[PATCH net] bridge: stp: Fix a potential use-after-free when deleting a bridge

[Ido Schimmel <idosch@nvidia.com>]

The three STP timers are not supposed to be armed while the bridge is
administratively down. They are synchronously deactivated when the
bridge is put administratively down and the various call sites check for
'IFF_UP' before arming them.

This check is missing from br_topology_change_detection() and it is
possible to engineer a situation in which the topology change timer is
armed while the bridge is administratively down, resulting in a
use-after-free [1] when the bridge is deleted.

Fix by adding the missing check and for good measures synchronously
shutdown the three timers when the bridge is deleted.

[1]
ODEBUG: free active (active state 0) object: ffff88811662b9b0 object type: timer_list hint: br_topology_change_timer_expired (net/bridge/br_stp_timer.c:120)
WARNING: lib/debugobjects.c:629 at debug_print_object+0x1bc/0x450, CPU#9: ip/359

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: Noam Rathaus <noamr@ssd-disclosure.com>
Reported-by: Neil Young <contact@ssd-disclosure.com>
Acked-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
---
 net/bridge/br_if.c  | 3 +++
 net/bridge/br_stp.c | 3 ++-
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/bridge/br_if.c b/net/bridge/br_if.c
index 7ed19aa8ae59..c52613431f88 100644
--- a/net/bridge/br_if.c
+++ b/net/bridge/br_if.c
@@ -392,6 +392,9 @@ void br_dev_delete(struct net_device *dev, struct list_head *head)
 
 	br_fdb_delete_by_port(br, NULL, 0, 1);
 
+	timer_shutdown_sync(&br->hello_timer);
+	timer_shutdown_sync(&br->topology_change_timer);
+	timer_shutdown_sync(&br->tcn_timer);
 	cancel_delayed_work_sync(&br->gc_work);
 
 	br_sysfs_delbr(br->dev);
diff --git a/net/bridge/br_stp.c b/net/bridge/br_stp.c
index 46919d73d42f..c7e7e924f155 100644
--- a/net/bridge/br_stp.c
+++ b/net/bridge/br_stp.c
@@ -382,7 +382,8 @@ void br_topology_change_detection(struct net_bridge *br)
 {
 	int isroot = br_is_root_bridge(br);
 
-	if (br->stp_enabled != BR_KERNEL_STP)
+	if (br->stp_enabled != BR_KERNEL_STP ||
+	    !(br->dev->flags & IFF_UP))
 		return;
 
 	br_info(br, "topology change detected, %s\n",
-- 
2.54.0