netlink: Add strict version of nlmsg_parse and nla_parse

nla_parse is currently lenient on message parsing, allowing type to be 0
or greater than max expected and only logging a message

    "netlink: %d bytes leftover after parsing attributes in process `%s'."

if the netlink message has unknown data at the end after parsing. What this
could mean is that the header at the front of the attributes is actually
wrong and the parsing is shifted from what is expected.

Add a new strict version that actually fails with EINVAL if there are any
bytes remaining after the parsing loop completes, if the atttrbitue type
is 0 or greater than max expected.
Signed-off-by: David Ahern <dsahern@gmail.com>
Acked-by: Christian Brauner <christian@brauner.io>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/netlink.h

@@ -373,6 +373,9 @@ int nla_validate(const struct nlattr *head, int len, int maxtype,
 int nla_parse(struct nlattr **tb, int maxtype, const struct nlattr *head,
 	      int len, const struct nla_policy *policy,
 	      struct netlink_ext_ack *extack);
+int nla_parse_strict(struct nlattr **tb, int maxtype, const struct nlattr *head,
+		     int len, const struct nla_policy *policy,
+		     struct netlink_ext_ack *extack);
 int nla_policy_len(const struct nla_policy *, int);
 struct nlattr *nla_find(const struct nlattr *head, int len, int attrtype);
 size_t nla_strlcpy(char *dst, const struct nlattr *nla, size_t dstsize);
@@ -525,6 +528,20 @@ static inline int nlmsg_parse(const struct nlmsghdr *nlh, int hdrlen,
 			 nlmsg_attrlen(nlh, hdrlen), policy, extack);
 }
 
+static inline int nlmsg_parse_strict(const struct nlmsghdr *nlh, int hdrlen,
+				     struct nlattr *tb[], int maxtype,
+				     const struct nla_policy *policy,
+				     struct netlink_ext_ack *extack)
+{
+	if (nlh->nlmsg_len < nlmsg_msg_size(hdrlen)) {
+		NL_SET_ERR_MSG(extack, "Invalid header length");
+		return -EINVAL;
+	}
+
+	return nla_parse_strict(tb, maxtype, nlmsg_attrdata(nlh, hdrlen),
+				nlmsg_attrlen(nlh, hdrlen), policy, extack);
+}
+
 /**
  * nlmsg_find_attr - find a specific attribute in a netlink message
  * @nlh: netlink message header

lib/nlattr.c

@@ -391,8 +391,9 @@ EXPORT_SYMBOL(nla_policy_len);
  *
  * Returns 0 on success or a negative error code.
  */
-int nla_parse(struct nlattr **tb, int maxtype, const struct nlattr *head,
-	      int len, const struct nla_policy *policy,
+static int __nla_parse(struct nlattr **tb, int maxtype,
+		       const struct nlattr *head, int len,
+		       bool strict, const struct nla_policy *policy,
 		       struct netlink_ext_ack *extack)
 {
 	const struct nlattr *nla;
@@ -403,10 +404,15 @@ int nla_parse(struct nlattr **tb, int maxtype, const struct nlattr *head,
 	nla_for_each_attr(nla, head, len, rem) {
 		u16 type = nla_type(nla);
 
-		if (type > 0 && type <= maxtype) {
+		if (type == 0 || type > maxtype) {
+			if (strict) {
+				NL_SET_ERR_MSG(extack, "Unknown attribute type");
+				return -EINVAL;
+			}
+			continue;
+		}
 		if (policy) {
-				int err = validate_nla(nla, maxtype, policy,
-						       extack);
+			int err = validate_nla(nla, maxtype, policy, extack);
 
 			if (err < 0)
 				return err;
@@ -414,16 +420,34 @@ int nla_parse(struct nlattr **tb, int maxtype, const struct nlattr *head,
 
 		tb[type] = (struct nlattr *)nla;
 	}
-	}
 
-	if (unlikely(rem > 0))
+	if (unlikely(rem > 0)) {
 		pr_warn_ratelimited("netlink: %d bytes leftover after parsing attributes in process `%s'.\n",
 				    rem, current->comm);
+		NL_SET_ERR_MSG(extack, "bytes leftover after parsing attributes");
+		if (strict)
+			return -EINVAL;
+	}
 
 	return 0;
 }
+
+int nla_parse(struct nlattr **tb, int maxtype, const struct nlattr *head,
+	      int len, const struct nla_policy *policy,
+	      struct netlink_ext_ack *extack)
+{
+	return __nla_parse(tb, maxtype, head, len, false, policy, extack);
+}
 EXPORT_SYMBOL(nla_parse);
 
+int nla_parse_strict(struct nlattr **tb, int maxtype, const struct nlattr *head,
+		     int len, const struct nla_policy *policy,
+		     struct netlink_ext_ack *extack)
+{
+	return __nla_parse(tb, maxtype, head, len, true, policy, extack);
+}
+EXPORT_SYMBOL(nla_parse_strict);
+
 /**
  * nla_find - Find a specific attribute in a stream of attributes
  * @head: head of attribute stream