stm class: Fix a use-after-free

BugLink: http://bugs.launchpad.net/bugs/1724783

commit fd085bb1 upstream.

For reasons unknown, the stm_source removal path uses device_destroy()
to kill the underlying device object. Because device_destroy() uses
devt to look for the device to destroy and the fact that stm_source
devices don't have one (or all have the same one), it just picks the
first device in the class, which may well be the wrong one.

That is, loading stm_console and stm_heartbeat and then removing both
will die in dereferencing a freed object.

Since this should have been device_unregister() in the first place,
use it instead of device_destroy().
Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Fixes: 7bd1d409 ("stm class: Introduce an abstraction for System Trace Module devices")
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>

drivers/hwtracing/stm/core.c

@@ -952,7 +952,7 @@ void stm_source_unregister_device(struct stm_source_data *data)
 
 	stm_source_link_drop(src);
 
-	device_destroy(&stm_source_class, src->dev.devt);
+	device_unregister(&src->dev);
 }
 EXPORT_SYMBOL_GPL(stm_source_unregister_device);