netlink: add ethernet address policy types

Commonly, ethernet addresses are just using a policy of
	{ .len = ETH_ALEN }
which leaves userspace free to send more data than it should,
which may hide bugs.

Introduce NLA_EXACT_LEN which checks for exact size, rejecting
the attribute if it's not exactly that length. Also add
NLA_EXACT_LEN_WARN which requires the minimum length and will
warn on longer attributes, for backward compatibility.

Use these to define NLA_POLICY_ETH_ADDR (new strict policy) and
NLA_POLICY_ETH_ADDR_COMPAT (compatible policy with warning);
these are used like this:

    static const struct nla_policy <name>[...] = {
        [NL_ATTR_NAME] = NLA_POLICY_ETH_ADDR,
        ...
    };
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Reviewed-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/netlink.h

@@ -181,6 +181,8 @@ enum {
 	NLA_S64,
 	NLA_BITFIELD32,
 	NLA_REJECT,
+	NLA_EXACT_LEN,
+	NLA_EXACT_LEN_WARN,
 	__NLA_TYPE_MAX,
 };
 
@@ -211,6 +213,10 @@ enum {
  *                         just like "All other"
  *    NLA_BITFIELD32       Unused
  *    NLA_REJECT           Unused
+ *    NLA_EXACT_LEN        Attribute must have exactly this length, otherwise
+ *                         it is rejected.
+ *    NLA_EXACT_LEN_WARN   Attribute should have exactly this length, a warning
+ *                         is logged if it is longer, shorter is rejected.
  *    All other            Minimum length of attribute payload
  *
  * Meaning of `validation_data' field:
@@ -236,6 +242,13 @@ struct nla_policy {
 	void            *validation_data;
 };
 
+#define NLA_POLICY_EXACT_LEN(_len)	{ .type = NLA_EXACT_LEN, .len = _len }
+#define NLA_POLICY_EXACT_LEN_WARN(_len)	{ .type = NLA_EXACT_LEN_WARN, \
+					  .len = _len }
+
+#define NLA_POLICY_ETH_ADDR		NLA_POLICY_EXACT_LEN(ETH_ALEN)
+#define NLA_POLICY_ETH_ADDR_COMPAT	NLA_POLICY_EXACT_LEN_WARN(ETH_ALEN)
+
 /**
  * struct nl_info - netlink source information
  * @nlh: Netlink message header of original request

lib/nlattr.c

@@ -82,12 +82,18 @@ static int validate_nla(const struct nlattr *nla, int maxtype,
 
 	BUG_ON(pt->type > NLA_TYPE_MAX);
 
-	if (nla_attr_len[pt->type] && attrlen != nla_attr_len[pt->type]) {
+	if ((nla_attr_len[pt->type] && attrlen != nla_attr_len[pt->type]) ||
+	    (pt->type == NLA_EXACT_LEN_WARN && attrlen != pt->len)) {
 		pr_warn_ratelimited("netlink: '%s': attribute type %d has an invalid length.\n",
 				    current->comm, type);
 	}
 
 	switch (pt->type) {
+	case NLA_EXACT_LEN:
+		if (attrlen != pt->len)
+			return -ERANGE;
+		break;
+
 	case NLA_REJECT:
 		if (pt->validation_data && error_msg)
 			*error_msg = pt->validation_data;