gtp: check skb_pull_data() return in gtp1u_send_echo_resp()

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
Signed-off-by: Xiang Mei (Microsoft) <xmei5@asu.edu>
Link: https://patch.msgid.link/20260710230724.942574-1-xmei5@asu.edu
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/drivers/net/gtp.c b/drivers/net/gtp.c
index c0e38878af51b300aa4da9a41230d50731df8e24..9a12cc53da0021406e029ef306c4872ce2c69d08 100644 (file)
--- 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));