netdev - Re: [PATCH net] sit: fix use after free of fb_tunnel

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Date:	Thu, 14 Nov 2013 06:47:19 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Willem de Bruijn <willemb@...gle.com>
Cc:	davem@...emloft.net, edumazet@...gle.com,
	nicolas.dichtel@...nd.com, netdev@...r.kernel.org
Subject: Re: [PATCH net] sit: fix use after free of fb_tunnel_dev

On Wed, 2013-11-13 at 21:27 -0500, Willem de Bruijn wrote:
> Bug: The fallback device is created in sit_init_net and assumed to be
> freed in sit_exit_net. First, it is dereferenced in that function, in
> sit_destroy_tunnels:
> 
>         struct net *net = dev_net(sitn->fb_tunnel_dev);
> 
> Prior to this, rtnl_unlink_register has removed all devices that match
> rtnl_link_ops == sit_link_ops.
> 
> Commit 205983c43700 added the line
> 
> +       sitn->fb_tunnel_dev->rtnl_link_ops = &sit_link_ops;
> 
> which cases the fallback device to match here and be freed before it
> is last dereferenced.
> 
> Fix: This commit adds an explicit .delllink callback to sit_link_ops
> that skips deallocation at rtnl_unlink_register for the fallback
> device. This mechanism is comparable to the one in ip_tunnel.
> 
> It also modifies sit_destroy_tunnels and its only caller sit_exit_net
> to avoid the offending dereference in the first place. That double
> lookup is more complicated than required.
> 
> Test: The bug is only triggered when CONFIG_NET_NS is enabled. It
> causes a GPF only when CONFIG_DEBUG_SLAB is enabled. Verified that
> this bug exists at the mentioned commit, at davem-net HEAD and at
> 3.11.y HEAD. Verified that it went away after applying this patch.
> 
> Fixes: 205983c43700 ("sit: allow to use rtnl ops on fb tunnel")
> 
> Signed-off-by: Willem de Bruijn <willemb@...gle.com>
> 
> ---

Acked-by: Eric Dumazet <edumazet@...gle.com>


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