[PATCH net] rxrpc: Prevent poking connections after final put

[Chengfeng Ye <nicoyip.dev@gmail.com>]

rxrpc_poke_conn() unconditionally gets a reference before adding a
connection to conn_attend_q.  This is unsafe for the connection timer
because timer deletion cannot stop a callback that has already started.

The following interleaving can occur:

  I/O thread                         Timer softirq
  rxrpc_put_connection()
    refcount reaches zero
                                     rxrpc_connection_timer()
                                       rxrpc_poke_conn()
                                         rxrpc_get_connection()
                                           increment from zero
                                         list_add_tail()
    timer_delete()
    rxrpc_clean_up_connection()
      timer_delete_sync()
      call_rcu()

The refcount increment saturates instead of resurrecting the connection,
so cleanup continues after the timer callback exits.  The RCU callback can
then free the connection while attend_link remains queued, and the I/O
thread later accesses the stale queue entry.

The kernel reported:

  refcount_t: addition on 0; use-after-free.
  WARNING: lib/refcount.c:25 at refcount_warn_saturate+0xc0/0xe0
  Call Trace:
   <IRQ>
   rxrpc_get_connection+0x185/0x1d0
   rxrpc_poke_conn+0x12e/0x2b0
   call_timer_fn+0x2b/0x220
   __run_timers+0x561/0x8d0
   run_timer_softirq+0x144/0x250
   handle_softirqs+0x18d/0x5b0
   </IRQ>
   <TASK>
   rxrpc_put_connection+0x2fe/0x3f0
   rxrpc_discard_expired_client_conns+0x396/0x670
   rxrpc_io_thread+0xbfc/0x1640
   </TASK>

Use rxrpc_get_connection_maybe() while holding the attend queue lock and
only enqueue the connection if a live reference was acquired.  The normal
path for a live connection is unchanged.

Fixes: 4241a702e0d0 ("rxrpc: Fix the rxrpc_connection attend queue handling")
Cc: stable@vger.kernel.org
Signed-off-by: Chengfeng Ye <nicoyip.dev@gmail.com>
---
 net/rxrpc/conn_object.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/net/rxrpc/conn_object.c b/net/rxrpc/conn_object.c
index 0ece717db0f8..3f4516115d37 100644
--- a/net/rxrpc/conn_object.c
+++ b/net/rxrpc/conn_object.c
@@ -33,10 +33,8 @@ void rxrpc_poke_conn(struct rxrpc_connection *conn, enum rxrpc_conn_trace why)
 
 	spin_lock_irq(&local->lock);
 	busy = !list_empty(&conn->attend_link);
-	if (!busy) {
-		rxrpc_get_connection(conn, why);
+	if (!busy && rxrpc_get_connection_maybe(conn, why))
 		list_add_tail(&conn->attend_link, &local->conn_attend_q);
-	}
 	spin_unlock_irq(&local->lock);
 	rxrpc_wake_up_io_thread(local);
 }
-- 
2.43.0