netdev - Re: [PATCH net 1/3] l2tp: hold reference on tunnels in netlink dumps

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Date:   Fri, 13 Apr 2018 10:57:03 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     g.nault@...halink.fr
Cc:     netdev@...r.kernel.org, jchapman@...alix.com
Subject: Re: [PATCH net 1/3] l2tp: hold reference on tunnels in netlink
 dumps

From: Guillaume Nault <g.nault@...halink.fr>
Date: Thu, 12 Apr 2018 20:50:33 +0200

> l2tp_tunnel_find_nth() is unsafe: no reference is held on the returned
> tunnel, therefore it can be freed whenever the caller uses it.
> This patch defines l2tp_tunnel_get_nth() which works similarly, but
> also takes a reference on the returned tunnel. The caller then has to
> drop it after it stops using the tunnel.
> 
> Convert netlink dumps to make them safe against concurrent tunnel
> deletion.
> 
> Fixes: 309795f4bec2 ("l2tp: Add netlink control API for L2TP")
> Signed-off-by: Guillaume Nault <g.nault@...halink.fr>

During the entire invocation of l2tp_nl_cmd_tunnel_dump(), the RTNL
mutex is held.

Therefore no tunnel configuration changes may occur and the tunnel
object will persist and is safe to access.

The netlink dump should be safe as-is.

Were you actually able to trigger a crash or KASAN warning or is
this purely from code inspection?