linux-kernel - Re: BUG: unable to handle kernel NULL pointer dereference at 0000000000000040

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Date:	Fri, 14 Jun 2013 23:30:56 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Ming Lei <ming.lei@...onical.com>
Cc:	nirinA raseliarison <nirina.raseliarison@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Francois Romieu <romieu@...zoreil.com>, nic_swsd@...ltek.com,
	Hayes Wang <hayeswang@...ltek.com>
Subject: Re: BUG: unable to handle kernel NULL pointer dereference at
 0000000000000040

On Sat, Jun 15, 2013 at 10:32:14AM +0800, Ming Lei wrote:
> On Sat, Jun 15, 2013 at 1:07 AM, nirinA raseliarison
> <nirina.raseliarison@...il.com> wrote:
> 
> > patch applied and no longer have the bug message when i
> > reboot and wake up the ethernet controller.
> 
> I am wondering if Guenter's patch can fix the race really, but I'd like to
> see Guenter's explanation on his patch.
> 
> The race should be caused by below:
> 
> - request timeout triggered by internal timer
> 
> - user space aborts the requests before the line in _request_firmware_load()
> 
>              fw_priv->buf = NULL
> 
> which is run in timeout path
> 
> - then the abort() called from firmware_loading_store() may use a freed fw buf
> since the timeout path will free the fw buffer.
> 
> Considered clearing 'fw_priv->buf' in _request_firmware_load()() isn't protected
> by fw_lock now, so Guenter's patch can't avoid the race entirely.
> 
I agree; my patch only protects one specific path, and was based on the
observation that access to fw_priv->buf is protected elsewhwere in the code.
My suspicion was that fw_priv->buf was freed while waiting for the mutex in
firmware_loading_store().

Your patch is more comprehensive.

Guenter
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