[PATCH v3 2/2] Make num_actions unsigned

[joao@overdrivepizza.com]

From: Joao Moreira <joao.moreira@intel.com>

Currently, in nft_flow_rule_create function, num_actions is a signed
integer. Yet, it is processed within a loop which increments its
value. To prevent an overflow from occurring, make it unsigned and
also check if it reaches 256 when being incremented.

Accordingly to discussions around v2, 256 actions are more than enough
for the frontend actions.

After checking with maintainers, it was mentioned that front-end will
cap the num_actions value and that it is not possible to reach such
condition for an overflow. Yet, for correctness, it is still better to
fix this.

This issue was observed by the commit author while reviewing a write-up
regarding a CVE within the same subsystem [1].

1 - https://nickgregory.me/post/2022/03/12/cve-2022-25636/

Signed-off-by: Joao Moreira <joao.moreira@intel.com>
---
 net/netfilter/nf_tables_offload.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/net/netfilter/nf_tables_offload.c b/net/netfilter/nf_tables_offload.c
index 12ab78fa5d84..9a86db1f0e07 100644
--- a/net/netfilter/nf_tables_offload.c
+++ b/net/netfilter/nf_tables_offload.c
@@ -90,7 +90,8 @@ struct nft_flow_rule *nft_flow_rule_create(struct net *net,
 {
 	struct nft_offload_ctx *ctx;
 	struct nft_flow_rule *flow;
-	int num_actions = 0, err;
+	unsigned int num_actions = 0;
+	int err;
 	struct nft_expr *expr;
 
 	expr = nft_expr_first(rule);
@@ -99,6 +100,10 @@ struct nft_flow_rule *nft_flow_rule_create(struct net *net,
 		    expr->ops->offload_action(expr))
 			num_actions++;
 
+		/* 2^8 is enough for frontend actions, avoid overflow */
+		if (num_actions == 256)
+			return ERR_PTR(-ENOMEM);
+
 		expr = nft_expr_next(expr);
 	}
 
-- 
2.42.0