[PATCH v2 net 2/2] net: tap: fix wrong transport_header when sending VLAN-tagged frame

From: Wei Fang <[email protected]>

In tap_get_user_xdp(), when processing a VLAN-tagged frame (e.g.
ETH_P_8021Q), skb_set_network_header() is called first to advance
network_header past the VLAN tag to the inner protocol header.
skb_probe_transport_header() is then called with skb->protocol still
set to ETH_P_8021Q, while nhoff (derived from skb_network_offset())
already points past the VLAN tag to the inner protocol header.

In __skb_flow_dissect(), proto is initialized to ETH_P_8021Q and nhoff
points past the VLAN tag. When the dissector hits case ETH_P_8021Q, it
reads a struct vlan_hdr at the current nhoff via __skb_header_pointer(),
but that offset contains the inner protocol header (e.g. an IP header).
The bytes are misinterpreted as a VLAN header, yielding a garbage
encapsulated EtherType that matches no known protocol. The dissector
returns false, so skb_probe_transport_header() never calls
skb_set_transport_header(), leaving transport_header at its uninitialized
sentinel value (~0U).

Move skb_set_network_header() to after skb_probe_transport_header(). At
the time skb_probe_transport_header() is called, network_header still
points to the VLAN header (offset ETH_HLEN), so nhoff is correct and the
flow dissector can parse the VLAN header, extract the inner EtherType,
and advance nhoff to the inner protocol header, allowing transport_header
to be set correctly.

Fixes: 8c76e77f9069 ("tap: call skb_probe_transport_header after setting skb->dev")
Assisted-by: WChat:claude-opus-4-8
Signed-off-by: Wei Fang <[email protected]>
---
 drivers/net/tap.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/tap.c b/drivers/net/tap.c
index 5d2d34d24ce8..9b5af08cdd47 100644
--- a/drivers/net/tap.c
+++ b/drivers/net/tap.c
@@ -1091,12 +1091,13 @@ static int tap_get_user_xdp(struct tap_queue *q, struct xdp_buff *xdp)
 		}
 	}
 
+	skb_probe_transport_header(skb);
+
 	/* Move network header to the right position for VLAN tagged packets */
 	if (eth_type_vlan(skb->protocol) &&
 	    vlan_get_protocol_and_depth(skb, skb->protocol, &depth) != 0)
 		skb_set_network_header(skb, depth);
 
-	skb_probe_transport_header(skb);
 	dev_queue_xmit(skb);
 	rcu_read_unlock();
 
-- 
2.34.1