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

[Jamal Hadi Salim]

On Wed, Jun 17, 2026 at 5:57 PM Michael Bommarito
<michael.bommarito@xxxxxxxxx> wrote:
>
> 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@xxxxxxxxx>
> ---
> 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]) {

Looks sane to me.
Acked-by: Jamal Hadi Salim <jhs@xxxxxxxxxxxx>

cheers,
jamal
> --
> 2.53.0
>