Patch "tipc: fix crash during node removal" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

tipc: fix crash during node removal

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
tipc-fix-crash-during-node-removal.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Jon Paul Maloy <[email protected]>
Date: Wed, 24 Feb 2016 11:10:48 -0500
Subject: tipc: fix crash during node removal

From: Jon Paul Maloy <[email protected]>

commit d25a01257e422a4bdeb426f69529d57c73b235fe upstream.

When the TIPC module is unloaded, we have identified a race condition that allows a node reference counter to go to zero and the node instance being freed before the node timer is finished with accessing it. This leads to occasional crashes, especially in multi-namespace environments.

The scenario goes as follows:

CPU0:(node_stop) CPU1:(node_timeout) // ref == 2

1: if(!mod_timer())
2: if (del_timer())
3: tipc_node_put() // ref -> 1
4: tipc_node_put() // ref -> 0
5: kfree_rcu(node);
6: tipc_node_get(node)
7: // BOOM!

We now clean up this functionality as follows:

1) We remove the node pointer from the node lookup table before we attempt deactivating the timer. This way, we reduce the risk that tipc_node_find() may obtain a valid pointer to an instance marked for deletion; a harmless but undesirable situation.

2) We use del_timer_sync() instead of del_timer() to safely deactivate the node timer without any risk that it might be reactivated by the timeout handler. There is no risk of deadlock here, since the two functions never touch the same spinlocks.

3: We remove a pointless tipc_node_get() + tipc_node_put() from the
timeout handler.

---
 net/tipc/node.c |   24 +++++++++++-------------
 1 file changed, 11 insertions(+), 13 deletions(-)

@@ -102,9 +102,10 @@ static unsigned int tipc_hashfn(u32 addr
 
 static void tipc_node_kref_release(struct kref *kref)
 {
-	struct tipc_node *node = container_of(kref, struct tipc_node, kref);
+	struct tipc_node *n = container_of(kref, struct tipc_node, kref);
 
-	tipc_node_delete(node);
+	kfree(n->bc_entry.link);
+	kfree_rcu(n, rcu);
 }
 
 void tipc_node_put(struct tipc_node *node)

@@ -216,21 +217,20 @@ static void tipc_node_delete(struct tipc
 {
 	list_del_rcu(&node->list);
 	hlist_del_rcu(&node->hash);
-	kfree(node->bc_entry.link);
-	kfree_rcu(node, rcu);
+	tipc_node_put(node);
+
+	del_timer_sync(&node->timer);
+	tipc_node_put(node);
 }
 
 void tipc_node_stop(struct net *net)
 {
-	struct tipc_net *tn = net_generic(net, tipc_net_id);
+	struct tipc_net *tn = tipc_net(net);
 	struct tipc_node *node, *t_node;
 
 	spin_lock_bh(&tn->node_list_lock);
-	list_for_each_entry_safe(node, t_node, &tn->node_list, list) {
-		if (del_timer(&node->timer))
-			tipc_node_put(node);
-		tipc_node_put(node);
-	}
+	list_for_each_entry_safe(node, t_node, &tn->node_list, list)
+		tipc_node_delete(node);
 	spin_unlock_bh(&tn->node_list_lock);
 }
 

@@ -313,9 +313,7 @@ static void tipc_node_timeout(unsigned l
 		if (rc & TIPC_LINK_DOWN_EVT)
 			tipc_node_link_down(n, bearer_id, false);
 	}
-	if (!mod_timer(&n->timer, jiffies + n->keepalive_intv))
-		tipc_node_get(n);
-	tipc_node_put(n);
+	mod_timer(&n->timer, jiffies + n->keepalive_intv);
 }
 
 /**

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/tipc-fix-crash-during-node-removal.patch