In the Linux kernel, the following vulnerability has been resolved: dmaengine: Fix device kref underflow in dma_chan_put() dma_chan_get()

In the Linux kernel, the following vulnerability has been resolved:

dmaengine: Fix device kref underflow in dma_chan_put()

dma_chan_get() takes chan->device->ref only on the slow path:

 1/* no kref on fast path */
 2if (chan->client_count) {
 3	__module_get(owner);
 4	chan->client_count++;
 5	return 0;
 6}
 7if (!try_module_get(owner))
 8	return -ENODEV;
 9if (!dma_device_get(chan->device)) { // calls kref_get_unless_zero()

dma_chan_put() drops the ref unconditionally, so every fast-path
get/put pair drops one extra device reference.

The bug fires when two conditions hold together: a non-private
provider has a persistent client holding chan->client_count > 0
and another client cycles dmaengine_get()/dmaengine_put().
When the kref hits zero, the subsequent dma_find_channel() returns
NULL even though the provider module is still loaded.

Fix this by dropping device->ref only on the last put, matching the
single slow-path get.