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

From: Dean Nelson
Date: Mon Jun 11 2018 - 07:22:21 EST

On 06/10/2018 02:35 PM, David Miller wrote:
From: Vadim Lomovtsev <Vadim.Lomovtsev@xxxxxxxxxxxxxxxxxx>
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-> 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->;

So the kfree() will be called with a NULL pointer and quickly return.

And you'll crash when you dereference it above via

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.