Merge branch 'netlink-export-policy-on-validation-failures'

Johannes Berg says:

====================
netlink: export policy on validation failures

Export the policy used for attribute validation when it fails,
so e.g. for an out-of-range attribute userspace immediately gets
the valid ranges back.

v2 incorporates the suggestion from Jakub to have a function to
estimate the size (netlink_policy_dump_attr_size_estimate()) and
check that it does the right thing on the *normal* policy dumps,
not (just) when calling it from the error scenario.

v3 only addresses a few minor style issues.

v4 fixes up a forgotten 'git add' ... sorry.

v5 is a resend, I messed up v4's cover letter subject (saying v3)
and apparently the second patch didn't go out at all.

Tested using nl80211/iw in a few scenarios, seems to work fine
and return the policy back, e.g.

kernel reports: integer out of range
policy: 04 00 0b 00 0c 00 04 00 01 00 00 00 00 00 00 00
        ^ padding
                    ^ minimum allowed value
policy: 04 00 0b 00 0c 00 05 00 ff ff ff ff 00 00 00 00
        ^ padding
                    ^ maximum allowed value
policy: 08 00 01 00 04 00 00 00
        ^ type 4 == U32

for an out-of-range case.
====================
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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)
 {

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

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

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;
 	}
 
@@ -377,8 +377,8 @@ 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;
 		}
 	}
@@ -386,14 +386,14 @@ 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;
 		}
 	}
@@ -550,7 +550,8 @@ 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;
 }
 

net/netlink/af_netlink.c

@@ -2420,6 +2420,8 @@ 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);
+	if (err && nlk_has_extack && extack && extack->policy)
+		tlvlen += netlink_policy_dump_attr_size_estimate(extack->policy);
 
 	if (tlvlen)
 		flags |= NLM_F_ACK_TLVS;
@@ -2452,6 +2454,9 @@ 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);

net/netlink/policy.c

@@ -196,49 +196,75 @@ bool netlink_policy_dump_loop(struct netlink_policy_dump_state *state)
 	return !netlink_policy_dump_finished(state);
 }
 
-/**
- * netlink_policy_dump_write - write current policy dump attributes
- * @skb: the message skb to write to
- * @state: the policy dump state
- *
- * Returns: 0 on success, an error code otherwise
- */
-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)
 {
-	const struct nla_policy *pt;
-	struct nlattr *policy, *attr;
-	enum netlink_attribute_type type;
-	bool again;
+	/* nested + type */
+	int common = 2 * nla_attr_size(sizeof(u32));
 
-send_attribute:
-	again = false;
+	switch (pt->type) {
+	case NLA_UNSPEC:
+	case NLA_REJECT:
+		/* these actually don't need any space */
+		return 0;
+	case NLA_NESTED:
+	case NLA_NESTED_ARRAY:
+		/* common, policy idx, policy maxattr */
+		return common + 2 * nla_attr_size(sizeof(u32));
+	case NLA_U8:
+	case NLA_U16:
+	case NLA_U32:
+	case NLA_U64:
+	case NLA_MSECS:
+	case NLA_S8:
+	case NLA_S16:
+	case NLA_S32:
+	case NLA_S64:
+		/* maximum is common, u64 min/max with padding */
+		return common +
+		       2 * (nla_attr_size(0) + nla_attr_size(sizeof(u64)));
+	case NLA_BITFIELD32:
+		return common + nla_attr_size(sizeof(u32));
+	case NLA_STRING:
+	case NLA_NUL_STRING:
+	case NLA_BINARY:
+		/* maximum is common, u32 min-length/max-length */
+		return common + 2 * nla_attr_size(sizeof(u32));
+	case NLA_FLAG:
+		return common;
+	}
 
-	pt = &state->policies[state->policy_idx].policy[state->attr_idx];
+	/* this should then cause a warning later */
+	return 0;
+}
 
-	policy = nla_nest_start(skb, state->policy_idx);
-	if (!policy)
-		return -ENOBUFS;
+static int
+__netlink_policy_dump_write_attr(struct netlink_policy_dump_state *state,
+				 struct sk_buff *skb,
+				 const struct nla_policy *pt,
+				 int nestattr)
+{
+	int estimate = netlink_policy_dump_attr_size_estimate(pt);
+	enum netlink_attribute_type type;
+	struct nlattr *attr;
 
-	attr = nla_nest_start(skb, state->attr_idx);
+	attr = nla_nest_start(skb, nestattr);
 	if (!attr)
-		goto nla_put_failure;
+		return -ENOBUFS;
 
 	switch (pt->type) {
 	default:
 	case NLA_UNSPEC:
 	case NLA_REJECT:
 		/* skip - use NLA_MIN_LEN to advertise such */
-		nla_nest_cancel(skb, policy);
-		again = true;
-		goto next;
+		nla_nest_cancel(skb, attr);
+		return -ENODATA;
 	case NLA_NESTED:
 		type = NL_ATTR_TYPE_NESTED;
 		fallthrough;
 	case NLA_NESTED_ARRAY:
 		if (pt->type == NLA_NESTED_ARRAY)
 			type = NL_ATTR_TYPE_NESTED_ARRAY;
-		if (pt->nested_policy && pt->len &&
+		if (state && pt->nested_policy && pt->len &&
 		    (nla_put_u32(skb, NL_POLICY_TYPE_ATTR_POLICY_IDX,
 				 netlink_policy_dump_get_policy_idx(state,
 								    pt->nested_policy,
@@ -349,8 +375,66 @@ int netlink_policy_dump_write(struct sk_buff *skb,
 	if (nla_put_u32(skb, NL_POLICY_TYPE_ATTR_TYPE, type))
 		goto nla_put_failure;
 
-	/* finish and move state to next attribute */
 	nla_nest_end(skb, attr);
+	WARN_ON(attr->nla_len > estimate);
+
+	return 0;
+nla_put_failure:
+	nla_nest_cancel(skb, attr);
+	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
+ * @state: the policy dump state
+ *
+ * Returns: 0 on success, an error code otherwise
+ */
+int netlink_policy_dump_write(struct sk_buff *skb,
+			      struct netlink_policy_dump_state *state)
+{
+	const struct nla_policy *pt;
+	struct nlattr *policy;
+	bool again;
+	int err;
+
+send_attribute:
+	again = false;
+
+	pt = &state->policies[state->policy_idx].policy[state->attr_idx];
+
+	policy = nla_nest_start(skb, state->policy_idx);
+	if (!policy)
+		return -ENOBUFS;
+
+	err = __netlink_policy_dump_write_attr(state, skb, pt, state->attr_idx);
+	if (err == -ENODATA) {
+		nla_nest_cancel(skb, policy);
+		again = true;
+		goto next;
+	} else if (err) {
+		goto nla_put_failure;
+	}
+
+	/* finish and move state to next attribute */
 	nla_nest_end(skb, policy);
 
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