netdev - Re: NULL pointer dereference at __ip_route_output

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Message-Id: <20120510.164946.154691168913135822.davem@davemloft.net>
Date:	Thu, 10 May 2012 16:49:46 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	yevgen.pronenko@...ymobile.com
Cc:	netdev@...r.kernel.org
Subject: Re: NULL pointer dereference at __ip_route_output_key

From: Yevgen Pronenko <yevgen.pronenko@...ymobile.com>
Date: Thu, 19 Apr 2012 16:58:52 +0200

> As you can see, there is a NULL in res.fi->fib_nh.nh_dev. One more
> thing which looks suspicious for me is that res.fi->fib_dead is 1
> here. And the crash happened just after shutting down a WLAN interface
> (the last string in the kernel log was "wlan: disconnected").
> 
> Having that, is it possible there is a race between network resources
> deallocation and a route lookup procedure?

Indeed this area is a mess.

Nothing actually gates on fi->fib_dead except for an assertion in
free_fib_info().

Therefore, fi->fib_dead is essentially useless, and doesn't block
usage of fib_info objects that we are about to liberate via RCU.

My initial impression is that we need to shift the cleanup code into
the RCU handler (free_fib_info_rcu), and add some checks on
fi->fib_dead to the fib_info lookup paths.

I suspect we might have lost the fi->fib_dead tests unintentionally
when we converted the fib_info lookup paths to be refcount-less and
use RCU.  Oddly enough, the code does check for things like
(fi->fib_flags & RTNH_F_DEAD).

Anyways, I'll do some data-mining to figure out what happened here
and then use that information to cons up a fix.

Thanks for the report.
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