Re: [PATCH v3] driver core: Fix use-after-free and double free on glue directory

[Muchun Song]

Hi rafael,

Thanks for your review.

Rafael J. Wysocki <rafael@xxxxxxxxxx> ä2019å5æ15æåä äå4:35åéï
>
> 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);
> > --