netfilter: conntrack: rework offload nf_conn timeout extension logic

netfilter: conntrack: rework offload nf_conn timeout extension logic

Offload nf_conn entries may not see traffic for a very long time.

To prevent incorrect 'ct is stale' checks during nf_conntrack table
lookup, the gc worker extends the timeout nf_conn entries marked for
offload to a large value.

The existing logic suffers from a few problems.

Garbage collection runs without locks, its unlikely but possible
that @ct is removed right after the 'offload' bit test.

In that case, the timeout of a new/reallocated nf_conn entry will
be increased.

Prevent this by obtaining a reference count on the ct object and
re-check of the confirmed and offload bits.

If those are not set, the ct is being removed, skip the timeout
extension in this case.

Parallel teardown is also problematic:
 cpu1                                cpu2
 gc_worker
                                     calls flow_offload_teardown()
 tests OFFLOAD bit, set
                                     clear OFFLOAD bit
                                     ct->timeout is repaired (e.g. set to timeout[UDP_CT_REPLIED])
 nf_ct_offload_timeout() called
 expire value is fetched
 <INTERRUPT>
-> NF_CT_DAY timeout for flow that isn't offloaded
(and might not see any further packets).

Use cmpxchg: if ct->timeout was repaired after the 2nd 'offload bit' test
passed, then ct->timeout will only be updated of ct->timeout was not
altered in between.

As we already have a gc worker for flowtable entries, ct->timeout repair
can be handled from the flowtable gc worker.

This avoids having flowtable specific logic in the conntrack core
and avoids checking entries that were never offloaded.

This allows to remove the nf_ct_offload_timeout helper.
Its safe to use in the add case, but not on teardown.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>