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>

2 files changed, 38 insertions, 12 deletions

diff --git a/include/net/netfilter/nf_tables.h b/include/net/netfilter/nf_tables.h
index 9eaaa7884586..decb9a095ae7 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];
 }
diff --git a/net/netfilter/nf_tables_api.c b/net/netfilter/nf_tables_api.c
index 7baafd5ab520..74e4b876c96e 100644
--- a/net/netfilter/nf_tables_api.c
+++ b/net/netfilter/nf_tables_api.c
@@ -1711,9 +1711,12 @@ static int nf_tables_fill_rule_info(struct sk_buff *skb, struct net *net,
         }
         nla_nest_end(skb, list);
 
-        if (rule->ulen &&
-            nla_put(skb, NFTA_RULE_USERDATA, rule->ulen, nft_userdata(rule)))
-                goto nla_put_failure;
+        if (rule->udata) {
+                struct nft_userdata *udata = nft_userdata(rule);
+                if (nla_put(skb, NFTA_RULE_USERDATA, udata->len + 1,
+                            udata->data) < 0)
+                        goto nla_put_failure;
+        }
 
         nlmsg_end(skb, nlh);
         return 0;
@@ -1896,11 +1899,12 @@ static int nf_tables_newrule(struct sock *nlsk, struct sk_buff *skb,
         struct nft_table *table;
         struct nft_chain *chain;
         struct nft_rule *rule, *old_rule = NULL;
+        struct nft_userdata *udata;
         struct nft_trans *trans = NULL;
         struct nft_expr *expr;
         struct nft_ctx ctx;
         struct nlattr *tmp;
-        unsigned int size, i, n, ulen = 0;
+        unsigned int size, i, n, ulen = 0, usize = 0;
         int err, rem;
         bool create;
         u64 handle, pos_handle;
@@ -1973,11 +1977,14 @@ static int nf_tables_newrule(struct sock *nlsk, struct sk_buff *skb,
         if (size >= 1 << 12)
                 goto err1;
 
-        if (nla[NFTA_RULE_USERDATA])
+        if (nla[NFTA_RULE_USERDATA]) {
                 ulen = nla_len(nla[NFTA_RULE_USERDATA]);
+                if (ulen > 0)
+                        usize = sizeof(struct nft_userdata) + ulen;
+        }
 
         err = -ENOMEM;
-        rule = kzalloc(sizeof(*rule) + size + ulen, GFP_KERNEL);
+        rule = kzalloc(sizeof(*rule) + size + usize, GFP_KERNEL);
         if (rule == NULL)
                 goto err1;
 
@@ -1985,10 +1992,13 @@ static int nf_tables_newrule(struct sock *nlsk, struct sk_buff *skb,
 
         rule->handle = handle;
         rule->dlen   = size;
-        rule->ulen   = ulen;
+        rule->udata  = ulen ? 1 : 0;
 
-        if (ulen)
-                nla_memcpy(nft_userdata(rule), nla[NFTA_RULE_USERDATA], ulen);
+        if (ulen) {
+                udata = nft_userdata(rule);
+                udata->len = ulen - 1;
+                nla_memcpy(udata->data, nla[NFTA_RULE_USERDATA], ulen);
+        }
 
         expr = nft_expr_first(rule);
         for (i = 0; i < n; i++) {