Hi rafael,
Thanks for your review.
Rafael J. Wysocki <rafael@xxxxxxxxxx> 于2019年5月15日周三 下午4:35写道：
>
> On Wed, May 15, 2019 at 2:57 AM Muchun Song <smuchun@xxxxxxxxx> 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@xxxxxxxxx>
> > ---
> >
> > 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().
AFAICT the __acquires() means that the specified lock is held on function
exit, but not entry. But the gdp_mutex lock is held in some cases on
__get_device_parent() exit, not all situations. So is it appropriate to annotate
the function with __acquires()?
According to your suggestions, I send the v4 patch. Please help me check thanks.
>
> >
> > /* 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);
> > --
