drivers/md/dm-builtin.c

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