[3.8.y.z extended stable] Patch "dm sysfs: fix a module unload race" has been added to staging queue
Kamal Mostafa
kamal at canonical.com
Fri Feb 7 21:37:07 UTC 2014
This is a note to let you know that I have just added a patch titled
dm sysfs: fix a module unload race
to the linux-3.8.y-queue branch of the 3.8.y.z extended stable tree
which can be found at:
http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.8.y-queue
This patch is scheduled to be released in version 3.8.13.18.
If you, or anyone else, feels it should not be added to this tree, please
reply to this email.
For more information about the 3.8.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable
Thanks.
-Kamal
------
>From a377ce349eed322ebcd37a54991194cb92e18f8b Mon Sep 17 00:00:00 2001
From: Mikulas Patocka <mpatocka at redhat.com>
Date: Mon, 13 Jan 2014 19:37:54 -0500
Subject: dm sysfs: fix a module unload race
commit 2995fa78e423d7193f3b57835f6c1c75006a0315 upstream.
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 at redhat.com>
Signed-off-by: Mike Snitzer <snitzer at redhat.com>
[ kamal: backport to 3.8 (dm-builtin.c needs "#include <linux/export.h>";
context changes) ]
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
drivers/md/Kconfig | 4 ++++
drivers/md/Makefile | 1 +
drivers/md/dm-builtin.c | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
drivers/md/dm-sysfs.c | 5 -----
drivers/md/dm.c | 22 +++++-----------------
drivers/md/dm.h | 17 ++++++++++++++++-
6 files changed, 75 insertions(+), 23 deletions(-)
create mode 100644 drivers/md/dm-builtin.c
diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
index 91a02ee..defbe5e1 100644
--- a/drivers/md/Kconfig
+++ b/drivers/md/Kconfig
@@ -185,8 +185,12 @@ config MD_FAULTY
In unsure, say N.
+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
diff --git a/drivers/md/Makefile b/drivers/md/Makefile
index 94dce8b..50ead1d 100644
--- a/drivers/md/Makefile
+++ b/drivers/md/Makefile
@@ -28,6 +28,7 @@ obj-$(CONFIG_MD_MULTIPATH) += multipath.o
obj-$(CONFIG_MD_FAULTY) += faulty.o
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
diff --git a/drivers/md/dm-builtin.c b/drivers/md/dm-builtin.c
new file mode 100644
index 0000000..8b82788
--- /dev/null
+++ b/drivers/md/dm-builtin.c
@@ -0,0 +1,49 @@
+#include <linux/export.h>
+#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);
diff --git a/drivers/md/dm-sysfs.c b/drivers/md/dm-sysfs.c
index e0cc5d6..c62c5ab 100644
--- a/drivers/md/dm-sysfs.c
+++ b/drivers/md/dm-sysfs.c
@@ -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
diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index a027513..9a7a409 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -185,11 +185,8 @@ 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;
@@ -1902,7 +1899,7 @@ 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;
@@ -2728,7 +2725,7 @@ struct gendisk *dm_disk(struct mapped_device *md)
struct kobject *dm_kobject(struct mapped_device *md)
{
- return &md->kobj;
+ return &md->kobj_holder.kobj;
}
/*
@@ -2739,9 +2736,7 @@ struct mapped_device *dm_get_from_kobject(struct kobject *kobj)
{
struct mapped_device *md;
- md = container_of(kobj, struct mapped_device, kobj);
- if (&md->kobj != kobj)
- return NULL;
+ md = container_of(kobj, struct mapped_device, kobj_holder.kobj);
if (test_bit(DMF_FREEING, &md->flags) ||
dm_deleting_md(md))
@@ -2751,13 +2746,6 @@ 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);
diff --git a/drivers/md/dm.h b/drivers/md/dm.h
index 66cfb7d..9b3222f 100644
--- a/drivers/md/dm.h
+++ b/drivers/md/dm.h
@@ -16,6 +16,7 @@
#include <linux/blkdev.h>
#include <linux/hdreg.h>
#include <linux/completion.h>
+#include <linux/kobject.h>
/*
* Suspend feature flags
@@ -126,11 +127,25 @@ 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
--
1.8.3.2
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