| // SPDX-License-Identifier: GPL-2.0 |
| #include "dm-core.h" |
| |
| /* |
| * The kobject release method must not be placed in the module itself, |
| * otherwise we are subject to module unload races. |
| * |
| * The release method is called when the last reference to the kobject is |
| * dropped. It may be called by any other kernel code that drops the last |
| * reference. |
| * |
| * The release method suffers from module unload race. We may prevent the |
| * module from being unloaded at the start of the release method (using |
| * increased module reference count or synchronizing against the release |
| * method), however there is no way to prevent the module from being |
| * unloaded at the end of the release method. |
| * |
| * If this code were placed in the dm module, the following race may |
| * happen: |
| * 1. Some other process takes a reference to dm kobject |
| * 2. The user issues ioctl function to unload the dm device |
| * 3. dm_sysfs_exit calls kobject_put, however the object is not released |
| * because of the other reference taken at step 1 |
| * 4. dm_sysfs_exit waits on the completion |
| * 5. The other process that took the reference in step 1 drops it, |
| * dm_kobject_release is called from this process |
| * 6. dm_kobject_release calls complete() |
| * 7. a reschedule happens before dm_kobject_release returns |
| * 8. dm_sysfs_exit continues, the dm device is unloaded, module reference |
| * count is decremented |
| * 9. The user unloads the dm module |
| * 10. The other process that was rescheduled in step 7 continues to run, |
| * it is now executing code in unloaded module, so it crashes |
| * |
| * Note that if the process that takes the foreign reference to dm kobject |
| * has a low priority and the system is sufficiently loaded with |
| * higher-priority processes that prevent the low-priority process from |
| * being scheduled long enough, this bug may really happen. |
| * |
| * In order to fix this module unload race, we place the release method |
| * into a helper code that is compiled directly into the kernel. |
| */ |
| |
| void dm_kobject_release(struct kobject *kobj) |
| { |
| complete(dm_get_completion_from_kobject(kobj)); |
| } |
| |
| EXPORT_SYMBOL(dm_kobject_release); |