[PATCH net 1/2] net/sched: Only allow act_ct to bind to clsact/ingress qdiscs and shared blocks

[PATCH net 1/2] net/sched: Only allow act_ct to bind to clsact/ingress qdiscs and shared blocks

[Victor Nogueira]

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: 3f14b377d01d ("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>
---
 include/net/act_api.h | 1 +
 net/sched/act_ct.c    | 6 ++++++
 net/sched/cls_api.c   | 7 +++++++
 3 files changed, 14 insertions(+)

diff --git a/include/net/act_api.h b/include/net/act_api.h
index 91a24b5e0b93..2ba40eb45aad 100644
--- a/include/net/act_api.h
+++ b/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.
diff --git a/net/sched/act_ct.c b/net/sched/act_ct.c
index 81d488655793..7de6eb3ff53b 100644
--- a/net/sched/act_ct.c
+++ b/net/sched/act_ct.c
@@ -1360,6 +1360,12 @@ 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;
diff --git a/net/sched/cls_api.c b/net/sched/cls_api.c
index ebca4b926dcf..8c72faf3314d 100644
--- a/net/sched/cls_api.c
+++ b/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) {
-- 
2.52.0

Re: [PATCH net 1/2] net/sched: Only allow act_ct to bind to clsact/ingress qdiscs and shared blocks

[Jamal Hadi Salim]

On Wed, Feb 25, 2026 at 8:44 AM Victor Nogueira <victor@mojatatu.com> wrote:
>
> 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: 3f14b377d01d ("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>

cheers,
jamal

> ---
>  include/net/act_api.h | 1 +
>  net/sched/act_ct.c    | 6 ++++++
>  net/sched/cls_api.c   | 7 +++++++
>  3 files changed, 14 insertions(+)
>
> diff --git a/include/net/act_api.h b/include/net/act_api.h
> index 91a24b5e0b93..2ba40eb45aad 100644
> --- a/include/net/act_api.h
> +++ b/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.
> diff --git a/net/sched/act_ct.c b/net/sched/act_ct.c
> index 81d488655793..7de6eb3ff53b 100644
> --- a/net/sched/act_ct.c
> +++ b/net/sched/act_ct.c
> @@ -1360,6 +1360,12 @@ 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;
> diff --git a/net/sched/cls_api.c b/net/sched/cls_api.c
> index ebca4b926dcf..8c72faf3314d 100644
> --- a/net/sched/cls_api.c
> +++ b/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) {
> --
> 2.52.0
>

Re: [PATCH net 1/2] net/sched: Only allow act_ct to bind to clsact/ingress qdiscs and shared blocks

[patchwork-bot+netdevbpf]

Hello:

This series was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Wed, 25 Feb 2026 10:43:48 -0300 you wrote:
> 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."
> 
> [...]

Here is the summary with links:
  - [net,1/2] net/sched: Only allow act_ct to bind to clsact/ingress qdiscs and shared blocks
    https://git.kernel.org/netdev/net/c/11cb63b0d1a0
  - [net,2/2] selftests/tc-testing: Create tests to exercise act_ct binding restrictions
    https://git.kernel.org/netdev/net/c/b14e82abf78a

You are awesome, thank you!
-- 
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