netdev - Re: [PATCH net v2] l2tp: fix race condition in l2tp_tunnel

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Date:   Thu, 21 Sep 2017 11:53:02 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     sd@...asysnail.net
Cc:     netdev@...r.kernel.org, g.nault@...halink.fr, lucien.xin@...il.com,
        tparkin@...alix.com
Subject: Re: [PATCH net v2] l2tp: fix race condition in l2tp_tunnel_delete

From: Sabrina Dubroca <sd@...asysnail.net>
Date: Tue, 19 Sep 2017 15:40:40 +0200

> If we try to delete the same tunnel twice, the first delete operation
> does a lookup (l2tp_tunnel_get), finds the tunnel, calls
> l2tp_tunnel_delete, which queues it for deletion by
> l2tp_tunnel_del_work.
> 
> The second delete operation also finds the tunnel and calls
> l2tp_tunnel_delete. If the workqueue has already fired and started
> running l2tp_tunnel_del_work, then l2tp_tunnel_delete will queue the
> same tunnel a second time, and try to free the socket again.
> 
> Add a dead flag to prevent firing the workqueue twice. Then we can
> remove the check of queue_work's result that was meant to prevent that
> race but doesn't.
> 
> Also check the flag in the tunnel lookup functions, to avoid returning a
> tunnel that is already scheduled for destruction.

Sabrina, please respond to Guillaume's feedback.

Thank you.