netdev - [PATCH net-next 1/7] netfilter: xt_mangle: only check verdict part of return value

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Message-ID: <20231018085118.10829-2-fw@strlen.de>
Date: Wed, 18 Oct 2023 10:51:05 +0200
From: Florian Westphal <fw@...len.de>
To: <netdev@...r.kernel.org>
Cc: Paolo Abeni <pabeni@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	<netfilter-devel@...r.kernel.org>
Subject: [PATCH net-next 1/7] netfilter: xt_mangle: only check verdict part of return value

These checks assume that the caller only returns NF_DROP without
any errno embedded in the upper bits.

This is fine right now, but followup patches will start to propagate
such errors to allow kfree_skb_drop_reason() in the called functions,
those would then indicate 'errno << 8 | NF_STOLEN'.

To not break things we have to mask those parts out.

Signed-off-by: Florian Westphal <fw@...len.de>
---
 net/ipv4/netfilter/iptable_mangle.c  | 9 +++++----
 net/ipv6/netfilter/ip6table_mangle.c | 9 +++++----
 2 files changed, 10 insertions(+), 8 deletions(-)

diff --git a/net/ipv4/netfilter/iptable_mangle.c b/net/ipv4/netfilter/iptable_mangle.c
index 3abb430af9e6..385d945d8ebe 100644
--- a/net/ipv4/netfilter/iptable_mangle.c
+++ b/net/ipv4/netfilter/iptable_mangle.c
@@ -36,12 +36,12 @@ static const struct xt_table packet_mangler = {
 static unsigned int
 ipt_mangle_out(void *priv, struct sk_buff *skb, const struct nf_hook_state *state)
 {
-	unsigned int ret;
+	unsigned int ret, verdict;
 	const struct iphdr *iph;
-	u_int8_t tos;
 	__be32 saddr, daddr;
-	u_int32_t mark;
+	u32 mark;
 	int err;
+	u8 tos;
 
 	/* Save things which could affect route */
 	mark = skb->mark;
@@ -51,8 +51,9 @@ ipt_mangle_out(void *priv, struct sk_buff *skb, const struct nf_hook_state *stat
 	tos = iph->tos;
 
 	ret = ipt_do_table(priv, skb, state);
+	verdict = ret & NF_VERDICT_MASK;
 	/* Reroute for ANY change. */
-	if (ret != NF_DROP && ret != NF_STOLEN) {
+	if (verdict != NF_DROP && verdict != NF_STOLEN) {
 		iph = ip_hdr(skb);
 
 		if (iph->saddr != saddr ||
diff --git a/net/ipv6/netfilter/ip6table_mangle.c b/net/ipv6/netfilter/ip6table_mangle.c
index a88b2ce4a3cb..8dd4cd0c47bd 100644
--- a/net/ipv6/netfilter/ip6table_mangle.c
+++ b/net/ipv6/netfilter/ip6table_mangle.c
@@ -31,10 +31,10 @@ static const struct xt_table packet_mangler = {
 static unsigned int
 ip6t_mangle_out(void *priv, struct sk_buff *skb, const struct nf_hook_state *state)
 {
-	unsigned int ret;
 	struct in6_addr saddr, daddr;
-	u_int8_t hop_limit;
-	u_int32_t flowlabel, mark;
+	unsigned int ret, verdict;
+	u32 flowlabel, mark;
+	u8 hop_limit;
 	int err;
 
 	/* save source/dest address, mark, hoplimit, flowlabel, priority,  */
@@ -47,8 +47,9 @@ ip6t_mangle_out(void *priv, struct sk_buff *skb, const struct nf_hook_state *sta
 	flowlabel = *((u_int32_t *)ipv6_hdr(skb));
 
 	ret = ip6t_do_table(priv, skb, state);
+	verdict = ret & NF_VERDICT_MASK;
 
-	if (ret != NF_DROP && ret != NF_STOLEN &&
+	if (verdict != NF_DROP && verdict != NF_STOLEN &&
 	    (!ipv6_addr_equal(&ipv6_hdr(skb)->saddr, &saddr) ||
 	     !ipv6_addr_equal(&ipv6_hdr(skb)->daddr, &daddr) ||
 	     skb->mark != mark ||
-- 
2.41.0