net: rtnetlink: fix info leak in RTM_GETSTATS call

When RTM_GETSTATS was added the fields of its header struct were not all
initialized when returning the result thus leaking 4 bytes of information
to user-space per rtnl_fill_statsinfo call, so initialize them now. Thanks
to Alexander Potapenko for the detailed report and bisection.
Reported-by: Alexander Potapenko <glider@google.com>
Fixes: 10c9ead9 ("rtnetlink: add new RTM_GETSTATS message to dump link stats")
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Acked-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -3854,6 +3854,9 @@ static int rtnl_fill_statsinfo(struct sk_buff *skb, struct net_device *dev,
 		return -EMSGSIZE;
 
 	ifsm = nlmsg_data(nlh);
+	ifsm->family = PF_UNSPEC;
+	ifsm->pad1 = 0;
+	ifsm->pad2 = 0;
 	ifsm->ifindex = dev->ifindex;
 	ifsm->filter_mask = filter_mask;