linux-kernel - Re: [PATCH net/next] bridge:Add rcu read lock when delete br port

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Date:	Mon, 4 Aug 2014 09:29:04 -0700
From:	Stephen Hemminger <stephen@...workplumber.org>
To:	<lichunhe@...wei.com>
Cc:	<vyasevic@...hat.com>, <xiyou.wangcong@...il.com>,
	<makita.toshiaki@....ntt.co.jp>, <mst@...hat.com>,
	<ebiederm@...ssion.com>, <f.fainelli@...il.com>,
	<linux-kernel@...r.kernel.org>, <wuyunfei@...wei.com>,
	<qianhuibin@...wei.com>
Subject: Re: [PATCH net/next] bridge:Add rcu read lock when delete br port

On Mon, 4 Aug 2014 11:37:56 +0800
<lichunhe@...wei.com> wrote:

> From: Chunhe Li <lichunhe@...wei.com>
> 
> In the br_hanle_frame function has a bug, when the bridge receive packets
> which go througth the br_handle_frame, get the net_bridge_port pointer "p",
> but don't check NULL pointer to use it. If somebody delete the bridge port
> at the same time, will call a NULL pointer, trigger kernel panic. I see the
> del_nbp comments, call del_nbp should via RCU, but the caller don't do this.
> 
> following steps will make bug happened
> 1.start vm and add the vm interface to a bridge br0,for example,
> brctl addbr br0 tap0
> 
> 2.configuer vm interface and br0 same ip subnet, vm ping br0.
> 
> 3.add and delete the vm interface port for endless loop.
> 
> Signed-off-by: Chunhe Li <lichunhe@...wei.com>

This is safe because:
 1. br_dev_delete is called with RTNL mutex therefore it is safe with other add/delete
    RCU does not block anything
 2. br_handle_frame is called with RCU lock (in frame rx)
 3. port is not freed until after grace period
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