dm sysfs: fix a module unload race

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

View File

@ -176,8 +176,12 @@ config MD_FAULTY
source "drivers/md/bcache/Kconfig"
config BLK_DEV_DM_BUILTIN
boolean
config BLK_DEV_DM
tristate "Device mapper support"
select BLK_DEV_DM_BUILTIN
---help---
Device-mapper is a low level volume manager. It works by allowing
people to specify mappings for ranges of logical sectors. Various

View File

@ -32,6 +32,7 @@ obj-$(CONFIG_MD_FAULTY) += faulty.o
obj-$(CONFIG_BCACHE) += bcache/
obj-$(CONFIG_BLK_DEV_MD) += md-mod.o
obj-$(CONFIG_BLK_DEV_DM) += dm-mod.o
obj-$(CONFIG_BLK_DEV_DM_BUILTIN) += dm-builtin.o
obj-$(CONFIG_DM_BUFIO) += dm-bufio.o
obj-$(CONFIG_DM_BIO_PRISON) += dm-bio-prison.o
obj-$(CONFIG_DM_CRYPT) += dm-crypt.o

48
drivers/md/dm-builtin.c Normal file
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@ -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);

View File

@ -79,11 +79,6 @@ static const struct sysfs_ops dm_sysfs_ops = {
.show = dm_attr_show,
};
static void dm_kobject_release(struct kobject *kobj)
{
complete(dm_get_completion_from_kobject(kobj));
}
/*
* dm kobject is embedded in mapped_device structure
* no need to define release function here

View File

@ -200,11 +200,8 @@ struct mapped_device {
/* forced geometry settings */
struct hd_geometry geometry;
/* sysfs handle */
struct kobject kobj;
/* wait until the kobject is released */
struct completion kobj_completion;
/* kobject and completion */
struct dm_kobject_holder kobj_holder;
/* zero-length flush that will be cloned and submitted to targets */
struct bio flush_bio;
@ -2044,7 +2041,7 @@ static struct mapped_device *alloc_dev(int minor)
init_waitqueue_head(&md->wait);
INIT_WORK(&md->work, dm_wq_work);
init_waitqueue_head(&md->eventq);
init_completion(&md->kobj_completion);
init_completion(&md->kobj_holder.completion);
md->disk->major = _major;
md->disk->first_minor = minor;
@ -2906,14 +2903,14 @@ struct gendisk *dm_disk(struct mapped_device *md)
struct kobject *dm_kobject(struct mapped_device *md)
{
return &md->kobj;
return &md->kobj_holder.kobj;
}
struct mapped_device *dm_get_from_kobject(struct kobject *kobj)
{
struct mapped_device *md;
md = container_of(kobj, struct mapped_device, kobj);
md = container_of(kobj, struct mapped_device, kobj_holder.kobj);
if (test_bit(DMF_FREEING, &md->flags) ||
dm_deleting_md(md))
@ -2923,13 +2920,6 @@ struct mapped_device *dm_get_from_kobject(struct kobject *kobj)
return md;
}
struct completion *dm_get_completion_from_kobject(struct kobject *kobj)
{
struct mapped_device *md = container_of(kobj, struct mapped_device, kobj);
return &md->kobj_completion;
}
int dm_suspended_md(struct mapped_device *md)
{
return test_bit(DMF_SUSPENDED, &md->flags);

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@ -16,6 +16,7 @@
#include <linux/blkdev.h>
#include <linux/hdreg.h>
#include <linux/completion.h>
#include <linux/kobject.h>
#include "dm-stats.h"
@ -149,11 +150,25 @@ void dm_interface_exit(void);
/*
* sysfs interface
*/
struct dm_kobject_holder {
struct kobject kobj;
struct completion completion;
};
static inline struct completion *dm_get_completion_from_kobject(struct kobject *kobj)
{
return &container_of(kobj, struct dm_kobject_holder, kobj)->completion;
}
int dm_sysfs_init(struct mapped_device *md);
void dm_sysfs_exit(struct mapped_device *md);
struct kobject *dm_kobject(struct mapped_device *md);
struct mapped_device *dm_get_from_kobject(struct kobject *kobj);
struct completion *dm_get_completion_from_kobject(struct kobject *kobj);
/*
* The kobject helper
*/
void dm_kobject_release(struct kobject *kobj);
/*
* Targets for linear and striped mappings