[PATCH 4.9 158/175] net/l2tp: Fix reference count leak in l2tp_udp_recv_core

From: Xiyu Yang <redacted>

commit 9b6ff7eb666415e1558f1ba8a742f5db6a9954de upstream.

The reference count leak issue may take place in an error handling
path. If both conditions of tunnel->version == L2TP_HDR_VER_3 and the
return value of l2tp_v3_ensure_opt_in_linear is nonzero, the function
would directly jump to label invalid, without decrementing the reference
count of the l2tp_session object session increased earlier by
l2tp_tunnel_get_session(). This may result in refcount leaks.

Fix this issue by decrease the reference count before jumping to the
label invalid.

Fixes: 4522a70db7aa ("l2tp: fix reading optional fields of L2TPv3")
Signed-off-by: Xiyu Yang <redacted>
Signed-off-by: Xin Xiong <redacted>
Signed-off-by: Xin Tan <redacted>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 net/l2tp/l2tp_core.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c

@@ -994,8 +994,10 @@ static int l2tp_udp_recv_core(struct l2t
 	}
 
 	if (tunnel->version == L2TP_HDR_VER_3 &&
-	    l2tp_v3_ensure_opt_in_linear(session, skb, &ptr, &optr))
+	    l2tp_v3_ensure_opt_in_linear(session, skb, &ptr, &optr)) {
+		l2tp_session_dec_refcount(session);
 		goto error;
+	}
 
 	l2tp_recv_common(session, skb, ptr, optr, hdrflags, length, payload_hook);
 	l2tp_session_dec_refcount(session);