1 // SPDX-License-Identifier: GPL-2.0
2 #include "dm-core.h"
3 
4 /*
5  * The kobject release method must not be placed in the module itself,
6  * otherwise we are subject to module unload races.
7  *
8  * The release method is called when the last reference to the kobject is
9  * dropped. It may be called by any other kernel code that drops the last
10  * reference.
11  *
12  * The release method suffers from module unload race. We may prevent the
13  * module from being unloaded at the start of the release method (using
14  * increased module reference count or synchronizing against the release
15  * method), however there is no way to prevent the module from being
16  * unloaded at the end of the release method.
17  *
18  * If this code were placed in the dm module, the following race may
19  * happen:
20  *  1. Some other process takes a reference to dm kobject
21  *  2. The user issues ioctl function to unload the dm device
22  *  3. dm_sysfs_exit calls kobject_put, however the object is not released
23  *     because of the other reference taken at step 1
24  *  4. dm_sysfs_exit waits on the completion
25  *  5. The other process that took the reference in step 1 drops it,
26  *     dm_kobject_release is called from this process
27  *  6. dm_kobject_release calls complete()
28  *  7. a reschedule happens before dm_kobject_release returns
29  *  8. dm_sysfs_exit continues, the dm device is unloaded, module reference
30  *     count is decremented
31  *  9. The user unloads the dm module
32  * 10. The other process that was rescheduled in step 7 continues to run,
33  *     it is now executing code in unloaded module, so it crashes
34  *
35  * Note that if the process that takes the foreign reference to dm kobject
36  * has a low priority and the system is sufficiently loaded with
37  * higher-priority processes that prevent the low-priority process from
38  * being scheduled long enough, this bug may really happen.
39  *
40  * In order to fix this module unload race, we place the release method
41  * into a helper code that is compiled directly into the kernel.
42  */
43 
dm_kobject_release(struct kobject * kobj)44 void dm_kobject_release(struct kobject *kobj)
45 {
46 	complete(dm_get_completion_from_kobject(kobj));
47 }
48 
49 EXPORT_SYMBOL(dm_kobject_release);
50