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dm sysfs: fix a module unload race

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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
This commit is contained in:
Mikulas Patocka
2014-01-13 19:37:54 -05:00
committed by Mike Snitzer
parent 55b082e614
commit 2995fa78e4
6 changed files with 74 additions and 21 deletions
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48
drivers/md/dm-builtin.c Normal file
View File

@@ -0,0 +1,48 @@
#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);