[PATCH net] ax25: fix use-after-free bugs caused by ax25_ds_del_timer

[Duoming Zhou]

When the ax25 device is detaching, the ax25_dev_device_down()
calls ax25_ds_del_timer() to cleanup the slave_timer. When
the timer handler is running, the ax25_ds_del_timer() that
calls del_timer() in it will return directly. As a result,
the use-after-free bugs could happen, one of the scenarios
is shown below:

      (Thread 1)          |      (Thread 2)
                          | ax25_ds_timeout()
ax25_dev_device_down()    |
  ax25_ds_del_timer()     |
    del_timer()           |
  ax25_dev_put() //FREE   |
                          |  ax25_dev-> //USE

In order to mitigate bugs, when the device is detaching, use
timer_shutdown_sync() to stop the timer.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Duoming Zhou <duoming@xxxxxxxxxx>
---
 net/ax25/ax25_ds_timer.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/net/ax25/ax25_ds_timer.c b/net/ax25/ax25_ds_timer.c
index c4f8adbf814..5624c0d174c 100644
--- a/net/ax25/ax25_ds_timer.c
+++ b/net/ax25/ax25_ds_timer.c
@@ -43,7 +43,12 @@ void ax25_ds_setup_timer(ax25_dev *ax25_dev)
 
 void ax25_ds_del_timer(ax25_dev *ax25_dev)
 {
-	if (ax25_dev)
+	if (!ax25_dev)
+		return;
+
+	if (!ax25_dev->device_up)
+		timer_shutdown_sync(&ax25_dev->dama.slave_timer);
+	else
 		del_timer(&ax25_dev->dama.slave_timer);
 }
 
-- 
2.17.1