[PATCH 2/3] rxrpc: wait for deferred conn destruction before conn_proc_list check

[Henry Martin <bsdhenrymartin@gmail.com>]

rxrpc_destroy_all_connections() flushes rxrpc_workqueue and then
immediately asserts that rxnet->conn_proc_list is empty. However,
connection destruction is normally deferred to system_wq: the
final-ACK timer is still pending when the last ref is dropped, so
rxrpc_put_connection() schedules conn->destructor instead of running
it inline. conn->proc_link is only removed by the deferred
destructor (rxrpc_clean_up_connection()), which the
flush_workqueue(rxrpc_workqueue) call does not cover - so the
assertion can fire on a perfectly healthy intermediate state:

	rxrpc: Assertion failed
	kernel BUG at net/rxrpc/conn_object.c:488!
	Workqueue: netns cleanup_net

The BUG kills the cleanup_net kworker mid-teardown, leaving the
netns half-destroyed and potentially wedging later netns operations.
This is easily reachable from an unprivileged userns+netns running
loopback AF_RXRPC traffic.

The existing wait_var_event() on nr_conns is exactly the right
synchronization: nr_conns only reaches zero after every destructor
and RCU free has completed, which implies proc_link has been removed
from every connection. It is, however, placed *after* the assertion
it is meant to make reliable. Move it between the service_conns
leak check (whose outcome is already final once the reaper has been
flushed) and the conn_proc_list assertion.

This also silences the spurious "AF_RXRPC: Leaked peer" messages
seen during netns teardown, which share the same root cause: peer
references are dropped by the same deferred destructors.

Found by the autokbug dynamic kernel fuzzer at Tencent Yunding Lab.

Signed-off-by: Henry Martin <bsdhenrymartin@gmail.com>
---
 net/rxrpc/conn_object.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/net/rxrpc/conn_object.c b/net/rxrpc/conn_object.c
index 0ece717db0f85..df3f92b1a42e9 100644
--- a/net/rxrpc/conn_object.c
+++ b/net/rxrpc/conn_object.c
@@ -485,11 +485,13 @@ void rxrpc_destroy_all_connections(struct rxrpc_net *rxnet)
 	write_unlock(&rxnet->conn_lock);
 	BUG_ON(leak);
 
-	ASSERT(list_empty(&rxnet->conn_proc_list));
-
-	/* We need to wait for the connections to be destroyed by RCU as they
-	 * pin things that we still need to get rid of.
+	/* Connection destruction is normally deferred to system_wq because
+	 * the final-ACK timer is still pending when the last ref is dropped.
+	 * Wait for the deferred destructors (and the RCU frees) to complete
+	 * before checking conn_proc_list; they remove conns from it.
 	 */
 	wait_var_event(&rxnet->nr_conns, !atomic_read(&rxnet->nr_conns));
+
+	ASSERT(list_empty(&rxnet->conn_proc_list));
 	_leave("");
 }
-- 
2.43.0