iptunnel_pull_header() (covering Geneve, GRE, IPIP, SIT, etc.)
2) vxlan_rcv() (covering VXLAN)

This restores skb invariants at the decapsulation boundary without adding
overhead to the Qdisc fast path.

Fixes: 7fb4c1967011 ("net: pull headers in qdisc_pkt_len_segs_init()")
Reported-by: syzbot+d5d0d598a4cfdfafdc3b@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/6a3b853b.52ae72c2.136ac7.000c.GAE@google.com/T/#u
Signed-off-by: Eric Dumazet <edumazet@google.com>
Assisted-by: Gemini:gemini-3.1-pro
---
 drivers/net/vxlan/vxlan_core.c | 1 +
 include/linux/skbuff.h         | 5 +++++
 net/ipv4/ip_tunnel_core.c      | 1 +
 3 files changed, 7 insertions(+)

diff --git a/drivers/net/vxlan/vxlan_core.c b/drivers/net/vxlan/vxlan_core.c
index 67c367cc566233e809b0f70e0d939dd1c1ac0d9f..49318ad8164a2f2572fc58c0ed449b68922ae71e 100644
--- a/drivers/net/vxlan/vxlan_core.c
+++ b/drivers/net/vxlan/vxlan_core.c
@@ -1799,6 +1799,7 @@ static int vxlan_rcv(struct sock *sk, struct sk_buff *skb)
 
 	dev_dstats_rx_add(vxlan->dev, skb->len);
 	vxlan_vnifilter_count(vxlan, vni, vninode, VXLAN_VNI_STATS_RX, skb->len);
+	skb_unset_transport_header(skb);
 	gro_cells_receive(&vxlan->gro_cells, skb);
 
 	rcu_read_unlock();
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 115db8c44db21383632dd150a17c9ddcc03508e4..e8305a0fd3857ab85da4c2e8322989ed93e88d87 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -3084,6 +3084,11 @@ static inline bool skb_transport_header_was_set(const struct sk_buff *skb)
 	return skb->transport_header != (typeof(skb->transport_header))~0U;
 }
 
+static inline void skb_unset_transport_header(struct sk_buff *skb)
+{
+	skb->transport_header = (typeof(skb->transport_header))~0U;
+}
+
 static inline unsigned char *skb_transport_header(const struct sk_buff *skb)
 {
 	DEBUG_NET_WARN_ON_ONCE(!skb_transport_header_was_set(skb));
diff --git a/net/ipv4/ip_tunnel_core.c b/net/ipv4/ip_tunnel_core.c
index d3c677e9bff2080e4760347a3d873da4e83ac3ca..59192f58da2e3aae19d00505cc3bb04b083b77c5 100644
--- a/net/ipv4/ip_tunnel_core.c
+++ b/net/ipv4/ip_tunnel_core.c
@@ -134,6 +134,7 @@ int __iptunnel_pull_header(struct sk_buff *skb, int hdr_len,
 	__vlan_hwaccel_clear_tag(skb);
 	skb_set_queue_mapping(skb, 0);
 	skb_scrub_packet(skb, xnet);
+	skb_unset_transport_header(skb);
 
 	return iptunnel_pull_offloads(skb);
 }
-- 
2.55.0.rc0.799.gd6f94ed593-goog