[PATCH net 1/8] netfilter: flowtable: publish HW_DEAD after worker is done

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

From: Jérémy Jean <Jeremy.Jean@oss.cyber.gouv.fr>

flow_offload_work_del() sets NF_FLOW_HW_DEAD before the work handler
clears NF_FLOW_HW_PENDING. Once a flow is both HW_DYING and HW_DEAD, a
concurrent garbage collection pass can remove it and schedule it for RCU
freeing.

The offload worker holds neither an RCU read lock nor a reference to the
flow. If it is preempted after publishing HW_DEAD, the RCU callback can
free the flow before the worker resumes and clears HW_PENDING, resulting
in a use-after-free.

Move HW_DEAD publication to the common worker epilogue after the pending
bit is cleared, making it the final flow access by destroy work. Order all
preceding flow accesses before publishing the bit that allows garbage
collection to free the object.

Fixes: 2c8897953f3b ("netfilter: flowtable: Add pending bit for offload work")
Assisted-by: Codex:gpt-5
Signed-off-by: Jérémy Jean <Jeremy.Jean@oss.cyber.gouv.fr>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
---
 net/netfilter/nf_flow_table_offload.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/net/netfilter/nf_flow_table_offload.c b/net/netfilter/nf_flow_table_offload.c
index 801a3dd9ceea..6757fd89c1f1 100644
--- a/net/netfilter/nf_flow_table_offload.c
+++ b/net/netfilter/nf_flow_table_offload.c
@@ -995,7 +995,6 @@ static void flow_offload_work_del(struct flow_offload_work *offload)
 	flow_offload_tuple_del(offload, FLOW_OFFLOAD_DIR_ORIGINAL);
 	if (test_bit(NF_FLOW_HW_BIDIRECTIONAL, &offload->flow->flags))
 		flow_offload_tuple_del(offload, FLOW_OFFLOAD_DIR_REPLY);
-	set_bit(NF_FLOW_HW_DEAD, &offload->flow->flags);
 }
 
 static void flow_offload_tuple_stats(struct flow_offload_work *offload,
@@ -1059,6 +1058,12 @@ static void flow_offload_work_handler(struct work_struct *work)
 	}
 
 	clear_bit(NF_FLOW_HW_PENDING, &offload->flow->flags);
+	if (offload->cmd == FLOW_CLS_DESTROY) {
+		/* Publish after the worker's last flow access. */
+		smp_mb__before_atomic();
+		set_bit(NF_FLOW_HW_DEAD, &offload->flow->flags);
+	}
+
 	kfree(offload);
 }
 
-- 
2.47.3