Re: [PATCH] net: thunderx: prevent concurrent data re-writing by nicvf_set_rx_mode

[David Miller <davem@davemloft.net>]

From: Dean Nelson <dnelson@redhat.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@caviumnetworks.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.