FAILED: patch "[PATCH] net/sched: act_ct: avoid modifying shared unconfirmed ct" failed to apply to 6.18-stable tree

The patch below does not apply to the 6.18-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.18.y
git checkout FETCH_HEAD
git cherry-pick -x f85009dfcd65e5969526b0db7a49b5413746e630
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026092927-flatness-surprise-1efc@gregkh' --subject-prefix 'PATCH 6.18.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From f85009dfcd65e5969526b0db7a49b5413746e630 Mon Sep 17 00:00:00 2001
From: Ilya Maximets <[email protected]>
Date: Mon, 21 Sep 2026 16:55:46 +0200
Subject: [PATCH] net/sched: act_ct: avoid modifying shared unconfirmed ct
entry

In a case where skb with an unconfirmed ct entry gets cloned, we may end up processing both again but with different sets of extensions.

The series of events:

1. The first clone wants to commit and runs the helpers wiring up the extension pointer into the expectation list.
2. Then it looses the confirmation keeping the entry unconfirmed.
3. Second clone now wants to commit labels or run NAT and adds the new extension for that breaking the pointer in the expectation list causing UAF on the destruction path later.

While this is possible to trigger, there should be no practical network pipeline where we need to process both clones without modifications in the same zone. So, let's just reset the entry in case for some reason we got an skb with a shared one. This doesn't affect any known use cases, but avoids any potential problems with sharing and modification of the unconfirmed ct entry.

Unlike openvswitch module, act_ct allows for NAT without commit. Changing that would be a uAPI break. So, act_ct needs to reset on NAT regardless of the commit flag to avoid reallocation of the extension space. This, however, doesn't really change the picture for sensible networking cases as there should be no need to run the same packet twice (before and after the clone) through conntrack without packet header or zone changes and without commit.

The fixes tag points to the introduction of helpers, since that's the main UAF trigger for the sharing.

index 55f3521edb4c..e72143d36b11 100644
@@ -979,11 +979,11 @@ TC_INDIRECT_SCOPE int tcf_ct_act(struct sk_buff *skb, const struct tc_action *a,
 				 struct tcf_result *res)
 {
 	struct net *net = dev_net(skb->dev);
+	bool cached, commit, clear, nat;
 	enum ip_conntrack_info ctinfo;
 	struct tcf_ct *c = to_ct(a);
 	struct nf_conn *tmpl = NULL;
 	struct nf_hook_state state;
-	bool cached, commit, clear;
 	int nh_ofs, err, retval;
 	struct tcf_ct_params *p;
 	bool add_helper = false;

@@ -998,6 +998,7 @@ TC_INDIRECT_SCOPE int tcf_ct_act(struct sk_buff *skb, const struct tc_action *a,
 	retval = p->action;
 	commit = p->ct_action & TCA_CT_ACT_COMMIT;
 	clear = p->ct_action & TCA_CT_ACT_CLEAR;
+	nat = p->ct_action & TCA_CT_ACT_NAT;
 	tmpl = p->tmpl;
 
 	tcf_lastuse_update(&c->tcf_tm);

@@ -1046,6 +1047,19 @@ TC_INDIRECT_SCOPE int tcf_ct_act(struct sk_buff *skb, const struct tc_action *a,
 	 * different zone.
 	 */
 	cached = tcf_ct_skb_nfct_cached(net, skb, p);
+
+	/* If the ct entry is not confirmed and shared with some other skb,
+	 * e.g., a cloned one, we can't just modify it with a commit or nat
+	 * as we must not modify the extension set.  Reset.
+	 */
+	if (cached && (commit || nat)) {
+		ct = nf_ct_get(skb, &ctinfo);
+		if (ct && !nf_ct_is_confirmed(ct) && nf_ct_shared(ct)) {
+			nf_reset_ct(skb);
+			cached = false;
+		}
+	}
+
 	if (!cached) {
 		if (tcf_ct_flow_table_lookup(p, skb, family)) {
 			skip_add = true;

@@ -1083,7 +1097,7 @@ TC_INDIRECT_SCOPE int tcf_ct_act(struct sk_buff *skb, const struct tc_action *a,
 		if (err)
 			goto drop;
 		add_helper = true;
-		if (p->ct_action & TCA_CT_ACT_NAT && !nfct_seqadj(ct)) {
+		if (nat && !nfct_seqadj(ct)) {
 			if (!nfct_seqadj_ext_add(ct))
 				goto drop;
 		}

From: Ilya Maximets <[email protected]>

[ Upstream commit 26b2bd70d22457556e2fa01cbf1192cb1a94d619 ]

In a case where skb with an unconfirmed ct entry gets cloned, we may end up committing both but with different sets of extensions.

The series of events:

1. The first clone wants to commit and runs the helpers wiring up the extension pointer into the expectation list.
2. Then it looses the confirmation keeping the entry unconfirmed.
3. Second clone now wants to commit labels and adds the new extension for that breaking the pointer in the expectation list causing UAF on the destruction path later.

While this is possible to trigger, there should be no practical network pipeline where committing both clones without modifications into the same zone is needed. So, let's just reset the entry in case for some reason we got an skb with a shared one during commit. This doesn't affect any known use cases, but avoids any potential problems with sharing and modification of the unconfirmed ct entry.

The fixes tag points to the introduction of helpers, since that's the main UAF trigger for the sharing.

Stable-dep-of: f85009dfcd65 ("net/sched: act_ct: avoid modifying shared unconfirmed ct entry")

---
 include/net/netfilter/nf_conntrack.h |  5 +++++
 net/openvswitch/conntrack.c          | 12 ++++++++++++
 2 files changed, 17 insertions(+)

index aa0a7c82199ec..7e26611eb3c4a 100644
@@ -184,6 +184,11 @@ static inline void nf_ct_put(struct nf_conn *ct)
 		nf_ct_destroy(&ct->ct_general);
 }
 
+static inline bool nf_ct_shared(const struct nf_conn *ct)
+{
+	return refcount_read(&ct->ct_general.use) > 1;
+}
+
 /* load module; enable/disable conntrack in this namespace */
 int nf_ct_netns_get(struct net *net, u8 nfproto);
 void nf_ct_netns_put(struct net *net, u8 nfproto);

index 89ef350ca91c4..a3d839099fdb3 100644
@@ -734,6 +734,18 @@ static int __ovs_ct_lookup(struct net *net, struct sw_flow_key *key,
 	enum ip_conntrack_info ctinfo;
 	struct nf_conn *ct;
 
+	/* If the ct entry is not confirmed and shared with some other skb,
+	 * e.g., a cloned one, we can't just modify it with the commit as we
+	 * must not modify the extension set.  Reset.
+	 */
+	if (cached && info->commit) {
+		ct = nf_ct_get(skb, &ctinfo);
+		if (ct && !nf_ct_is_confirmed(ct) && nf_ct_shared(ct)) {
+			nf_reset_ct(skb);
+			cached = false;
+		}
+	}
+
 	if (!cached) {
 		struct nf_hook_state state = {
 			.hook = NF_INET_PRE_ROUTING,

2.53.0

From: Ilya Maximets <[email protected]>

[ Upstream commit f85009dfcd65e5969526b0db7a49b5413746e630 ]

In a case where skb with an unconfirmed ct entry gets cloned, we may end up processing both again but with different sets of extensions.

The series of events:

1. The first clone wants to commit and runs the helpers wiring up the extension pointer into the expectation list.
2. Then it looses the confirmation keeping the entry unconfirmed.
3. Second clone now wants to commit labels or run NAT and adds the new extension for that breaking the pointer in the expectation list causing UAF on the destruction path later.

While this is possible to trigger, there should be no practical network pipeline where we need to process both clones without modifications in the same zone. So, let's just reset the entry in case for some reason we got an skb with a shared one. This doesn't affect any known use cases, but avoids any potential problems with sharing and modification of the unconfirmed ct entry.

Unlike openvswitch module, act_ct allows for NAT without commit. Changing that would be a uAPI break. So, act_ct needs to reset on NAT regardless of the commit flag to avoid reallocation of the extension space. This, however, doesn't really change the picture for sensible networking cases as there should be no need to run the same packet twice (before and after the clone) through conntrack without packet header or zone changes and without commit.

The fixes tag points to the introduction of helpers, since that's the main UAF trigger for the sharing.

---
 net/sched/act_ct.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

index cbac27ba76afc..931ea6e99505b 100644
@@ -978,11 +978,11 @@ TC_INDIRECT_SCOPE int tcf_ct_act(struct sk_buff *skb, const struct tc_action *a,
 				 struct tcf_result *res)
 {
 	struct net *net = dev_net(skb->dev);
+	bool cached, commit, clear, nat;
 	enum ip_conntrack_info ctinfo;
 	struct tcf_ct *c = to_ct(a);
 	struct nf_conn *tmpl = NULL;
 	struct nf_hook_state state;
-	bool cached, commit, clear;
 	int nh_ofs, err, retval;
 	struct tcf_ct_params *p;
 	bool add_helper = false;

@@ -997,6 +997,7 @@ TC_INDIRECT_SCOPE int tcf_ct_act(struct sk_buff *skb, const struct tc_action *a,
 	retval = p->action;
 	commit = p->ct_action & TCA_CT_ACT_COMMIT;
 	clear = p->ct_action & TCA_CT_ACT_CLEAR;
+	nat = p->ct_action & TCA_CT_ACT_NAT;
 	tmpl = p->tmpl;
 
 	tcf_lastuse_update(&c->tcf_tm);

@@ -1045,6 +1046,19 @@ TC_INDIRECT_SCOPE int tcf_ct_act(struct sk_buff *skb, const struct tc_action *a,
 	 * different zone.
 	 */
 	cached = tcf_ct_skb_nfct_cached(net, skb, p);
+
+	/* If the ct entry is not confirmed and shared with some other skb,
+	 * e.g., a cloned one, we can't just modify it with a commit or nat
+	 * as we must not modify the extension set.  Reset.
+	 */
+	if (cached && (commit || nat)) {
+		ct = nf_ct_get(skb, &ctinfo);
+		if (ct && !nf_ct_is_confirmed(ct) && nf_ct_shared(ct)) {
+			nf_reset_ct(skb);
+			cached = false;
+		}
+	}
+
 	if (!cached) {
 		if (tcf_ct_flow_table_lookup(p, skb, family)) {
 			skip_add = true;

@@ -1082,7 +1096,7 @@ TC_INDIRECT_SCOPE int tcf_ct_act(struct sk_buff *skb, const struct tc_action *a,
 		if (err)
 			goto drop;
 		add_helper = true;
-		if (p->ct_action & TCA_CT_ACT_NAT && !nfct_seqadj(ct)) {
+		if (nat && !nfct_seqadj(ct)) {
 			if (!nfct_seqadj_ext_add(ct))
 				goto drop;
 		}

2.53.0