net: limit altnames to 64k total

Property list (altname is a link "property") is wrapped
in a nlattr. nlattrs length is 16bit so practically
speaking the list of properties can't be longer than
that, otherwise user space would have to interpret
broken netlink messages.

Prevent the problem from occurring by checking the length
of the property list before adding new entries.
Reported-by: George Shuklin <george.shuklin@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/rtnetlink.c

@@ -3652,12 +3652,23 @@ static int rtnl_alt_ifname(int cmd, struct net_device *dev, struct nlattr *attr,
 			   bool *changed, struct netlink_ext_ack *extack)
 {
 	char *alt_ifname;
+	size_t size;
 	int err;
 
 	err = nla_validate(attr, attr->nla_len, IFLA_MAX, ifla_policy, extack);
 	if (err)
 		return err;
 
+	if (cmd == RTM_NEWLINKPROP) {
+		size = rtnl_prop_list_size(dev);
+		size += nla_total_size(ALTIFNAMSIZ);
+		if (size >= U16_MAX) {
+			NL_SET_ERR_MSG(extack,
+				       "effective property list too long");
+			return -EINVAL;
+		}
+	}
+
 	alt_ifname = nla_strdup(attr, GFP_KERNEL_ACCOUNT);
 	if (!alt_ifname)
 		return -ENOMEM;