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:
<pre>
/* no kref on fast path */
if (chan->client_count) {
__module_get(owner);
chan->client_count++;
return 0;
}
if (!try_module_get(owner))
return -ENODEV;
if (!dma_device_get(chan->device)) { // calls kref_get_unless_zero()
</pre>
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.