netfilter: nf_tables: report use refcount overflow

Overflow use refcount checks are not complete.

Add helper function to deal with object reference counter tracking.
Report -EMFILE in case UINT_MAX is reached.

nft_use_dec() splats in case that reference counter underflows,
which should not ever happen.

Add nft_use_inc_restore() and nft_use_dec_restore() which are used
to restore reference counter from error and abort paths.

Use u32 in nft_flowtable and nft_object since helper functions cannot
work on bitfields.

Remove the few early incomplete checks now that the helper functions
are in place and used to check for refcount overflow.

Fixes: 96518518 ("netfilter: add nftables")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/net/netfilter/nf_tables.h

@@ -1211,6 +1211,29 @@ int __nft_release_basechain(struct nft_ctx *ctx);
 
 unsigned int nft_do_chain(struct nft_pktinfo *pkt, void *priv);
 
+static inline bool nft_use_inc(u32 *use)
+{
+	if (*use == UINT_MAX)
+		return false;
+
+	(*use)++;
+
+	return true;
+}
+
+static inline void nft_use_dec(u32 *use)
+{
+	WARN_ON_ONCE((*use)-- == 0);
+}
+
+/* For error and abort path: restore use counter to previous state. */
+static inline void nft_use_inc_restore(u32 *use)
+{
+	WARN_ON_ONCE(!nft_use_inc(use));
+}
+
+#define nft_use_dec_restore	nft_use_dec
+
 /**
  *	struct nft_table - nf_tables table
  *
@@ -1296,8 +1319,8 @@ struct nft_object {
 	struct list_head		list;
 	struct rhlist_head		rhlhead;
 	struct nft_object_hash_key	key;
-	u32				genmask:2,
-					use:30;
+	u32				genmask:2;
+	u32				use;
 	u64				handle;
 	u16				udlen;
 	u8				*udata;
@@ -1399,8 +1422,8 @@ struct nft_flowtable {
 	char				*name;
 	int				hooknum;
 	int				ops_len;
-	u32				genmask:2,
-					use:30;
+	u32				genmask:2;
+	u32				use;
 	u64				handle;
 	/* runtime data below here */
 	struct list_head		hook_list ____cacheline_aligned;

net/netfilter/nft_flow_offload.c

@@ -408,8 +408,10 @@ static int nft_flow_offload_init(const struct nft_ctx *ctx,
 	if (IS_ERR(flowtable))
 		return PTR_ERR(flowtable);
 
+	if (!nft_use_inc(&flowtable->use))
+		return -EMFILE;
+
 	priv->flowtable = flowtable;
-	flowtable->use++;
 
 	return nf_ct_netns_get(ctx->net, ctx->family);
 }
@@ -428,7 +430,7 @@ static void nft_flow_offload_activate(const struct nft_ctx *ctx,
 {
 	struct nft_flow_offload *priv = nft_expr_priv(expr);
 
-	priv->flowtable->use++;
+	nft_use_inc_restore(&priv->flowtable->use);
 }
 
 static void nft_flow_offload_destroy(const struct nft_ctx *ctx,

net/netfilter/nft_immediate.c

@@ -159,7 +159,7 @@ static void nft_immediate_deactivate(const struct nft_ctx *ctx,
 			default:
 				nft_chain_del(chain);
 				chain->bound = false;
-				chain->table->use--;
+				nft_use_dec(&chain->table->use);
 				break;
 			}
 			break;
@@ -198,7 +198,7 @@ static void nft_immediate_destroy(const struct nft_ctx *ctx,
 		 * let the transaction records release this chain and its rules.
 		 */
 		if (chain->bound) {
-			chain->use--;
+			nft_use_dec(&chain->use);
 			break;
 		}
 
@@ -206,9 +206,9 @@ static void nft_immediate_destroy(const struct nft_ctx *ctx,
 		chain_ctx = *ctx;
 		chain_ctx.chain = chain;
 
-		chain->use--;
+		nft_use_dec(&chain->use);
 		list_for_each_entry_safe(rule, n, &chain->rules, list) {
-			chain->use--;
+			nft_use_dec(&chain->use);
 			list_del(&rule->list);
 			nf_tables_rule_destroy(&chain_ctx, rule);
 		}

net/netfilter/nft_objref.c

@@ -41,8 +41,10 @@ static int nft_objref_init(const struct nft_ctx *ctx,
 	if (IS_ERR(obj))
 		return -ENOENT;
 
+	if (!nft_use_inc(&obj->use))
+		return -EMFILE;
+
 	nft_objref_priv(expr) = obj;
-	obj->use++;
 
 	return 0;
 }
@@ -72,7 +74,7 @@ static void nft_objref_deactivate(const struct nft_ctx *ctx,
 	if (phase == NFT_TRANS_COMMIT)
 		return;
 
-	obj->use--;
+	nft_use_dec(&obj->use);
 }
 
 static void nft_objref_activate(const struct nft_ctx *ctx,
@@ -80,7 +82,7 @@ static void nft_objref_activate(const struct nft_ctx *ctx,
 {
 	struct nft_object *obj = nft_objref_priv(expr);
 
-	obj->use++;
+	nft_use_inc_restore(&obj->use);
 }
 
 static const struct nft_expr_ops nft_objref_ops = {