[PATCH net 08/15] netfilter: flowtable: extend ct timeout of offloaded GRE flows

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

From: Michail Andreas Michalopoulos <mike.michalo@gmail.com>

Since commit 03428ca5cee9 ("netfilter: conntrack: rework offload nf_conn
timeout extension logic"), the flowtable gc worker extends the timeout
of an offloaded conntrack entry from nf_flow_table_extend_ct_timeout()
once it falls below 5 minutes. That function only knows about TCP and
UDP and hits WARN_ON_ONCE() for any other layer 4 protocol. However,
GRE flows can be offloaded too, both via nft_flow_offload and act_ct,
since commit 4e8d9584d154 ("netfilter: flowtable: Support GRE").

The timeout of an offloaded GRE entry drops below that threshold as soon
as a packet of the flow still goes through conntrack. With ip6gre
tunnels using the default encapsulation limit this is ordinary traffic:
the flowtable does not parse the Destination Options header in front of
GRE, so every packet of the flow takes the classic forwarding path. It
also happens for a packet with ttl <= 1, which the flowtable fast path
passes up to the classic forwarding path. nf_conntrack_gre_packet() then
refreshes ct->timeout to the GRE stream timeout (180 seconds by default),
and the next gc run triggers, as reported by syzbot:

  WARNING: net/netfilter/nf_flow_table_core.c:553 at nf_flow_table_extend_ct_timeout+0x1ee/0x2f0
  Workqueue: events_power_efficient nf_flow_offload_work_gc
  Call Trace:
   nf_flow_offload_gc_step+0x545/0x7b0
   nf_flow_table_iterate+0x13c/0x1f0
   nf_flow_offload_work_gc+0x22/0x70
   process_scheduled_works+0xc3d/0x1630
   worker_thread+0xa47/0xfb0
   kthread+0x38b/0x480

Besides the splat, which panics the machine with panic_on_warn, the
default branch sets the new timeout to 1 jiffy, so the conntrack entry
expires right away and the offloaded flow is torn down.

Handle GRE like UDP and extend the timeout to NF_CT_DAY. GRE conntrack
has no protocol state to take into account, and NF_CT_DAY is also what
flow_offload_add() already sets for every offloaded entry, GRE included.
This also restores the behaviour from before commit 03428ca5cee9, when
the conntrack gc worker extended offloaded entries of any protocol to
NF_CT_DAY.

Fixes: 03428ca5cee9 ("netfilter: conntrack: rework offload nf_conn timeout extension logic")
Reported-by: syzbot+521f7e7e2b974abde99a@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=521f7e7e2b974abde99a
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Michail Andreas Michalopoulos <mike.michalo@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
---
 net/netfilter/nf_flow_table_core.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/netfilter/nf_flow_table_core.c b/net/netfilter/nf_flow_table_core.c
index 0d72dc8ae47a..2aa2647ed9df 100644
--- a/net/netfilter/nf_flow_table_core.c
+++ b/net/netfilter/nf_flow_table_core.c
@@ -544,6 +544,7 @@ static void nf_flow_table_extend_ct_timeout(struct nf_conn *ct)
 
 		switch (l4proto) {
 		case IPPROTO_UDP:
+		case IPPROTO_GRE:
 			new_timeout = NF_CT_DAY;
 			break;
 		case IPPROTO_TCP:
-- 
2.47.3