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Date:   Tue,  6 Oct 2020 14:32:02 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     netdev@...r.kernel.org
Cc:     dsahern@...il.com, Jakub Kicinski <kuba@...nel.org>,
        Johannes Berg <johannes.berg@...el.com>
Subject: [PATCH 2/2] netlink: export policy in extended ACK

From: Johannes Berg <johannes.berg@...el.com>

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.

Signed-off-by: Johannes Berg <johannes.berg@...el.com>
---
 include/linux/netlink.h      | 30 ++++++++++++++++++++----------
 include/net/netlink.h        |  3 +++
 include/uapi/linux/netlink.h |  2 ++
 lib/nlattr.c                 | 35 ++++++++++++++++++-----------------
 net/netlink/af_netlink.c     |  6 ++++++
 net/netlink/policy.c         | 17 +++++++++++++++++
 6 files changed, 66 insertions(+), 27 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 5a5ff97cc596..6c9b08c0f071 100644
--- a/include/net/netlink.h
+++ b/include/net/netlink.h
@@ -1946,6 +1946,9 @@ 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_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 eac8a6a648ea..b9c28c558698 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
diff --git a/lib/nlattr.c b/lib/nlattr.c
index 80ff9fe83696..cdd78c52c3c4 100644
--- a/lib/nlattr.c
+++ b/lib/nlattr.c
@@ -96,8 +96,8 @@ static int nla_validate_array(const struct nlattr *head, int len, int maxtype,
 			continue;
 
 		if (nla_len(entry) < NLA_HDRLEN) {
-			NL_SET_ERR_MSG_ATTR(extack, entry,
-					    "Array element too short");
+			NL_SET_ERR_MSG_ATTR_POL(extack, entry, policy,
+						"Array element too short");
 			return -ERANGE;
 		}
 
@@ -195,8 +195,8 @@ static int nla_validate_range_unsigned(const struct nla_policy *pt,
 		pr_warn_ratelimited("netlink: '%s': attribute type %d has an invalid length.\n",
 				    current->comm, pt->type);
 		if (validate & NL_VALIDATE_STRICT_ATTRS) {
-			NL_SET_ERR_MSG_ATTR(extack, nla,
-					    "invalid attribute length");
+			NL_SET_ERR_MSG_ATTR_POL(extack, nla, pt,
+						"invalid attribute length");
 			return -EINVAL;
 		}
 
@@ -208,11 +208,11 @@ static int nla_validate_range_unsigned(const struct nla_policy *pt,
 		bool binary = pt->type == NLA_BINARY;
 
 		if (binary)
-			NL_SET_ERR_MSG_ATTR(extack, nla,
-					    "binary attribute size out of range");
+			NL_SET_ERR_MSG_ATTR_POL(extack, nla, pt,
+						"binary attribute size out of range");
 		else
-			NL_SET_ERR_MSG_ATTR(extack, nla,
-					    "integer out of range");
+			NL_SET_ERR_MSG_ATTR_POL(extack, nla, pt,
+						"integer out of range");
 
 		return -ERANGE;
 	}
@@ -291,8 +291,8 @@ static int nla_validate_int_range_signed(const struct nla_policy *pt,
 	nla_get_range_signed(pt, &range);
 
 	if (value < range.min || value > range.max) {
-		NL_SET_ERR_MSG_ATTR(extack, nla,
-				    "integer out of range");
+		NL_SET_ERR_MSG_ATTR_POL(extack, nla, pt,
+					"integer out of range");
 		return -ERANGE;
 	}
 
@@ -346,8 +346,8 @@ static int validate_nla(const struct nlattr *nla, int maxtype,
 		pr_warn_ratelimited("netlink: '%s': attribute type %d has an invalid length.\n",
 				    current->comm, type);
 		if (validate & NL_VALIDATE_STRICT_ATTRS) {
-			NL_SET_ERR_MSG_ATTR(extack, nla,
-					    "invalid attribute length");
+			NL_SET_ERR_MSG_ATTR_POL(extack, nla, pt,
+						"invalid attribute length");
 			return -EINVAL;
 		}
 	}
@@ -355,14 +355,14 @@ static int validate_nla(const struct nlattr *nla, int maxtype,
 	if (validate & NL_VALIDATE_NESTED) {
 		if ((pt->type == NLA_NESTED || pt->type == NLA_NESTED_ARRAY) &&
 		    !(nla->nla_type & NLA_F_NESTED)) {
-			NL_SET_ERR_MSG_ATTR(extack, nla,
-					    "NLA_F_NESTED is missing");
+			NL_SET_ERR_MSG_ATTR_POL(extack, nla, pt,
+						"NLA_F_NESTED is missing");
 			return -EINVAL;
 		}
 		if (pt->type != NLA_NESTED && pt->type != NLA_NESTED_ARRAY &&
 		    pt->type != NLA_UNSPEC && (nla->nla_type & NLA_F_NESTED)) {
-			NL_SET_ERR_MSG_ATTR(extack, nla,
-					    "NLA_F_NESTED not expected");
+			NL_SET_ERR_MSG_ATTR_POL(extack, nla, pt,
+						"NLA_F_NESTED not expected");
 			return -EINVAL;
 		}
 	}
@@ -514,7 +514,8 @@ static int validate_nla(const struct nlattr *nla, int maxtype,
 
 	return 0;
 out_err:
-	NL_SET_ERR_MSG_ATTR(extack, nla, "Attribute failed policy validation");
+	NL_SET_ERR_MSG_ATTR_POL(extack, nla, pt,
+				"Attribute failed policy validation");
 	return err;
 }
 
diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index df675a8e1918..8273d5e70594 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -2420,6 +2420,9 @@ void netlink_ack(struct sk_buff *in_skb, struct nlmsghdr *nlh, int err,
 		tlvlen += nla_total_size(sizeof(u32));
 	if (nlk_has_extack && extack && extack->cookie_len)
 		tlvlen += nla_total_size(extack->cookie_len);
+	/* the max policy content is currently ~44 bytes for range min/max */
+	if (err && nlk_has_extack && extack && extack->policy)
+		tlvlen += 64;
 
 	if (tlvlen)
 		flags |= NLM_F_ACK_TLVS;
@@ -2452,6 +2455,9 @@ void netlink_ack(struct sk_buff *in_skb, struct nlmsghdr *nlh, int err,
 		if (extack->cookie_len)
 			WARN_ON(nla_put(skb, NLMSGERR_ATTR_COOKIE,
 					extack->cookie_len, extack->cookie));
+		if (extack->policy)
+			netlink_policy_dump_write_attr(skb, extack->policy,
+						       NLMSGERR_ATTR_POLICY);
 	}
 
 	nlmsg_end(skb, rep);
diff --git a/net/netlink/policy.c b/net/netlink/policy.c
index bb4e3ffb4924..b19f2a3f2533 100644
--- a/net/netlink/policy.c
+++ b/net/netlink/policy.c
@@ -332,6 +332,23 @@ __netlink_policy_dump_write_attr(struct netlink_policy_dump_state *state,
 	return -ENOBUFS;
 }
 
+/**
+ * netlink_policy_dump_write_attr - write a given attribute policy
+ * @skb: the message skb to write to
+ * @pt: the attribute's policy
+ * @nestattr: the nested attribute ID to use
+ *
+ * Returns: 0 on success, an error code otherwise; -%ENODATA is
+ *	    special, indicating that there's no policy data and
+ *	    the attribute is generally rejected.
+ */
+int netlink_policy_dump_write_attr(struct sk_buff *skb,
+				   const struct nla_policy *pt,
+				   int nestattr)
+{
+	return __netlink_policy_dump_write_attr(NULL, skb, pt, nestattr);
+}
+
 /**
  * netlink_policy_dump_write - write current policy dump attributes
  * @skb: the message skb to write to
-- 
2.26.2

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