[PATCH net] net/sched: act_ct: fix nf_connlabels leak on two error paths

[Michael Bommarito <michael.bommarito@gmail.com>]

tcf_ct_fill_params() calls nf_connlabels_get() (setting put_labels) when
TCA_CT_LABELS is present, but two later error sites use a bare return
instead of "goto err", skipping the err: nf_connlabels_put() cleanup.
They also precede the "p->put_labels = put_labels" assignment, so the
tcf_ct_params_free() fallback does not release the count either. Each
failed RTM_NEWACTION on these paths leaks one nf_connlabels reference:
net->ct.labels_used is incremented and never released. The action is
reachable with CAP_NET_ADMIN over the netns, i.e. from an unprivileged
user namespace on default-userns kernels.

Impact: an unprivileged user with CAP_NET_ADMIN over a network namespace
(e.g. via user namespaces) leaks one nf_connlabels reference per failed
RTM_NEWACTION on the two error paths; net->ct.labels_used is never
released.

The err: label is safe to reach from both sites: p->tmpl is still NULL
there (kzalloc'd, not yet assigned) and nf_ct_put(NULL) is a no-op, so
no inline release is needed.

Fixes: 70f06c115bcc ("sched: act_ct: switch to per-action label counting")
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
---
Testing: refcount/counter leak (CWE-772); no sanitizer for this class, so
the oracle is the nf_connlabels accounting counter net->ct.labels_used.

Reproduction (UML, before/after, same trigger): CONFIG_NET_ACT_CT=y,
NF_CONNTRACK_LABELS=y, NF_CONNTRACK_ZONES=n (forces the zone-disabled
path). A raw RTM_NEWACTION trigger adds "action ct label 0x1/0x1 zone 1"
20 times; each returns -EOPNOTSUPP.
  stock:   net->ct.labels_used climbs 1,2,...,20 (get, then bare return,
           no put) -- 20 leaked counts, never recovered.
  patched: counter stays balanced (get then goto err -> put); baseline.
Control: the same loop without "label" (no nf_connlabels_get) leaves the
counter unchanged on both trees -- the trigger reached the labels path and
the synthesis is not itself the cause.

Conditions: reachable via RTM_NEWACTION with CAP_NET_ADMIN over the netns,
i.e. an unprivileged user in a fresh user+net namespace on default-userns
distros. The easy path needs CONFIG_NF_CONNTRACK_ZONES=n; the
nf_ct_tmpl_alloc ENOMEM path leaks on any config under memory pressure.

Mitigations: restrict unprivileged user namespaces; otherwise none short of
the fix. Harness (trigger.c, init) available on request.

 net/sched/act_ct.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/sched/act_ct.c b/net/sched/act_ct.c
index 6158e13c98d35..f5866a364a74a 100644
--- a/net/sched/act_ct.c
+++ b/net/sched/act_ct.c
@@ -1295,7 +1295,8 @@ static int tcf_ct_fill_params(struct net *net,
 	if (tb[TCA_CT_ZONE]) {
 		if (!IS_ENABLED(CONFIG_NF_CONNTRACK_ZONES)) {
 			NL_SET_ERR_MSG_MOD(extack, "Conntrack zones isn't enabled.");
-			return -EOPNOTSUPP;
+			err = -EOPNOTSUPP;
+			goto err;
 		}
 
 		tcf_ct_set_key_val(tb,
@@ -1308,7 +1309,8 @@ static int tcf_ct_fill_params(struct net *net,
 	tmpl = nf_ct_tmpl_alloc(net, &zone, GFP_KERNEL);
 	if (!tmpl) {
 		NL_SET_ERR_MSG_MOD(extack, "Failed to allocate conntrack template");
-		return -ENOMEM;
+		err = -ENOMEM;
+		goto err;
 	}
 	p->tmpl = tmpl;
 	if (tb[TCA_CT_HELPER_NAME]) {
-- 
2.53.0