[PATCH linux-5.9 1/1] net: netfilter: fix KASAN: slab-out-of-bounds Read in nft_flow_rule_create

[saeed . mirzamohammadi]

From: Saeed Mirzamohammadi <saeed.mirzamohammadi@xxxxxxxxxx>

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.

Signed-off-by: Saeed Mirzamohammadi <saeed.mirzamohammadi@xxxxxxxxxx>
---
 net/netfilter/nf_tables_offload.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/netfilter/nf_tables_offload.c b/net/netfilter/nf_tables_offload.c
index 9ef37c1b7b3b..1273e3c0d4b8 100644
--- a/net/netfilter/nf_tables_offload.c
+++ b/net/netfilter/nf_tables_offload.c
@@ -37,7 +37,7 @@ struct nft_flow_rule *nft_flow_rule_create(struct net *net,
 	struct nft_expr *expr;
 
 	expr = nft_expr_first(rule);
-	while (expr->ops && expr != nft_expr_last(rule)) {
+	while (expr != nft_expr_last(rule) && expr->ops) {
 		if (expr->ops->offload_flags & NFT_OFFLOAD_F_ACTION)
 			num_actions++;
 
@@ -61,7 +61,7 @@ struct nft_flow_rule *nft_flow_rule_create(struct net *net,
 	ctx->net = net;
 	ctx->dep.type = NFT_OFFLOAD_DEP_UNSPEC;
 
-	while (expr->ops && expr != nft_expr_last(rule)) {
+	while (expr != nft_expr_last(rule) && expr->ops) {
 		if (!expr->ops->offload) {
 			err = -EOPNOTSUPP;
 			goto err_out;
-- 
2.27.0