[SRU][J][PATCH 1/1] netfilter: nf_conntrack_expect: use expect->helper

[Cengiz Can]

From: Pablo Neira Ayuso <pablo at netfilter.org>

Use expect->helper in ctnetlink and /proc to dump the helper name.
Using nfct_help() without holding a reference to the master conntrack
is unsafe.

Use exp->master->helper in ctnetlink path if userspace does not provide
an explicit helper when creating an expectation to retain the existing
behaviour. The ctnetlink expectation path holds the reference on the
master conntrack and nf_conntrack_expect lock and the nfnetlink glue
path refers to the master ct that is attached to the skb.

Reported-by: Hyunwoo Kim <imv4bel at gmail.com>
Signed-off-by: Florian Westphal <fw at strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo at netfilter.org>
(backported from commit f01794106042ee27e54af6fdf5b319a2fe3df94d)
[bot_kybele: In ctnetlink_alloc_expect this tree assigns exp->helper with a
 plain assignment (exp->helper is not __rcu here), so I kept "exp->helper =
 helper;" instead of upstream's rcu_assign_pointer while still adding the "if
 (!helper) helper = rcu_dereference(help->helper);" fallback; expect_iter_name
 here lacked the m_help/helper NULL-check form, so I replaced its simpler body
 with the upstream exp->helper version.]
CVE-2026-31414
Assisted-by: kybele:claude-opus-4.8
Signed-off-by: Cengiz Can <cengiz.can at canonical.com>
---
 net/netfilter/nf_conntrack_expect.c  |  2 +-
 net/netfilter/nf_conntrack_helper.c  |  6 +-----
 net/netfilter/nf_conntrack_netlink.c | 27 ++++++++++++++-------------
 net/netfilter/nf_conntrack_sip.c     |  2 +-
 4 files changed, 17 insertions(+), 20 deletions(-)

diff --git a/net/netfilter/nf_conntrack_expect.c b/net/netfilter/nf_conntrack_expect.c
index 43bbfdfc4093..758440a1bb35 100644
--- a/net/netfilter/nf_conntrack_expect.c
+++ b/net/netfilter/nf_conntrack_expect.c
@@ -654,7 +654,7 @@ static int exp_seq_show(struct seq_file *s, void *v)
 	if (expect->flags & NF_CT_EXPECT_USERSPACE)
 		seq_printf(s, "%sUSERSPACE", delim);
 
-	helper = rcu_dereference(nfct_help(expect->master)->helper);
+	helper = rcu_dereference(expect->helper);
 	if (helper) {
 		seq_printf(s, "%s%s", expect->flags ? " " : "", helper->name);
 		if (helper->expect_policy[expect->class].name[0])
diff --git a/net/netfilter/nf_conntrack_helper.c b/net/netfilter/nf_conntrack_helper.c
index b5f4bfc60f89..3ce1a4a6fe03 100644
--- a/net/netfilter/nf_conntrack_helper.c
+++ b/net/netfilter/nf_conntrack_helper.c
@@ -445,14 +445,10 @@ EXPORT_SYMBOL_GPL(nf_conntrack_helper_register);
 
 static bool expect_iter_me(struct nf_conntrack_expect *exp, void *data)
 {
-	struct nf_conn_help *help = nfct_help(exp->master);
 	const struct nf_conntrack_helper *me = data;
 	const struct nf_conntrack_helper *this;
 
-	if (exp->helper == me)
-		return true;
-
-	this = rcu_dereference_protected(help->helper,
+	this = rcu_dereference_protected(exp->helper,
 					 lockdep_is_held(&nf_conntrack_expect_lock));
 	return this == me;
 }
diff --git a/net/netfilter/nf_conntrack_netlink.c b/net/netfilter/nf_conntrack_netlink.c
index a6d6ba88c247..2e8fb88e9a6a 100644
--- a/net/netfilter/nf_conntrack_netlink.c
+++ b/net/netfilter/nf_conntrack_netlink.c
@@ -3024,7 +3024,7 @@ ctnetlink_exp_dump_expect(struct sk_buff *skb,
 {
 	struct nf_conn *master = exp->master;
 	long timeout = ((long)exp->timeout.expires - (long)jiffies) / HZ;
-	struct nf_conn_help *help;
+	struct nf_conntrack_helper *helper;
 #if IS_ENABLED(CONFIG_NF_NAT)
 	struct nlattr *nest_parms;
 	struct nf_conntrack_tuple nat_tuple = {};
@@ -3069,15 +3069,12 @@ ctnetlink_exp_dump_expect(struct sk_buff *skb,
 	    nla_put_be32(skb, CTA_EXPECT_FLAGS, htonl(exp->flags)) ||
 	    nla_put_be32(skb, CTA_EXPECT_CLASS, htonl(exp->class)))
 		goto nla_put_failure;
-	help = nfct_help(master);
-	if (help) {
-		struct nf_conntrack_helper *helper;
 
-		helper = rcu_dereference(help->helper);
-		if (helper &&
-		    nla_put_string(skb, CTA_EXPECT_HELP_NAME, helper->name))
-			goto nla_put_failure;
-	}
+	helper = rcu_dereference(exp->helper);
+	if (helper &&
+	    nla_put_string(skb, CTA_EXPECT_HELP_NAME, helper->name))
+		goto nla_put_failure;
+
 	expfn = nf_ct_helper_expectfn_find_by_symbol(exp->expectfn);
 	if (expfn != NULL &&
 	    nla_put_string(skb, CTA_EXPECT_FN, expfn->name))
@@ -3405,12 +3402,14 @@ static int ctnetlink_get_expect(struct sk_buff *skb,
 
 static bool expect_iter_name(struct nf_conntrack_expect *exp, void *data)
 {
-	const struct nf_conn_help *m_help;
+	struct nf_conntrack_helper *helper;
 	const char *name = data;
 
-	m_help = nfct_help(exp->master);
+	helper = rcu_dereference(exp->helper);
+	if (!helper)
+		return false;
 
-	return strcmp(m_help->helper->name, name) == 0;
+	return strcmp(helper->name, name) == 0;
 }
 
 static bool expect_iter_all(struct nf_conntrack_expect *exp, void *data)
@@ -3541,9 +3540,9 @@ ctnetlink_alloc_expect(const struct nlattr * const cda[], struct nf_conn *ct,
 		       struct nf_conntrack_tuple *tuple,
 		       struct nf_conntrack_tuple *mask)
 {
-	u_int32_t class = 0;
 	struct nf_conntrack_expect *exp;
 	struct nf_conn_help *help;
+	u32 class = 0;
 	int err;
 
 	help = nfct_help(ct);
@@ -3580,6 +3579,8 @@ ctnetlink_alloc_expect(const struct nlattr * const cda[], struct nf_conn *ct,
 
 	exp->class = class;
 	exp->master = ct;
+	if (!helper)
+		helper = rcu_dereference(help->helper);
 	exp->helper = helper;
 	exp->tuple = *tuple;
 	exp->mask.src.u3 = mask->src.u3;
diff --git a/net/netfilter/nf_conntrack_sip.c b/net/netfilter/nf_conntrack_sip.c
index 4326d5ea0400..4ddae15cfcca 100644
--- a/net/netfilter/nf_conntrack_sip.c
+++ b/net/netfilter/nf_conntrack_sip.c
@@ -1003,7 +1003,7 @@ static int set_expected_rtp_rtcp(struct sk_buff *skb, unsigned int protoff,
 		exp = __nf_ct_expect_find(net, nf_ct_zone(ct), &tuple);
 
 		if (!exp || exp->master == ct ||
-		    nfct_help(exp->master)->helper != nfct_help(ct)->helper ||
+		    exp->helper != nfct_help(ct)->helper ||
 		    exp->class != class)
 			break;
 #if IS_ENABLED(CONFIG_NF_NAT)
-- 
2.43.0