mirror of
https://github.com/torvalds/linux.git
synced 2024-11-18 10:01:43 +00:00
2995fa78e4
This reverts commit be35f48610 ("dm: wait until embedded kobject is
released before destroying a device") and provides an improved fix.
The kobject release code that calls the completion must be placed in a
non-module file, otherwise there is a module unload race (if the process
calling dm_kobject_release is preempted and the DM module unloaded after
the completion is triggered, but before dm_kobject_release returns).
To fix this race, this patch moves the completion code to dm-builtin.c
which is always compiled directly into the kernel if BLK_DEV_DM is
selected.
The patch introduces a new dm_kobject_holder structure, its purpose is
to keep the completion and kobject in one place, so that it can be
accessed from non-module code without the need to export the layout of
struct mapped_device to that code.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org
49 lines
2.0 KiB
C
49 lines
2.0 KiB
C
#include "dm.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);
|