[PATCH nf 2/2] netfilter: flowtable: bump ct refcount before teardown

[Pablo Neira Ayuso <pablo@netfilter.org>]

The existing ct state and timeout fix up after setting the teardown bit
is unsafe. The flowtable GC owns the flow entry once the teardown bit is
set on. Therefore, it might release the flow entry and drop the
reference on the ct while the ct fix up is being performed from either
packet path or netdevice notifier.

Bump the ct refcount, in combination with the teardown bit, as a
synchronization point, if refcount is zero, then this flow is being
teardown. Furthermore, if the teardown bit has been already set, then
this CPU lost race to tear down this flow.

Fixes: d2d31ea8cd80 ("netfilter: conntrack: fix erronous removal of offload bit")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
---
v1: new in this series.

 net/netfilter/nf_flow_table_core.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/net/netfilter/nf_flow_table_core.c b/net/netfilter/nf_flow_table_core.c
index 625caaa8addf..1a9fd5442b0c 100644
--- a/net/netfilter/nf_flow_table_core.c
+++ b/net/netfilter/nf_flow_table_core.c
@@ -392,9 +392,15 @@ static void flow_offload_del(struct nf_flowtable *flow_table,
 
 void flow_offload_teardown(struct flow_offload *flow)
 {
-	clear_bit(IPS_OFFLOAD_BIT, &flow->ct->status);
-	if (!test_and_set_bit(NF_FLOW_TEARDOWN, &flow->flags))
+	if (unlikely(!refcount_inc_not_zero(&flow->ct->ct_general.use)))
+		return;
+
+	if (!test_and_set_bit(NF_FLOW_TEARDOWN, &flow->flags)) {
 		flow_offload_fixup_ct(flow);
+		smp_mb__before_atomic();
+		clear_bit(IPS_OFFLOAD_BIT, &flow->ct->status);
+	}
+	nf_ct_put(flow->ct);
 }
 EXPORT_SYMBOL_GPL(flow_offload_teardown);
 
-- 
2.47.3