Subject: [PATCH] netfilter: ctnetlink: validate expect class against master helper

From: Qi Tang <[email-masked]>
Date: Mon, 30 Mar 2026 00:51:31 +0800

ctnetlink_alloc_expect() validates CTA_EXPECT_CLASS against the
helper specified by CTA_EXPECT_HELP_NAME.  However,
__nf_ct_expect_check() and nf_ct_expect_insert() later index the
expect_policy array using the master conntrack's actual helper.

When the supplied helper has a larger expect_class_max than the
master's helper, the class passes validation but produces an
out-of-bounds read on the master helper's heap-allocated policy
array during expectation insertion.

Validate the class against the master conntrack's own helper
instead, since that is the helper whose policy array will actually
be indexed.

  BUG: KASAN: slab-out-of-bounds in nf_ct_expect_related_report+0x2479/0x27c0
  Read of size 4 at addr ffff8880043fe408 by task poc/102
  Call Trace:
   nf_ct_expect_related_report+0x2479/0x27c0
   ctnetlink_create_expect+0x22b/0x3b0
   ctnetlink_new_expect+0x4bd/0x5c0
   nfnetlink_rcv_msg+0x67a/0x950
   netlink_rcv_skb+0x120/0x350

Fixes: b8c5e52c13ed ("netfilter: ctnetlink: allow to set expectation class")
Signed-off-by: Qi Tang <[email-masked]>
---
 net/netfilter/nf_conntrack_netlink.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/net/netfilter/nf_conntrack_netlink.c b/net/netfilter/nf_conntrack_netlink.c
index 3f408f3713bb..c57c665363e0 100644
--- a/net/netfilter/nf_conntrack_netlink.c
+++ b/net/netfilter/nf_conntrack_netlink.c
@@ -3542,9 +3542,14 @@ ctnetlink_alloc_expect(const struct nlattr * const cda[], struct nf_conn *ct,
 	if (!help)
 		return ERR_PTR(-EOPNOTSUPP);
 
-	if (cda[CTA_EXPECT_CLASS] && helper) {
+	if (cda[CTA_EXPECT_CLASS]) {
+		struct nf_conntrack_helper *master_helper;
+
+		master_helper = rcu_dereference(help->helper);
+		if (!master_helper)
+			return ERR_PTR(-EOPNOTSUPP);
 		class = ntohl(nla_get_be32(cda[CTA_EXPECT_CLASS]));
-		if (class > helper->expect_class_max)
+		if (class > master_helper->expect_class_max)
 			return ERR_PTR(-EINVAL);
 	}
 	exp = nf_ct_expect_alloc(ct);
-- 
2.43.0