Re: [PATCH net] netfilter: flowtable: Fix expired flow not being deleted from software

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

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On Mon, May 11, 2020 at 10:24:44AM +0300, Paul Blakey wrote:
> 
> 
> On 5/11/2020 1:26 AM, Pablo Neira Ayuso wrote:
> > On Wed, May 06, 2020 at 02:27:29PM +0300, Paul Blakey wrote:
> >> Once a flow is considered expired, it is marked as DYING, and
> >> scheduled a delete from hardware. The flow will be deleted from
> >> software, in the next gc_step after hardware deletes the flow
> >> (and flow is marked DEAD). Till that happens, the flow's timeout
> >> might be updated from a previous scheduled stats, or software packets
> >> (refresh). This will cause the gc_step to no longer consider the flow
> >> expired, and it will not be deleted from software.
> >>
> >> Fix that by looking at the DYING flag as in deciding
> >> a flow should be deleted from software.
> > Would this work for you?
> >
> > The idea is to skip the refresh if this has already expired.
> >
> > Thanks.
> 
> The idea is ok, but timeout check + update isn't atomic (need atomic_inc_unlesss
> or something like that), and there is also
> the hardware stats which if comes too late (after gc finds it expired) might
> bring a flow back to life.

Right. Once the entry has expired, there should not be a way turning
back.

I'm attaching a new sketch, it's basically using the teardown state to
specify that the gc already made the decision to remove this entry.

Thanks.

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diff --git a/net/netfilter/nf_flow_table_core.c b/net/netfilter/nf_flow_table_core.c
index 4344e572b7f9..42da6e337276 100644
--- a/net/netfilter/nf_flow_table_core.c
+++ b/net/netfilter/nf_flow_table_core.c
@@ -284,7 +284,7 @@ static void flow_offload_del(struct nf_flowtable *flow_table,
 
 	if (nf_flow_has_expired(flow))
 		flow_offload_fixup_ct(flow->ct);
-	else if (test_bit(NF_FLOW_TEARDOWN, &flow->flags))
+	else
 		flow_offload_fixup_ct_timeout(flow->ct);
 
 	flow_offload_free(flow);
@@ -361,8 +361,10 @@ static void nf_flow_offload_gc_step(struct flow_offload *flow, void *data)
 {
 	struct nf_flowtable *flow_table = data;
 
-	if (nf_flow_has_expired(flow) || nf_ct_is_dying(flow->ct) ||
-	    test_bit(NF_FLOW_TEARDOWN, &flow->flags)) {
+	if (nf_flow_has_expired(flow) || nf_ct_is_dying(flow->ct))
+		set_bit(NF_FLOW_TEARDOWN, &flow->flags);
+
+	if (test_bit(NF_FLOW_TEARDOWN, &flow->flags)) {
 		if (test_bit(NF_FLOW_HW, &flow->flags)) {
 			if (!test_bit(NF_FLOW_HW_DYING, &flow->flags))
 				nf_flow_offload_del(flow_table, flow);