audit: use nlmsg_len() to get message payload length

Using the nlmsg_len member of the netlink header to test if the message
is valid is wrong as it includes the size of the netlink header itself.
Thereby allowing to send short netlink messages that pass those checks.

Use nlmsg_len() instead to test for the right message length. The result
of nlmsg_len() is guaranteed to be non-negative as the netlink message
already passed the checks of nlmsg_ok().

Also switch to min_t() to please checkpatch.pl.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Eric Paris <eparis@redhat.com>
Cc: stable@vger.kernel.org  # v2.6.6+ for the 1st hunk, v2.6.23+ for the 2nd
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Eric Paris <eparis@redhat.com>

kernel/audit.c

@@ -771,7 +771,7 @@ static int audit_receive_msg(struct sk_buff *skb, struct nlmsghdr *nlh)
 				 &status_set, sizeof(status_set));
 		break;
 	case AUDIT_SET:
-		if (nlh->nlmsg_len < sizeof(struct audit_status))
+		if (nlmsg_len(nlh) < sizeof(struct audit_status))
 			return -EINVAL;
 		status_get   = (struct audit_status *)data;
 		if (status_get->mask & AUDIT_STATUS_ENABLED) {
@@ -944,7 +944,7 @@ static int audit_receive_msg(struct sk_buff *skb, struct nlmsghdr *nlh)
 
 		memset(&s, 0, sizeof(s));
 		/* guard against past and future API changes */
-		memcpy(&s, data, min(sizeof(s), (size_t)nlh->nlmsg_len));
+		memcpy(&s, data, min_t(size_t, sizeof(s), nlmsg_len(nlh)));
 		if ((s.enabled != 0 && s.enabled != 1) ||
 		    (s.log_passwd != 0 && s.log_passwd != 1))
 			return -EINVAL;