netdev - Re: [NETLINK] Don't attach callback to a going-away netlink socket

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Message-ID: <4625D637.2040308@trash.net>
Date:	Wed, 18 Apr 2007 10:26:31 +0200
From:	Patrick McHardy <kaber@...sh.net>
To:	Evgeniy Polyakov <johnpol@....mipt.ru>
CC:	Pavel Emelianov <xemul@...ru>, David Miller <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	devel@...nvz.org, Kirill Korotaev <dev@...nvz.org>
Subject: Re: [NETLINK] Don't attach callback to a going-away netlink socket

Evgeniy Polyakov wrote:
> On Wed, Apr 18, 2007 at 12:16:18PM +0400, Pavel Emelianov (xemul@...ru) wrote:
> 
>>Sorry, I forgot to put netdev and David in Cc when I first sent it.
>>
>>There is a race between netlink_dump_start() and netlink_release()
>>that can lead to the situation when a netlink socket with non-zero
>>callback is freed.
> 
> 
> Out of curiosity, why not to fix a netlink_dump_start() to remove
> callback in error path, since in 'no-error' path it removes it in
> netlink_dump().

It already does (netlink_destroy_callback), but that doesn't help
with this race though since without this patch we don't enter the
error path.

> And, btw, can release method be called while socket is being used, I
> thought about proper reference counters should prevent this, but not
> 100% sure with RCU dereferencing of the descriptor.

The problem is asynchronous processing of the dump request in the
context of a different process. Process requests a dump, message
is queued and process returns from sendmsg since some other process
is already processing the queue. Then the process closes the socket,
resulting in netlink_release being called. When the dump request
is finally processed the race Pavel described might happen. This
can only happen for netlink families that use mutex_try_lock for
queue processing of course.

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