1 | // SPDX-License-Identifier: GPL-2.0-only |
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 | |
44 | void dm_kobject_release(struct kobject *kobj) |
45 | { |
46 | complete(dm_get_completion_from_kobject(kobj)); |
47 | } |
48 | EXPORT_SYMBOL(dm_kobject_release); |
49 | |