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

From: Pablo Neira Ayuso <[email-masked]>
Date: Tue, 20 Oct 2020 13:50:47 +0200

On Mon, Oct 19, 2020 at 10:25:32AM -0700, [email-masked] wrote:
> From: Saeed Mirzamohammadi <[email-masked]>
> 
> 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.

Thanks. I made a slight variant of your patch.

I'm attaching it, it is also fixing the problem but it introduced
nft_expr_more() and use it everywhere.

Let me know if this looks fine to you.
>From 3f60e5f489ec44e8b0a7e9e622c93be4df335fb6 Mon Sep 17 00:00:00 2001
From: Saeed Mirzamohammadi <[email-masked]>
Date: Tue, 20 Oct 2020 13:41:36 +0200
Subject: [PATCH nf] netfilter: fix 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 <[email-masked]>
Signed-off-by: Pablo Neira Ayuso <[email-masked]>
---
 include/net/netfilter/nf_tables.h | 6 ++++++
 net/netfilter/nf_tables_api.c     | 6 +++---
 net/netfilter/nf_tables_offload.c | 4 ++--
 3 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/include/net/netfilter/nf_tables.h b/include/net/netfilter/nf_tables.h
index 3f7e56b1171e..55b4cadf290a 100644
--- a/include/net/netfilter/nf_tables.h
+++ b/include/net/netfilter/nf_tables.h
@@ -891,6 +891,12 @@ static inline struct nft_expr *nft_expr_last(const struct nft_rule *rule)
 	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];
diff --git a/net/netfilter/nf_tables_api.c b/net/netfilter/nf_tables_api.c
index 9957e0ed8658..65cb8e3c13d9 100644
--- a/net/netfilter/nf_tables_api.c
+++ b/net/netfilter/nf_tables_api.c
@@ -302,7 +302,7 @@ static void nft_rule_expr_activate(const struct nft_ctx *ctx,
 	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);
 
@@ -317,7 +317,7 @@ static void nft_rule_expr_deactivate(const struct nft_ctx *ctx,
 	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);
 
@@ -3080,7 +3080,7 @@ static void nf_tables_rule_destroy(const struct nft_ctx *ctx,
 	 * 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;
diff --git a/net/netfilter/nf_tables_offload.c b/net/netfilter/nf_tables_offload.c
index 7c7e06624dc3..9f625724a20f 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 (nft_expr_more(rule, expr)) {
 		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 (nft_expr_more(rule, expr)) {
 		if (!expr->ops->offload) {
 			err = -EOPNOTSUPP;
 			goto err_out;
-- 
2.20.1