netlink: add NLA_REJECT policy type

In some situations some netlink attributes may be used for output
only (kernel->userspace) or may be reserved for future use. It's
then helpful to be able to prevent userspace from using them in
messages sent to the kernel, since they'd otherwise be ignored and
any future will become impossible if this happens.

Add NLA_REJECT to the policy which does nothing but reject (with
EINVAL) validation of any messages containing this attribute.
Allow for returning a specific extended ACK error message in the
validation_data pointer.

While at it clear up the documentation a bit - the NLA_BITFIELD32
documentation was added to the list of len field descriptions.

Also, use NL_SET_BAD_ATTR() in one place where it's open-coded.

The specific case I have in mind now is a shared nested attribute
containing request/response data, and it would be pointless and
potentially confusing to have userspace include response data in
the messages that actually contain a request.
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

@@ -180,6 +180,7 @@ enum {
 	NLA_S32,
 	NLA_S64,
 	NLA_BITFIELD32,
+	NLA_REJECT,
 	__NLA_TYPE_MAX,
 };
 
@@ -208,9 +209,19 @@ enum {
  *    NLA_MSECS            Leaving the length field zero will verify the
  *                         given type fits, using it verifies minimum length
  *                         just like "All other"
- *    NLA_BITFIELD32      A 32-bit bitmap/bitselector attribute
+ *    NLA_BITFIELD32       Unused
+ *    NLA_REJECT           Unused
  *    All other            Minimum length of attribute payload
  *
+ * Meaning of `validation_data' field:
+ *    NLA_BITFIELD32       This is a 32-bit bitmap/bitselector attribute and
+ *                         validation data must point to a u32 value of valid
+ *                         flags
+ *    NLA_REJECT           This attribute is always rejected and validation data
+ *                         may point to a string to report as the error instead
+ *                         of the generic one in extended ACK.
+ *    All other            Unused
+ *
  * Example:
  * static const struct nla_policy my_policy[ATTR_MAX+1] = {
  * 	[ATTR_FOO] = { .type = NLA_U16 },

lib/nlattr.c

@@ -69,7 +69,8 @@ static int validate_nla_bitfield32(const struct nlattr *nla,
 }
 
 static int validate_nla(const struct nlattr *nla, int maxtype,
-			const struct nla_policy *policy)
+			const struct nla_policy *policy,
+			const char **error_msg)
 {
 	const struct nla_policy *pt;
 	int minlen = 0, attrlen = nla_len(nla), type = nla_type(nla);
@@ -87,6 +88,11 @@ static int validate_nla(const struct nlattr *nla, int maxtype,
 	}
 
 	switch (pt->type) {
+	case NLA_REJECT:
+		if (pt->validation_data && error_msg)
+			*error_msg = pt->validation_data;
+		return -EINVAL;
+
 	case NLA_FLAG:
 		if (attrlen > 0)
 			return -ERANGE;
@@ -180,11 +186,10 @@ int nla_validate(const struct nlattr *head, int len, int maxtype,
 	int rem;
 
 	nla_for_each_attr(nla, head, len, rem) {
-		int err = validate_nla(nla, maxtype, policy);
+		int err = validate_nla(nla, maxtype, policy, NULL);
 
 		if (err < 0) {
-			if (extack)
-				extack->bad_attr = nla;
+			NL_SET_BAD_ATTR(extack, nla);
 			return err;
 		}
 	}
@@ -250,11 +255,15 @@ int nla_parse(struct nlattr **tb, int maxtype, const struct nlattr *head,
 		u16 type = nla_type(nla);
 
 		if (type > 0 && type <= maxtype) {
+			static const char _msg[] = "Attribute failed policy validation";
+			const char *msg = _msg;
+
 			if (policy) {
-				err = validate_nla(nla, maxtype, policy);
+				err = validate_nla(nla, maxtype, policy, &msg);
 				if (err < 0) {
-					NL_SET_ERR_MSG_ATTR(extack, nla,
-							    "Attribute failed policy validation");
+					NL_SET_BAD_ATTR(extack, nla);
+					if (extack)
+						extack->_msg = msg;
 					goto errout;
 				}
 			}