[PATCH net] gtp: fix NULL pointer dereference in gtp0_handle_echo_resp()

["Cen Zhang (Microsoft)" <blbllhy@gmail.com>]

In gtp_create_sockets(), gtp->sk_created is set to true before gtp->sk0
and gtp->sk1u are assigned. A concurrent GTP Echo Response packet on
another CPU observes sk_created == true and dereferences the still-NULL
gtp->sk0 in gtp0_handle_echo_resp(), causing a kernel panic.

  KASAN: null-ptr-deref in range [0x0000000000000010-0x0000000000000017]
  RIP: 0010:gtp_encap_recv  (drivers/net/gtp.c:542 gtp0_handle_echo_resp)
  Call Trace:
   <IRQ>
   udp_queue_rcv_one_skb
   ip_protocol_deliver_rcu
   ip_local_deliver
  Kernel panic - not syncing: Fatal exception in interrupt

Reorder the assignments so that gtp->sk0 and gtp->sk1u are fully visible
before gtp->sk_created is set to true. This ensures no concurrent packet
path can observe the flag without valid socket pointers.

Fixes: b20dc3c68458 ("gtp: Allow to create GTP device without FDs")
Reported-by: AutonomousCodeSecurity@microsoft.com
Reported-by: Xiang Mei (Microsoft) <xmei5@asu.edu>
Reported-by: Cen Zhang (Microsoft) <blbllhy@gmail.com>
Signed-off-by: Cen Zhang (Microsoft) <blbllhy@gmail.com>
---
 drivers/net/gtp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/gtp.c b/drivers/net/gtp.c
index 9a12cc53da00..2b5a8f6d24d0 100644
--- a/drivers/net/gtp.c
+++ b/drivers/net/gtp.c
@@ -1456,9 +1456,9 @@ static int gtp_create_sockets(struct gtp_dev *gtp, const struct nlattr *nla,
 		return PTR_ERR(sk1u);
 	}
 
-	gtp->sk_created = true;
 	gtp->sk0 = sk0;
 	gtp->sk1u = sk1u;
+	gtp->sk_created = true;
 
 	return 0;
 }
-- 
2.52.0