netlink: add nested array policy validation

Sometimes nested netlink attributes are just used as arrays, with
the nla_type() of each not being used; we have this in nl80211 and
e.g. NFTA_SET_ELEM_LIST_ELEMENTS.

Add the ability to validate this type of message directly in the
policy, by adding the type NLA_NESTED_ARRAY which does exactly
this: require a first level of nesting but ignore the attribute
type, and then inside each require a second level of nested and
validate those attributes against a given policy (if present).

Note that some nested array types actually require that all of
the entries have the same index, this is possible to express in
a nested policy already, apart from the validation that only the
one allowed type is used.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/netlink.h

@@ -172,6 +172,7 @@ enum {
 	NLA_FLAG,
 	NLA_MSECS,
 	NLA_NESTED,
+	NLA_NESTED_ARRAY,
 	NLA_NUL_STRING,
 	NLA_BINARY,
 	NLA_S8,
@@ -200,7 +201,8 @@ enum {
  *    NLA_NUL_STRING       Maximum length of string (excluding NUL)
  *    NLA_FLAG             Unused
  *    NLA_BINARY           Maximum length of attribute payload
- *    NLA_NESTED           Length verification is done by checking len of
+ *    NLA_NESTED,
+ *    NLA_NESTED_ARRAY     Length verification is done by checking len of
  *                         nested header (or empty); len field is used if
  *                         validation_data is also used, for the max attr
  *                         number in the nested policy.
@@ -230,6 +232,12 @@ enum {
  *                         `len' to the max attribute number.
  *                         Note that nla_parse() will validate, but of course not
  *                         parse, the nested sub-policies.
+ *    NLA_NESTED_ARRAY     Points to a nested policy to validate, must also set
+ *                         `len' to the max attribute number. The difference to
+ *                         NLA_NESTED is the structure - NLA_NESTED has the
+ *                         nested attributes directly inside, while an array has
+ *                         the nested attributes at another level down and the
+ *                         attributes directly in the nesting don't matter.
  *    All other            Unused
  *
  * Example:
@@ -255,6 +263,8 @@ struct nla_policy {
 
 #define NLA_POLICY_NESTED(maxattr, policy) \
 	{ .type = NLA_NESTED, .validation_data = policy, .len = maxattr }
+#define NLA_POLICY_NESTED_ARRAY(maxattr, policy) \
+	{ .type = NLA_NESTED_ARRAY, .validation_data = policy, .len = maxattr }
 
 /**
  * struct nl_info - netlink source information