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SubjectRe: [PATCH v3] driver core: Fix use-after-free and double free on glue directory
On Wed, May 15, 2019 at 2:57 AM Muchun Song <smuchun@gmail.com> wrote:
>
> 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 we should not call kobject_del() on
> path2 when the reference count of glue_dir is greater than 1. So we add a
> conditional statement to fix it.
>
> 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 v3:
> add change log.
> Change in v2:
> Fix device_move() also.
>
> drivers/base/core.c | 47 ++++++++++++++++++++++++++++++++++++---------
> 1 file changed, 38 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/base/core.c b/drivers/base/core.c
> index 4aeaa0c92bda..e7810329223a 100644
> --- a/drivers/base/core.c
> +++ b/drivers/base/core.c
> @@ -1739,8 +1739,9 @@ 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)
> +static struct kobject *__get_device_parent(struct device *dev,
> + struct device *parent,
> + bool lock)
> {
> if (dev->class) {
> struct kobject *kobj = NULL;
> @@ -1779,14 +1780,16 @@ static struct kobject *get_device_parent(struct device *dev,
> }
> spin_unlock(&dev->class->p->glue_dirs.list_lock);
> if (kobj) {
> - mutex_unlock(&gdp_mutex);
> + if (!lock)
> + mutex_unlock(&gdp_mutex);
> return kobj;
> }

There is an unconditional mutex_lock(&gdp_mutex) above this, so the
idea appears to be to leave the function with the mutex locked in some
cases.

That is far away from straightforward, so there needs to be a
kerneldoc comment explaining that behavior.

It also may be necessary to annotate the function with __acquires().

>
> /* 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);
> + if (!lock || IS_ERR(k))
> + mutex_unlock(&gdp_mutex);

So it needs to be unlocked in the error case too, because the
conditional unlock down the road doesn't work then, right?

There needs to be a comment to that effect here, please.

> return k;
> }
>
> @@ -1799,6 +1802,19 @@ 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);
> +}
> +
> +static inline struct kobject *
> +get_device_parent_locked_if_glue_dir(struct device *dev,
> + struct device *parent)

The name is long and it doesn't explain much IMO.

I guess the idea is to pair this wrapper with unlock_if_glue_dir() to
produce a kind of lock/unlock pattern, but this doesn't work anyway
IMO because of the extra glue dir arg that needs to be passed to
unlock_if_glue_dir().

I would just use the raw __get_device_parent(dev, parent, true)
instead which should be clear enough as long as the (missing now)
kerneldoc comment (and possibly annotation) is added to it.

> +{
> + return __get_device_parent(dev, parent, true);
> +}
> +
> static inline bool live_in_glue_dir(struct kobject *kobj,
> struct device *dev)
> {
> @@ -1831,6 +1847,16 @@ static void cleanup_glue_dir(struct device *dev, struct kobject *glue_dir)
> mutex_unlock(&gdp_mutex);
> }
>
> +static inline void unlock_if_glue_dir(struct device *dev,
> + struct kobject *glue_dir)
> +{
> + /* see if we live in a "glue" directory */
> + if (!live_in_glue_dir(glue_dir, dev))
> + return;
> +
> + mutex_unlock(&gdp_mutex);

This can be written in fewer lines of code and one negation less as follows:

if (live_in_glue_dir(glue_dir, dev))
mutex_unlock(&gdp_mutex);

which IMO is self-explanatory, so the comment is redundant.

And I would use it directly, without the static wrapper around it,
possibly with a comment to say that it drops the mutex possibly
acquired by __get_device_parent().

> +}
> +
> static int device_add_class_symlinks(struct device *dev)
> {
> struct device_node *of_node = dev_of_node(dev);
> @@ -2040,7 +2066,7 @@ 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_if_glue_dir(dev, parent);
> if (IS_ERR(kobj)) {
> error = PTR_ERR(kobj);
> goto parent_error;
> @@ -2055,10 +2081,12 @@ 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);
> + unlock_if_glue_dir(dev, glue_dir);
> +
> + if (error)
> goto Error;
> - }
>
> /* notify platform of device entry */
> error = device_platform_notify(dev, KOBJ_ADD);
> @@ -2972,7 +3000,7 @@ 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_if_glue_dir(dev, new_parent);
> if (IS_ERR(new_parent_kobj)) {
> error = PTR_ERR(new_parent_kobj);
> put_device(new_parent);
> @@ -2982,6 +3010,7 @@ 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);
> + unlock_if_glue_dir(dev, new_parent_kobj);
> if (error) {
> cleanup_glue_dir(dev, new_parent_kobj);
> put_device(new_parent);
> --

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