[PATCH net v3] gtp: parse extension headers before reading inner protocol

[Zhixing Chen <running910@gmail.com>]

GTPv1-U packets may carry a chain of extension headers before the inner
IP packet. The receive path already parses and skips these extension
headers, but it currently reads the inner protocol before doing so.

As a result, the first extension header byte is interpreted as the inner
IP version. Packets with extension headers are then dropped before PDP
lookup.

Parse the extension header chain before calling gtp_inner_proto(), so the
inner protocol is read from the actual inner IP header.

Fixes: c75fc0b9e5be ("gtp: identify tunnel via GTP device + GTP version + TEID + family")
Signed-off-by: Zhixing Chen <running910@gmail.com>
---

Changes in v3:
- Refetch the GTP header after pskb_may_pull() before checking the
  extension-header flag.

Changes in v2:
- Add missing Fixes tag.

v2: https://lore.kernel.org/netdev/20260703093708.18141-1-running910@gmail.com/T/ 
v1: https://lore.kernel.org/netdev/20260703084244.59077-1-running910@gmail.com/T/

---
 drivers/net/gtp.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/drivers/net/gtp.c b/drivers/net/gtp.c
index a60ef32b35b8..c0e38878af51 100644
--- a/drivers/net/gtp.c
+++ b/drivers/net/gtp.c
@@ -826,13 +826,17 @@ static int gtp1u_udp_encap_recv(struct gtp_dev *gtp, struct sk_buff *skb)
 	if (!pskb_may_pull(skb, hdrlen))
 		return -1;
 
+	gtp1 = (struct gtp1_header *)(skb->data + sizeof(struct udphdr));
+
+	if (gtp1->flags & GTP1_F_EXTHDR &&
+	    gtp_parse_exthdrs(skb, &hdrlen) < 0)
+		return -1;
+
 	if (gtp_inner_proto(skb, hdrlen, &inner_proto) < 0) {
 		netdev_dbg(gtp->dev, "GTP packet does not encapsulate an IP packet\n");
 		return -1;
 	}
 
-	gtp1 = (struct gtp1_header *)(skb->data + sizeof(struct udphdr));
-
 	pctx = gtp1_pdp_find(gtp, ntohl(gtp1->tid),
 			     gtp_proto_to_family(inner_proto));
 	if (!pctx) {
@@ -840,10 +844,6 @@ static int gtp1u_udp_encap_recv(struct gtp_dev *gtp, struct sk_buff *skb)
 		return 1;
 	}
 
-	if (gtp1->flags & GTP1_F_EXTHDR &&
-	    gtp_parse_exthdrs(skb, &hdrlen) < 0)
-		return -1;
-
 	return gtp_rx(pctx, skb, hdrlen, gtp->role, inner_proto);
 }
 
-- 
2.34.1