Re: [PATCH v6] driver core: Fix use-after-free and double free on glue directory

From: Prateek Sood
Date: Thu Jul 25 2019 - 14:03:27 EST

On 7/24/19 9:30 PM, 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_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 also should make sure that kernfs_node
> for glue_dir is released in path2 only when refcount for glue_dir kobj is
> 1 to fix this race.
>
> 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>
> Reviewed-by: Mukesh Ojha <mojha@xxxxxxxxxxxxxx>
> ---
>
> Change in v6:
> 1. Remove hardcoding "1 "
> Change in v5:
> 1. Revert to the v1 fix.
> 2. Add some comment to explain why we need do this in
> cleanup_glue_dir().
> 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 | 57 ++++++++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 56 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/base/core.c b/drivers/base/core.c
> index 4aeaa0c92bda..49bcb01e44cd 100644
> --- a/drivers/base/core.c
> +++ b/drivers/base/core.c
> @@ -1820,12 +1820,67 @@ static inline struct kobject *get_glue_dir(struct device *dev)
> */
> static void cleanup_glue_dir(struct device *dev, struct kobject *glue_dir)
> {
> + unsigned int ref;
> +
> /* see if we live in a "glue" directory */
> if (!live_in_glue_dir(glue_dir, dev))
> return;
>
> mutex_lock(&gdp_mutex);
> - if (!kobject_has_children(glue_dir))
> + /**
> + * There is a race condition between removing glue directory
> + * and adding a new device under the glue directory.
> + *
> + * path 1: Add the child device under glue dir
> + * device_add()
> + * get_device_parent()
> + * mutex_lock(&gdp_mutex);
> + * ....
> + * 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()
> + * ....
> + * if (kobj->parent)
> + * parent = kobj->parent->sd;
> + * ....
> + * kernfs_create_dir_ns(parent)
> + * ....
> + *
> + * path2: Remove last child device under glue dir
> + * device_del()
> + * cleanup_glue_dir()
> + * ....
> + * mutex_lock(&gdp_mutex);
> + * if (!kobject_has_children(glue_dir))
> + * kobject_del(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_create_dir_ns(). Meanwhile,
> + * path2 call kobject_del(glue_dir) beacause 0 is returned by
> + * kobject_has_children(). This result in glue_dir->sd is freed.
> + * Then the path1 will see a stale "empty" but still potentially used
> + * glue dir around.
> + *
> + * In order to avoid this happening, we also should make sure that
> + * kernfs_node for glue_dir is released in path2 only when refcount
> + * for glue_dir kobj is 1.
> + */
> + ref = kref_read(&glue_dir->kref);
> + if (!kobject_has_children(glue_dir) && !--ref)
> kobject_del(glue_dir);
> kobject_put(glue_dir);
> mutex_unlock(&gdp_mutex);
>

I would recommend to describe the issue as done in patch
https://lkml.org/lkml/2019/5/1/3

It helps in visualizing the race condition.


Thanks
Prateek


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