netlink: export policy in extended ACK

Add a new attribute NLMSGERR_ATTR_POLICY to the extended ACK
to advertise the policy, e.g. if an attribute was out of range,
you'll know the range that's permissible.

Add new NL_SET_ERR_MSG_ATTR_POL() and NL_SET_ERR_MSG_ATTR_POL()
macros to set this, since realistically it's only useful to do
this when the bad attribute (offset) is also returned.

Use it in lib/nlattr.c which practically does all the policy
validation.

v2:
 - add and use netlink_policy_dump_attr_size_estimate()
v3:
 - remove redundant break
v4:
 - really remove redundant break ... sorry

Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

3 files changed, 26 insertions, 10 deletions

diff --git a/include/linux/netlink.h b/include/linux/netlink.h
index e3e49f0e5c13..666cd0390699 100644
--- a/include/linux/netlink.h
+++ b/include/linux/netlink.h
@@ -68,12 +68,14 @@ netlink_kernel_create(struct net *net, int unit, struct netlink_kernel_cfg *cfg)
  * @_msg: message string to report - don't access directly, use
  *	%NL_SET_ERR_MSG
  * @bad_attr: attribute with error
+ * @policy: policy for a bad attribute
  * @cookie: cookie data to return to userspace (for success)
  * @cookie_len: actual cookie data length
  */
 struct netlink_ext_ack {
 	const char *_msg;
 	const struct nlattr *bad_attr;
+	const struct nla_policy *policy;
 	u8 cookie[NETLINK_MAX_COOKIE_LEN];
 	u8 cookie_len;
 };
@@ -95,21 +97,29 @@ struct netlink_ext_ack {
 #define NL_SET_ERR_MSG_MOD(extack, msg)			\
 	NL_SET_ERR_MSG((extack), KBUILD_MODNAME ": " msg)
 
-#define NL_SET_BAD_ATTR(extack, attr) do {		\
-	if ((extack))					\
+#define NL_SET_BAD_ATTR_POLICY(extack, attr, pol) do {	\
+	if ((extack)) {					\
 		(extack)->bad_attr = (attr);		\
+		(extack)->policy = (pol);		\
+	}						\
 } while (0)
 
-#define NL_SET_ERR_MSG_ATTR(extack, attr, msg) do {	\
-	static const char __msg[] = msg;		\
-	struct netlink_ext_ack *__extack = (extack);	\
-							\
-	if (__extack) {					\
-		__extack->_msg = __msg;			\
-		__extack->bad_attr = (attr);		\
-	}						\
+#define NL_SET_BAD_ATTR(extack, attr) NL_SET_BAD_ATTR_POLICY(extack, attr, NULL)
+
+#define NL_SET_ERR_MSG_ATTR_POL(extack, attr, pol, msg) do {	\
+	static const char __msg[] = msg;			\
+	struct netlink_ext_ack *__extack = (extack);		\
+								\
+	if (__extack) {						\
+		__extack->_msg = __msg;				\
+		__extack->bad_attr = (attr);			\
+		__extack->policy = (pol);			\
+	}							\
 } while (0)
 
+#define NL_SET_ERR_MSG_ATTR(extack, attr, msg)		\
+	NL_SET_ERR_MSG_ATTR_POL(extack, attr, NULL, msg)
+
 static inline void nl_set_extack_cookie_u64(struct netlink_ext_ack *extack,
 					    u64 cookie)
 {
diff --git a/include/net/netlink.h b/include/net/netlink.h
index 2b9e41075f19..7356f41d23ba 100644
--- a/include/net/netlink.h
+++ b/include/net/netlink.h
@@ -1957,6 +1957,10 @@ int netlink_policy_dump_get_policy_idx(struct netlink_policy_dump_state *state,
 bool netlink_policy_dump_loop(struct netlink_policy_dump_state *state);
 int netlink_policy_dump_write(struct sk_buff *skb,
 			      struct netlink_policy_dump_state *state);
+int netlink_policy_dump_attr_size_estimate(const struct nla_policy *pt);
+int netlink_policy_dump_write_attr(struct sk_buff *skb,
+				   const struct nla_policy *pt,
+				   int nestattr);
 void netlink_policy_dump_free(struct netlink_policy_dump_state *state);
 
 #endif
diff --git a/include/uapi/linux/netlink.h b/include/uapi/linux/netlink.h
index d02e472ba54c..c3816ff7bfc3 100644
--- a/include/uapi/linux/netlink.h
+++ b/include/uapi/linux/netlink.h
@@ -129,6 +129,7 @@ struct nlmsgerr {
  * @NLMSGERR_ATTR_COOKIE: arbitrary subsystem specific cookie to
  *	be used - in the success case - to identify a created
  *	object or operation or similar (binary)
+ * @NLMSGERR_ATTR_POLICY: policy for a rejected attribute
  * @__NLMSGERR_ATTR_MAX: number of attributes
  * @NLMSGERR_ATTR_MAX: highest attribute number
  */
@@ -137,6 +138,7 @@ enum nlmsgerr_attrs {
 	NLMSGERR_ATTR_MSG,
 	NLMSGERR_ATTR_OFFS,
 	NLMSGERR_ATTR_COOKIE,
+	NLMSGERR_ATTR_POLICY,
 
 	__NLMSGERR_ATTR_MAX,
 	NLMSGERR_ATTR_MAX = __NLMSGERR_ATTR_MAX - 1