netfilter: nftables_offload: KASAN slab-out-of-bounds Read in nft_flow_rule_create

This patch fixes the issue due to:

BUG: KASAN: slab-out-of-bounds in nft_flow_rule_create+0x622/0x6a2
net/netfilter/nf_tables_offload.c:40
Read of size 8 at addr ffff888103910b58 by task syz-executor227/16244

The error happens when expr->ops is accessed early on before performing the boundary check and after nft_expr_next() moves the expr to go out-of-bounds.

This patch checks the boundary condition before expr->ops that fixes the slab-out-of-bounds Read issue.

Add nft_expr_more() and use it to fix this problem.

Signed-off-by: Saeed Mirzamohammadi <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

include/net/netfilter/nf_tables.h

···
 	return (struct nft_expr *)&rule->data[rule->dlen];
 }
 
+static inline bool nft_expr_more(const struct nft_rule *rule,
+				 const struct nft_expr *expr)
+{
+	return expr != nft_expr_last(rule) && expr->ops;
+}
+
 static inline struct nft_userdata *nft_userdata(const struct nft_rule *rule)
 {
 	return (void *)&rule->data[rule->dlen];

net/netfilter/nf_tables_api.c

···
 	struct nft_expr *expr;
 
 	expr = nft_expr_first(rule);
-	while (expr != nft_expr_last(rule) && expr->ops) {
+	while (nft_expr_more(rule, expr)) {
 		if (expr->ops->activate)
 			expr->ops->activate(ctx, expr);
 
···
 	struct nft_expr *expr;
 
 	expr = nft_expr_first(rule);
-	while (expr != nft_expr_last(rule) && expr->ops) {
+	while (nft_expr_more(rule, expr)) {
 		if (expr->ops->deactivate)
 			expr->ops->deactivate(ctx, expr, phase);
 
···
 	 * is called on error from nf_tables_newrule().
 	 */
 	expr = nft_expr_first(rule);
-	while (expr != nft_expr_last(rule) && expr->ops) {
+	while (nft_expr_more(rule, expr)) {
 		next = nft_expr_next(expr);
 		nf_tables_expr_destroy(ctx, expr);
 		expr = next;

net/netfilter/nf_tables_offload.c

···
 	struct nft_expr *expr;
 
 	expr = nft_expr_first(rule);
-	while (expr->ops && expr != nft_expr_last(rule)) {
+	while (nft_expr_more(rule, expr)) {
 		if (expr->ops->offload_flags & NFT_OFFLOAD_F_ACTION)
 			num_actions++;
 
···
 	ctx->net = net;
 	ctx->dep.type = NFT_OFFLOAD_DEP_UNSPEC;
 
-	while (expr->ops && expr != nft_expr_last(rule)) {
+	while (nft_expr_more(rule, expr)) {
 		if (!expr->ops->offload) {
 			err = -EOPNOTSUPP;
 			goto err_out;