sysfs: only allow one scheduled removal callback per kobj

The only way for a sysfs attribute to remove itself (without
deadlock) is to use the sysfs_schedule_callback() interface.

Vegard Nossum discovered that a poorly written sysfs ->store
callback can repeatedly schedule remove callbacks on the same
device over and over, e.g.

	$ while true ; do echo 1 > /sys/devices/.../remove ; done

If the 'remove' attribute uses the sysfs_schedule_callback API
and also does not protect itself from concurrent accesses, its
callback handler will be called multiple times, and will
eventually attempt to perform operations on a freed kobject,
leading to many problems.

Instead of requiring all callers of sysfs_schedule_callback to
implement their own synchronization, provide the protection in
the infrastructure.

Now, sysfs_schedule_callback will only allow one scheduled
callback per kobject. On subsequent calls with the same kobject,
return -EAGAIN.

This is a short term fix. The long term fix is to allow sysfs
attributes to remove themselves directly, without any of this
callback hokey pokey.

[cornelia.huck@de.ibm.com: s390 ccwgroup bits]

Reported-by: vegard.nossum@gmail.com
Signed-off-by: Alex Chiang <achiang@hp.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

2 files changed, 26 insertions, 5 deletions

diff --git a/drivers/s390/cio/ccwgroup.c b/drivers/s390/cio/ccwgroup.c
index 918e6fce2573..b91c1719b075 100644
--- a/drivers/s390/cio/ccwgroup.c
+++ b/drivers/s390/cio/ccwgroup.c
@@ -104,8 +104,9 @@ ccwgroup_ungroup_store(struct device *dev, struct device_attribute *attr, const
         rc = device_schedule_callback(dev, ccwgroup_ungroup_callback);
 out:
         if (rc) {
-                /* Release onoff "lock" when ungrouping failed. */
-                atomic_set(&gdev->onoff, 0);
+                if (rc != -EAGAIN)
+                        /* Release onoff "lock" when ungrouping failed. */
+                        atomic_set(&gdev->onoff, 0);
                 return rc;
         }
         return count;
diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
index 1f4a3f877262..289c43a47263 100644
--- a/fs/sysfs/file.c
+++ b/fs/sysfs/file.c
@@ -659,13 +659,16 @@ void sysfs_remove_file_from_group(struct kobject *kobj,
 EXPORT_SYMBOL_GPL(sysfs_remove_file_from_group);
 
 struct sysfs_schedule_callback_struct {
-        struct kobject          *kobj;
+        struct list_head        workq_list;
+        struct kobject          *kobj;
         void                    (*func)(void *);
         void                    *data;
         struct module           *owner;
         struct work_struct      work;
 };
 
+static DEFINE_MUTEX(sysfs_workq_mutex);
+static LIST_HEAD(sysfs_workq);
 static void sysfs_schedule_callback_work(struct work_struct *work)
 {
         struct sysfs_schedule_callback_struct *ss = container_of(work,
@@ -674,6 +677,9 @@ static void sysfs_schedule_callback_work(struct work_struct *work)
         (ss->func)(ss->data);
         kobject_put(ss->kobj);
         module_put(ss->owner);
+        mutex_lock(&sysfs_workq_mutex);
+        list_del(&ss->workq_list);
+        mutex_unlock(&sysfs_workq_mutex);
         kfree(ss);
 }
 
@@ -695,15 +701,25 @@ static void sysfs_schedule_callback_work(struct work_struct *work)
  * until @func returns.
  *
  * Returns 0 if the request was submitted, -ENOMEM if storage could not
- * be allocated, -ENODEV if a reference to @owner isn't available.
+ * be allocated, -ENODEV if a reference to @owner isn't available,
+ * -EAGAIN if a callback has already been scheduled for @kobj.
  */
 int sysfs_schedule_callback(struct kobject *kobj, void (*func)(void *),
                 void *data, struct module *owner)
 {
-        struct sysfs_schedule_callback_struct *ss;
+        struct sysfs_schedule_callback_struct *ss, *tmp;
 
         if (!try_module_get(owner))
                 return -ENODEV;
+
+        mutex_lock(&sysfs_workq_mutex);
+        list_for_each_entry_safe(ss, tmp, &sysfs_workq, workq_list)
+                if (ss->kobj == kobj) {
+                        mutex_unlock(&sysfs_workq_mutex);
+                        return -EAGAIN;
+                }
+        mutex_unlock(&sysfs_workq_mutex);
+
         ss = kmalloc(sizeof(*ss), GFP_KERNEL);
         if (!ss) {
                 module_put(owner);
@@ -715,6 +731,10 @@ int sysfs_schedule_callback(struct kobject *kobj, void (*func)(void *),
         ss->data = data;
         ss->owner = owner;
         INIT_WORK(&ss->work, sysfs_schedule_callback_work);
+        INIT_LIST_HEAD(&ss->workq_list);
+        mutex_lock(&sysfs_workq_mutex);
+        list_add_tail(&ss->workq_list, &sysfs_workq);
+        mutex_unlock(&sysfs_workq_mutex);
         schedule_work(&ss->work);
         return 0;
 }