dccp: check ccid before dereferencing

ccid_hc_rx_getsockopt() and ccid_hc_tx_getsockopt() might be called with
a NULL ccid pointer leading to a NULL pointer dereference. This could
lead to a privilege escalation if the attacker is able to map page 0 and
prepare it with a fake ccid_ops pointer.
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Cc: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Cc: stable@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dccp/ccid.h

@@ -246,7 +246,7 @@ static inline int ccid_hc_rx_getsockopt(struct ccid *ccid, struct sock *sk,
 					u32 __user *optval, int __user *optlen)
 {
 	int rc = -ENOPROTOOPT;
-	if (ccid->ccid_ops->ccid_hc_rx_getsockopt != NULL)
+	if (ccid != NULL && ccid->ccid_ops->ccid_hc_rx_getsockopt != NULL)
 		rc = ccid->ccid_ops->ccid_hc_rx_getsockopt(sk, optname, len,
 						 optval, optlen);
 	return rc;
@@ -257,7 +257,7 @@ static inline int ccid_hc_tx_getsockopt(struct ccid *ccid, struct sock *sk,
 					u32 __user *optval, int __user *optlen)
 {
 	int rc = -ENOPROTOOPT;
-	if (ccid->ccid_ops->ccid_hc_tx_getsockopt != NULL)
+	if (ccid != NULL && ccid->ccid_ops->ccid_hc_tx_getsockopt != NULL)
 		rc = ccid->ccid_ops->ccid_hc_tx_getsockopt(sk, optname, len,
 						 optval, optlen);
 	return rc;