sendmmsg/sendmsg: fix unsafe user pointer access

Dereferencing a user pointer directly from kernel-space without going
through the copy_from_user family of functions is a bad idea. Two of
such usages can be found in the sendmsg code path called from sendmmsg,
added by

commit c71d8ebe7a4496fb7231151cb70a6baa0cb56f9a upstream.
commit 5b47b8038f183b44d2d8ff1c7d11a5c1be706b34 in the 3.0-stable tree.

Usages are performed through memcmp() and memcpy() directly. Fix those
by using the already copied msg_sys structure instead of the __user *msg
structure. Note that msg_sys can be set to NULL by verify_compat_iovec()
or verify_iovec(), which requires additional NULL pointer checks.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <dgoulet@ev0ke.net>
CC: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
CC: Anton Blanchard <anton@samba.org>
CC: David S. Miller <davem@davemloft.net>
CC: stable <stable@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/socket.c

@@ -1965,8 +1965,9 @@ static int __sys_sendmsg(struct socket *sock, struct msghdr __user *msg,
 	 * used_address->name_len is initialized to UINT_MAX so that the first
 	 * destination address never matches.
 	 */
-	if (used_address && used_address->name_len == msg_sys->msg_namelen &&
-	    !memcmp(&used_address->name, msg->msg_name,
+	if (used_address && msg_sys->msg_name &&
+	    used_address->name_len == msg_sys->msg_namelen &&
+	    !memcmp(&used_address->name, msg_sys->msg_name,
 		    used_address->name_len)) {
 		err = sock_sendmsg_nosec(sock, msg_sys, total_len);
 		goto out_freectl;
@@ -1978,8 +1979,9 @@ static int __sys_sendmsg(struct socket *sock, struct msghdr __user *msg,
 	 */
 	if (used_address && err >= 0) {
 		used_address->name_len = msg_sys->msg_namelen;
-		memcpy(&used_address->name, msg->msg_name,
-		       used_address->name_len);
+		if (msg_sys->msg_name)
+			memcpy(&used_address->name, msg_sys->msg_name,
+			       used_address->name_len);
 	}
 
 out_freectl: