gtp1u_send_echo_resp() ignores skb_pull_data()'s return value. Its
caller gtp1u_udp_encap_recv() only guarantees 16 bytes (udphdr +
gtp1_header), but the pull requests 20 (gtp1_header_long + udphdr). For
a 16-19 byte echo request the pull fails and returns NULL without
advancing skb->data; execution continues, and the following skb_push()
plus the IP header pushed by iptunnel_xmit() move skb->data below
skb->head, tripping skb_under_panic().

Fix it by dropping the packet when skb_pull_data() fails.

  skbuff: skb_under_panic: ...
  kernel BUG at net/core/skbuff.c:214!
  Call Trace:
   skb_push (net/core/skbuff.c:2648)
   iptunnel_xmit (net/ipv4/ip_tunnel_core.c:82)
   gtp_encap_recv (drivers/net/gtp.c:701 drivers/net/gtp.c:808 drivers/net/gtp.c:920)
   udp_queue_rcv_one_skb (net/ipv4/udp.c:2388)
   ...
  Kernel panic - not syncing: Fatal exception in interrupt

Fixes: 9af41cc33471 ("gtp: Implement GTP echo response")
Reported-by: AutonomousCodeSecurity@microsoft.com
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Xiang Mei (Microsoft) <redacted>
---
 drivers/net/gtp.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/net/gtp.c b/drivers/net/gtp.c
index a60ef32b35b8..f8c7b532b10c 100644
--- a/drivers/net/gtp.c
+++ b/drivers/net/gtp.c

@@ -669,8 +669,9 @@ static int gtp1u_send_echo_resp(struct gtp_dev *gtp, struct sk_buff *skb)
 		return -1;
 
 	/* pull GTP and UDP headers */
-	skb_pull_data(skb,
-		      sizeof(struct gtp1_header_long) + sizeof(struct udphdr));
+	if (!skb_pull_data(skb, sizeof(struct gtp1_header_long) +
+				sizeof(struct udphdr)))
+		return -1;
 
 	gtp_pkt = skb_push(skb, sizeof(struct gtp1u_packet));
 	memset(gtp_pkt, 0, sizeof(struct gtp1u_packet));

-- 
2.43.0