linux-kernel - Re: [PATCH] net: thunderx: prevent concurrent data re-writing by nicvf_set_rx

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Date:   Wed, 13 Jun 2018 02:15:39 -0700
From:   Vadim Lomovtsev <Vadim.Lomovtsev@...iumnetworks.com>
To:     David Miller <davem@...emloft.net>
Cc:     dnelson@...hat.com, rric@...nel.org, sgoutham@...ium.com,
        linux-arm-kernel@...ts.infradead.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, Vadim.Lomovtsev@...ium.com
Subject: Re: [PATCH] net: thunderx: prevent concurrent data re-writing by
 nicvf_set_rx_mode

Sorry for delay.

On Tue, Jun 12, 2018 at 03:25:40PM -0700, David Miller wrote:
> From: Dean Nelson <dnelson@...hat.com>
> Date: Mon, 11 Jun 2018 06:22:14 -0500
> 
> > On 06/10/2018 02:35 PM, David Miller wrote:
> >> From: Vadim Lomovtsev <Vadim.Lomovtsev@...iumnetworks.com>
> >> Date: Fri,  8 Jun 2018 02:27:59 -0700
> >> 
> >>> +	/* Save message data locally to prevent them from
> >>> +	 * being overwritten by next ndo_set_rx_mode call().
> >>> +	 */
> >>> +	spin_lock(&nic->rx_mode_wq_lock);
> >>> +	mode = vf_work->mode;
> >>> +	mc = vf_work->mc;
> >>> +	vf_work->mc = NULL;
> > 
> > If I'm reading this code correctly, I believe nic->rx_mode_work.mc
> > will
> > have been set to NULL before the lock is dropped by
> > nicvf_set_rx_mode_task() and acquired by nicvf_set_rx_mode().
> > 
> > 
> >>> +	spin_unlock(&nic->rx_mode_wq_lock);
> >> At the moment you drop this lock, the memory behind 'mc' can be
> >> freed up by:
> >> 
> >>> +	spin_lock(&nic->rx_mode_wq_lock);
> >>> +	kfree(nic->rx_mode_work.mc);
> > 
> > So the kfree() will be called with a NULL pointer and quickly return.
> > 
> > 
> >> And you'll crash when you dereference it above via
> >> __nicvf_set_rx_mode_task().
> >> 
> > 
> > I believe the call to kfree() in nicvf_set_rx_mode() is there to free
> > up a mc_list that has been allocated by nicvf_set_rx_mode() during a
> > previous callback to the function, one that has not yet been processed
> > by nicvf_set_rx_mode_task().
> > 
> > In this way only the last 'unprocessed' callback to
> > nicvf_set_rx_mode()
> > gets processed should there be multiple callbacks occurring between
> > the
> > times the nicvf_set_rx_mode_task() runs.
> > 
> > In my testing with this patch, this is what I see happening.
> 
> You're right, my bad.
> 
> Patch applied.

Thank you for your time.

WBR,
Vadim