Reported by sashiko: there is a small race window. If a helper module is unloaded or a userspace-defined helper is removed, nf_conntrack_helper_unregister() sets ->helper to NULL. Handle this safely. This needs a second patch to close related race during nf_conntrack_helper_unregister(). Fixes: b20ab9cc63ca ("netfilter: nf_ct_helper: better logging for dropped packets") Signed-off-by: Florian Westphal --- net/netfilter/nf_conntrack_helper.c | 13 ++++++++----- 1 file changed, 8 insertions(+), 5 deletions(-) diff --git a/net/netfilter/nf_conntrack_helper.c b/net/netfilter/nf_conntrack_helper.c index b594cd244fe1..17e971bd4c74 100644 --- a/net/netfilter/nf_conntrack_helper.c +++ b/net/netfilter/nf_conntrack_helper.c @@ -321,8 +321,8 @@ __printf(3, 4) void nf_ct_helper_log(struct sk_buff *skb, const struct nf_conn *ct, const char *fmt, ...) { + const char *helper_name = "(null)"; const struct nf_conn_help *help; - const struct nf_conntrack_helper *helper; struct va_format vaf; va_list args; @@ -331,14 +331,17 @@ void nf_ct_helper_log(struct sk_buff *skb, const struct nf_conn *ct, vaf.fmt = fmt; vaf.va = &args; - /* Called from the helper function, this call never fails */ help = nfct_help(ct); + if (help) { + const struct nf_conntrack_helper *helper; - /* rcu_read_lock()ed by nf_hook_thresh */ - helper = rcu_dereference(help->helper); + helper = rcu_dereference(help->helper); + if (helper) + helper_name = helper->name; + } nf_log_packet(nf_ct_net(ct), nf_ct_l3num(ct), 0, skb, NULL, NULL, NULL, - "nf_ct_%s: dropping packet: %pV ", helper->name, &vaf); + "helper %s dropping packet: %pV ", helper_name, &vaf); va_end(args); } -- 2.54.0