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Subject[PATCH v4] driver core: Fix use-after-free and double free on glue directory
Date
There is a race condition between removing glue directory and adding a new
device under the glue directory. It can be reproduced in following test:

path 1: Add the child device under glue dir
device_add()
get_device_parent()
mutex_lock(&gdp_mutex);
....
/*find parent from glue_dirs.list*/
list_for_each_entry(k, &dev->class->p->glue_dirs.list, entry)
if (k->parent == parent_kobj) {
kobj = kobject_get(k);
break;
}
....
mutex_unlock(&gdp_mutex);
....
....
kobject_add()
kobject_add_internal()
create_dir()
sysfs_create_dir_ns()
if (kobj->parent)
parent = kobj->parent->sd;
....
kernfs_create_dir_ns(parent)
kernfs_new_node()
kernfs_get(parent)
....
/* link in */
rc = kernfs_add_one(kn);
if (!rc)
return kn;

kernfs_put(kn)
....
repeat:
kmem_cache_free(kn)
kn = parent;

if (kn) {
if (atomic_dec_and_test(&kn->count))
goto repeat;
}
....

path2: Remove last child device under glue dir
device_del()
cleanup_device_parent()
cleanup_glue_dir()
mutex_lock(&gdp_mutex);
if (!kobject_has_children(glue_dir))
kobject_del(glue_dir);
kobject_put(glue_dir);
mutex_unlock(&gdp_mutex);

Before path2 remove last child device under glue dir, If path1 add a new
device under glue dir, the glue_dir kobject reference count will be
increase to 2 via kobject_get(k) in get_device_parent(). And path1 has
been called kernfs_new_node(), but not call kernfs_get(parent).
Meanwhile, path2 call kobject_del(glue_dir) beacause 0 is returned by
kobject_has_children(). This result in glue_dir->sd is freed and it's
reference count will be 0. Then path1 call kernfs_get(parent) will trigger
a warning in kernfs_get()(WARN_ON(!atomic_read(&kn->count))) and increase
it's reference count to 1. Because glue_dir->sd is freed by path2, the next
call kernfs_add_one() by path1 will fail(This is also use-after-free)
and call atomic_dec_and_test() to decrease reference count. Because the
reference count is decremented to 0, it will also call kmem_cache_free()
to free glue_dir->sd again. This will result in double free.

In order to avoid this happening, we can ensure the lookup of the glue
dir and creation of the child object(s) are done under a single instance
of gdp_mutex so we never see a stale "empty" but still poentially used
glue dir around.

The following calltrace is captured in kernel 4.14 with the following patch
applied:

commit 726e41097920 ("drivers: core: Remove glue dirs from sysfs earlier")

--------------------------------------------------------------------------
[ 3.633703] WARNING: CPU: 4 PID: 513 at .../fs/kernfs/dir.c:494
Here is WARN_ON(!atomic_read(&kn->count) in kernfs_get().
....
[ 3.633986] Call trace:
[ 3.633991] kernfs_create_dir_ns+0xa8/0xb0
[ 3.633994] sysfs_create_dir_ns+0x54/0xe8
[ 3.634001] kobject_add_internal+0x22c/0x3f0
[ 3.634005] kobject_add+0xe4/0x118
[ 3.634011] device_add+0x200/0x870
[ 3.634017] _request_firmware+0x958/0xc38
[ 3.634020] request_firmware_into_buf+0x4c/0x70
....
[ 3.634064] kernel BUG at .../mm/slub.c:294!
Here is BUG_ON(object == fp) in set_freepointer().
....
[ 3.634346] Call trace:
[ 3.634351] kmem_cache_free+0x504/0x6b8
[ 3.634355] kernfs_put+0x14c/0x1d8
[ 3.634359] kernfs_create_dir_ns+0x88/0xb0
[ 3.634362] sysfs_create_dir_ns+0x54/0xe8
[ 3.634366] kobject_add_internal+0x22c/0x3f0
[ 3.634370] kobject_add+0xe4/0x118
[ 3.634374] device_add+0x200/0x870
[ 3.634378] _request_firmware+0x958/0xc38
[ 3.634381] request_firmware_into_buf+0x4c/0x70
--------------------------------------------------------------------------

Fixes: 726e41097920 ("drivers: core: Remove glue dirs from sysfs earlier")

Signed-off-by: Muchun Song <smuchun@gmail.com>
---

Change in v4:
1. Add some kerneldoc comment.
2. Remove unlock_if_glue_dir().
3. Rename get_device_parent_locked_if_glue_dir() to
get_device_parent_locked.
4. Update commit message.
Change in v3:
Add change log.
Change in v2:
Fix device_move() also.

drivers/base/core.c | 108 +++++++++++++++++++++++++++++++++++++-------
1 file changed, 92 insertions(+), 16 deletions(-)

diff --git a/drivers/base/core.c b/drivers/base/core.c
index 4aeaa0c92bda..2251e391a352 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -1739,8 +1739,23 @@ class_dir_create_and_add(struct class *class, struct kobject *parent_kobj)

static DEFINE_MUTEX(gdp_mutex);

-static struct kobject *get_device_parent(struct device *dev,
- struct device *parent)
+/**
+ * __get_device_parent() - Get the parent device kobject.
+ * @dev: Pointer to the device structure.
+ * @parent: Pointer to the parent device structure.
+ * @lock: When we live in a glue directory, should we hold the
+ * gdp_mutex lock when this function returns? If @lock
+ * is true, this function returns with the gdp_mutex
+ * holed. Otherwise it will not.
+ *
+ * Note: Only when we live in a glue directory and @lock is
+ * true, the function will return with the gdp_mutex holed.
+ * In this case, The caller is responsible for releasing the
+ * gdp_mutex lock.
+ */
+static struct kobject *__get_device_parent(struct device *dev,
+ struct device *parent,
+ bool lock)
{
if (dev->class) {
struct kobject *kobj = NULL;
@@ -1778,16 +1793,32 @@ static struct kobject *get_device_parent(struct device *dev,
break;
}
spin_unlock(&dev->class->p->glue_dirs.list_lock);
- if (kobj) {
- mutex_unlock(&gdp_mutex);
- return kobj;
- }

- /* or create a new class-directory at the parent device */
- k = class_dir_create_and_add(dev->class, parent_kobj);
- /* do not emit an uevent for this simple "glue" directory */
- mutex_unlock(&gdp_mutex);
- return k;
+ /**
+ * If not found, create a new class-directory at the
+ * parent device and do not emit an uevent for this
+ * simple "glue" directory.
+ */
+ if (!kobj)
+ kobj = class_dir_create_and_add(dev->class,
+ parent_kobj);
+
+ /**
+ * If the caller want to add/move a new directory
+ * under the glue directory next. We should leave
+ * the function with the gdp_mutex holed. And then
+ * release the gdp_mutex lock after adding/moving
+ * the new directory.
+ *
+ * Because we should ensure the lookup of the glue
+ * dir and creation of the child object(s) are done
+ * under a single instance of gdp_mutex so we never
+ * see a stale "empty" but still poentially used
+ * glue dir around.
+ */
+ if (!lock)
+ mutex_unlock(&gdp_mutex);
+ return kobj;
}

/* subsystems can specify a default root directory for their devices */
@@ -1799,6 +1830,36 @@ static struct kobject *get_device_parent(struct device *dev,
return NULL;
}

+static inline struct kobject *get_device_parent(struct device *dev,
+ struct device *parent)
+{
+ return __get_device_parent(dev, parent, false);
+}
+
+/**
+ * Note: When this function returns successfully, the gdp_mutex
+ * lock may be held (when we live in a glue directory). The caller
+ * can determine wheather we hold the lock by live_in_glue_dir().
+ *
+ * If true is returned by live_in_glue_dir(), the caller should
+ * drop the gdp_mutex lock.
+ */
+static inline struct kobject *get_device_parent_locked(struct device *dev,
+ struct device *parent)
+{
+ struct kobject *kobj = __get_device_parent(dev, parent, true);
+
+ /**
+ * When we create a new glue directory fail, there
+ * is no need for us to leave the function with the
+ * the gdp_mutex holed.
+ */
+ if (IS_ERR(kobj))
+ mutex_unlock(&gdp_mutex);
+
+ return kobj;
+}
+
static inline bool live_in_glue_dir(struct kobject *kobj,
struct device *dev)
{
@@ -2040,7 +2101,7 @@ int device_add(struct device *dev)
pr_debug("device: '%s': %s\n", dev_name(dev), __func__);

parent = get_device(dev->parent);
- kobj = get_device_parent(dev, parent);
+ kobj = get_device_parent_locked(dev, parent);
if (IS_ERR(kobj)) {
error = PTR_ERR(kobj);
goto parent_error;
@@ -2055,10 +2116,17 @@ int device_add(struct device *dev)
/* first, register with generic layer. */
/* we require the name to be set before, and pass NULL */
error = kobject_add(&dev->kobj, dev->kobj.parent, NULL);
- if (error) {
- glue_dir = get_glue_dir(dev);
+
+ glue_dir = get_glue_dir(dev);
+ /**
+ * Drops the mutex possibly acquired by get_device_parent_locked().
+ * See the comment in __get_device_parent().
+ */
+ if (live_in_glue_dir(glue_dir, dev))
+ mutex_unlock(&gdp_mutex);
+
+ if (error)
goto Error;
- }

/* notify platform of device entry */
error = device_platform_notify(dev, KOBJ_ADD);
@@ -2972,7 +3040,7 @@ int device_move(struct device *dev, struct device *new_parent,

device_pm_lock();
new_parent = get_device(new_parent);
- new_parent_kobj = get_device_parent(dev, new_parent);
+ new_parent_kobj = get_device_parent_locked(dev, new_parent);
if (IS_ERR(new_parent_kobj)) {
error = PTR_ERR(new_parent_kobj);
put_device(new_parent);
@@ -2982,6 +3050,14 @@ int device_move(struct device *dev, struct device *new_parent,
pr_debug("device: '%s': %s: moving to '%s'\n", dev_name(dev),
__func__, new_parent ? dev_name(new_parent) : "<NULL>");
error = kobject_move(&dev->kobj, new_parent_kobj);
+
+ /**
+ * Drops the mutex possibly acquired by get_device_parent_locked().
+ * See the comment in __get_device_parent().
+ */
+ if (live_in_glue_dir(new_parent_kobj, dev))
+ mutex_unlock(&gdp_mutex);
+
if (error) {
cleanup_glue_dir(dev, new_parent_kobj);
put_device(new_parent);
--
2.17.1
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