ipv6: IPV6_PKTINFO relied userspace providing correct length

Signed-off-by: NIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
Reported-by: NEric Sesterhenn <snakebyte@gmx.de>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/ipv6_sockglue.c

@@ -404,7 +404,7 @@ static int do_ipv6_setsockopt(struct sock *sk, int level, int optname,
 		else if (optlen < sizeof(struct in6_pktinfo) || optval == NULL)
 			goto e_inval;
 
-		if (copy_from_user(&pkt, optval, optlen)) {
+		if (copy_from_user(&pkt, optval, sizeof(struct in6_pktinfo))) {
 				retv = -EFAULT;
 				break;
 		}