From: Johannes Berg <johannes.berg@intel.com>
Date: Wed, 26 Sep 2018 11:15:33 +0200
Subject: [PATCH] netlink: allow NLA_NESTED to specify nested policy to
 validate
Patch-mainline: v4.20-rc1
Git-commit: 9a659a35ba177cec30676e170fb6ed98157bcb0d
References: bsc#1152107 CVE-2019-16746

Now that we have a validation_data pointer, and the len field in
the policy is unused for NLA_NESTED, we can allow using them both
to have nested validation. This can be nice in code, although we
still have to use nla_parse_nested() or similar which would also
take a policy; however, it also serves as documentation in the
policy without requiring a look at the code.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Cho, Yu-Chen <acho@suse.com>
---
 include/net/netlink.h |   13 +++++++++++--
 lib/nlattr.c          |   14 ++++++++++++++
 2 files changed, 25 insertions(+), 2 deletions(-)

--- a/include/net/netlink.h
+++ b/include/net/netlink.h
@@ -198,8 +198,10 @@ enum {
  *    NLA_NUL_STRING       Maximum length of string (excluding NUL)
  *    NLA_FLAG             Unused
  *    NLA_BINARY           Maximum length of attribute payload
- *    NLA_NESTED           Don't use `len' field -- length verification is
- *                         done by checking len of nested header (or empty)
+ *    NLA_NESTED           Length verification is done by checking len of
+ *                         nested header (or empty); len field is used if
+ *                         validation_data is also used, for the max attr
+ *                         number in the nested policy.
  *    NLA_NESTED_COMPAT    Minimum length of structure payload
  *    NLA_U8, NLA_U16,
  *    NLA_U32, NLA_U64,
@@ -219,6 +221,10 @@ enum {
  *    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.
+ *    NLA_NESTED           Points to a nested policy to validate, must also set
+ *                         `len' to the max attribute number.
+ *                         Note that nla_parse() will validate, but of course not
+ *                         parse, the nested sub-policies.
  *    All other            Unused
  *
  * Example:
@@ -235,6 +241,9 @@ struct nla_policy {
 	const void     *validation_data;
 };
 
+#define NLA_POLICY_NESTED(maxattr, policy) \
+	{ .type = NLA_NESTED, .validation_data = policy, .len = maxattr }
+
 /**
  * struct nl_info - netlink source information
  * @nlh: Netlink message header of original request
--- a/lib/nlattr.c
+++ b/lib/nlattr.c
@@ -138,6 +138,20 @@ static int validate_nla(const struct nla
 		 */
 		if (attrlen == 0)
 			break;
+		if (attrlen < NLA_HDRLEN)
+			goto out_err;
+		if (pt->validation_data) {
+			err = nla_validate(nla_data(nla), nla_len(nla), pt->len,
+					   pt->validation_data, extack);
+			if (err < 0) {
+				/*
+				 * return directly to preserve the inner
+				 * error message/attribute pointer
+				 */
+				return err;
+			}
+		}
+		break;
 	default:
 		if (pt->len)
 			minlen = pt->len;