Line data    Source code

       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             : 
      44           0 : void dm_kobject_release(struct kobject *kobj)
      45             : {
      46           0 :         complete(dm_get_completion_from_kobject(kobj));
      47           0 : }
      48             : 
      49             : EXPORT_SYMBOL(dm_kobject_release);