sctp: copy the optval from user space in sctp_setsockopt

Prepare for for moving the copy_from_user from the individual sockopts
to the main setsockopt helper.  As of this commit the kopt variable
is not used yet, but the following commits will start using it.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sctp/socket.c

@@ -4677,6 +4677,7 @@ static int sctp_setsockopt_pf_expose(struct sock *sk,
 static int sctp_setsockopt(struct sock *sk, int level, int optname,
 			   char __user *optval, unsigned int optlen)
 {
+	void *kopt = NULL;
 	int retval = 0;
 
 	pr_debug("%s: sk:%p, optname:%d\n", __func__, sk, optname);
@@ -4693,6 +4694,12 @@ static int sctp_setsockopt(struct sock *sk, int level, int optname,
 		goto out_nounlock;
 	}
 
+	if (optlen > 0) {
+		kopt = memdup_user(optval, optlen);
+		if (IS_ERR(kopt))
+			return PTR_ERR(kopt);
+	}
+
 	lock_sock(sk);
 
 	switch (optname) {
@@ -4878,6 +4885,7 @@ static int sctp_setsockopt(struct sock *sk, int level, int optname,
 	}
 
 	release_sock(sk);
+	kfree(kopt);
 
 out_nounlock:
 	return retval;