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 <[email protected]>
---
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
On Wed, Jun 26, 2019 at 10:40:21PM +0800, Muchun Song 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 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 <[email protected]>
> ---
>
> 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.
Ugh, if you are trying to get me to hate one version of these patches,
this is how you do it :)
A function should not "sometimes takes a lock, sometimes does not,
depending on a parameter passed into it" That way lies madness...
> + *
> + * 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;
> + /**
Note, in the future, you don't need to use /** for comments in a
function, those are only for kernel doc markers for a _global_ function
that gets pulled into the larger documentation documents. So even
__get_device_parent() here does not need this.
Anyway, this is a mess.
Ugh I hate glue dirs...
greg k-h
On Wed, 2019-07-03 at 21:36 +0200, Greg KH wrote:
>
> > -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.
>
> Ugh, if you are trying to get me to hate one version of these patches,
> this is how you do it :)
>
> A function should not "sometimes takes a lock, sometimes does not,
> depending on a parameter passed into it" That way lies madness...
Yes, I prefer this approach to the fix but I dont like the patch either
for the same reason...
...
> Anyway, this is a mess.
>
> Ugh I hate glue dirs...
Amen...
Ben.
On Thu, Jul 04, 2019 at 08:57:13AM +1000, Benjamin Herrenschmidt wrote:
> On Wed, 2019-07-03 at 21:36 +0200, Greg KH wrote:
> >
> > > -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.
> >
> > Ugh, if you are trying to get me to hate one version of these patches,
> > this is how you do it :)
> >
> > A function should not "sometimes takes a lock, sometimes does not,
> > depending on a parameter passed into it" That way lies madness...
>
> Yes, I prefer this approach to the fix but I dont like the patch either
> for the same reason...
>
> ...
>
> > Anyway, this is a mess.
> >
> > Ugh I hate glue dirs...
>
> Amen...
Well, can we just remove them? Who relies on them anymore?
thanks,
greg k-h
On Thu, 2019-07-04 at 07:40 +0200, Greg KH wrote:
> On Thu, Jul 04, 2019 at 08:57:13AM +1000, Benjamin Herrenschmidt
> wrote:
> > On Wed, 2019-07-03 at 21:36 +0200, Greg KH wrote:
> > >
> > > > -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.
> > >
> > > Ugh, if you are trying to get me to hate one version of these
> > > patches,
> > > this is how you do it :)
> > >
> > > A function should not "sometimes takes a lock, sometimes does
> > > not,
> > > depending on a parameter passed into it" That way lies
> > > madness...
> >
> > Yes, I prefer this approach to the fix but I dont like the patch
> > either
> > for the same reason...
> >
> > ...
> >
> > > Anyway, this is a mess.
> > >
> > > Ugh I hate glue dirs...
> >
> > Amen...
>
> Well, can we just remove them? Who relies on them anymore?
Isn't it an ABI ? I'm sure there are going to be userspace things that
break if we do...
Cheers,
Ben.