netfilter: nf_tables: fix userdata length overflow

The NFT_USERDATA_MAXLEN is defined to 256, however we only have a u8
to store its size. Introduce a struct nft_userdata which contains a
length field and indicate its presence using a single bit in the rule.

The length field of struct nft_userdata is also a u8, however we don't
store zero sized data, so the actual length is udata->len + 1.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 19 insertions, 3 deletions

diff --git a/include/net/netfilter/nf_tables.h b/include/net/netfilter/nf_tables.h
index 9eaaa78..decb9a0 100644
--- a/include/net/netfilter/nf_tables.h
+++ b/include/net/netfilter/nf_tables.h
@@ -119,6 +119,22 @@ int nft_validate_data_load(const struct nft_ctx *ctx, enum nft_registers reg,
 			   const struct nft_data *data,
 			   enum nft_data_types type);
 
+
+/**
+ *	struct nft_userdata - user defined data associated with an object
+ *
+ *	@len: length of the data
+ *	@data: content
+ *
+ *	The presence of user data is indicated in an object specific fashion,
+ *	so a length of zero can't occur and the value "len" indicates data
+ *	of length len + 1.
+ */
+struct nft_userdata {
+	u8			len;
+	unsigned char		data[0];
+};
+
 /**
  *	struct nft_set_elem - generic representation of set elements
  *
@@ -380,7 +396,7 @@ static inline void *nft_expr_priv(const struct nft_expr *expr)
  *	@handle: rule handle
  *	@genmask: generation mask
  *	@dlen: length of expression data
- *	@ulen: length of user data (used for comments)
+ *	@udata: user data is appended to the rule
  *	@data: expression data
  */
 struct nft_rule {
@@ -388,7 +404,7 @@ struct nft_rule {
 	u64				handle:42,
 					genmask:2,
 					dlen:12,
-					ulen:8;
+					udata:1;
 	unsigned char			data[]
 		__attribute__((aligned(__alignof__(struct nft_expr))));
 };
@@ -476,7 +492,7 @@ static inline struct nft_expr *nft_expr_last(const struct nft_rule *rule)
 	return (struct nft_expr *)&rule->data[rule->dlen];
 }
 
-static inline void *nft_userdata(const struct nft_rule *rule)
+static inline struct nft_userdata *nft_userdata(const struct nft_rule *rule)
 {
 	return (void *)&rule->data[rule->dlen];
 }