net/sched: Only allow act_ct to bind to clsact/ingress qdiscs and shared blocks

commit 11cb63b upstream.

As Paolo said earlier [1]:

"Since the blamed commit below, classify can return TC_ACT_CONSUMED while
the current skb being held by the defragmentation engine. As reported by
GangMin Kim, if such packet is that may cause a UaF when the defrag engine
later on tries to tuch again such packet."

act_ct was never meant to be used in the egress path, however some users
are attaching it to egress today [2]. Attempting to reach a middle
ground, we noticed that, while most qdiscs are not handling
TC_ACT_CONSUMED, clsact/ingress qdiscs are. With that in mind, we
address the issue by only allowing act_ct to bind to clsact/ingress
qdiscs and shared blocks. That way it's still possible to attach act_ct to
egress (albeit only with clsact).

[1] https://lore.kernel.org/netdev/674b8cbfc385c6f37fb29a1de08d8fe5c2b0fbee.1771321118.git.pabeni@redhat.com/
[2] https://lore.kernel.org/netdev/cc6bfb4a-4a2b-42d8-b9ce-7ef6644fb22b@ovn.org/

Reported-by: GangMin Kim <km.kim1503@gmail.com>
Fixes: 3f14b37 ("net/sched: act_ct: fix skb leak and crash on ooo frags")
CC: stable@vger.kernel.org
Signed-off-by: Victor Nogueira <victor@mojatatu.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://patch.msgid.link/20260225134349.1287037-1-victor@mojatatu.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: vbnogueira

include/net/act_api.h

@@ -70,6 +70,7 @@ struct tc_action {
 #define TCA_ACT_FLAGS_REPLACE	(1U << (TCA_ACT_FLAGS_USER_BITS + 2))
 #define TCA_ACT_FLAGS_NO_RTNL	(1U << (TCA_ACT_FLAGS_USER_BITS + 3))
 #define TCA_ACT_FLAGS_AT_INGRESS	(1U << (TCA_ACT_FLAGS_USER_BITS + 4))
+#define TCA_ACT_FLAGS_AT_INGRESS_OR_CLSACT	(1U << (TCA_ACT_FLAGS_USER_BITS + 5))
 
 /* Update lastuse only if needed, to avoid dirtying a cache line.
  * We use a temp variable to avoid fetching jiffies twice.

net/sched/act_ct.c

@@ -1358,6 +1358,12 @@ static int tcf_ct_init(struct net *net, struct nlattr *nla,
 		return -EINVAL;
 	}
 
+	if (bind && !(flags & TCA_ACT_FLAGS_AT_INGRESS_OR_CLSACT)) {
+		NL_SET_ERR_MSG_MOD(extack,
+				   "Attaching ct to a non ingress/clsact qdisc is unsupported");
+		return -EOPNOTSUPP;
+	}
+
 	err = nla_parse_nested(tb, TCA_CT_MAX, nla, ct_policy, extack);
 	if (err < 0)
 		return err;

net/sched/cls_api.c

@@ -2228,6 +2228,11 @@ static bool is_qdisc_ingress(__u32 classid)
 	return (TC_H_MIN(classid) == TC_H_MIN(TC_H_MIN_INGRESS));
 }
 
+static bool is_ingress_or_clsact(struct tcf_block *block, struct Qdisc *q)
+{
+	return tcf_block_shared(block) || (q && !!(q->flags & TCQ_F_INGRESS));
+}
+
 static int tc_new_tfilter(struct sk_buff *skb, struct nlmsghdr *n,
 			  struct netlink_ext_ack *extack)
 {
@@ -2420,6 +2425,8 @@ static int tc_new_tfilter(struct sk_buff *skb, struct nlmsghdr *n,
 		flags |= TCA_ACT_FLAGS_NO_RTNL;
 	if (is_qdisc_ingress(parent))
 		flags |= TCA_ACT_FLAGS_AT_INGRESS;
+	if (is_ingress_or_clsact(block, q))
+		flags |= TCA_ACT_FLAGS_AT_INGRESS_OR_CLSACT;
 	err = tp->ops->change(net, skb, tp, cl, t->tcm_handle, tca, &fh,
 			      flags, extack);
 	if (err == 0) {