From: Muchun Song smuchun@gmail.com
commit ac43432cb1f5c2950408534987e57c2071e24d8f upstream.
There is a race condition between removing glue directory and adding a new device under the glue dir. It can be reproduced in following test:
CPU1: CPU2:
device_add() get_device_parent() class_dir_create_and_add() kobject_add_internal() create_dir() // create glue_dir
device_add() get_device_parent() kobject_get() // get glue_dir
device_del() cleanup_glue_dir() kobject_del(glue_dir)
kobject_add() kobject_add_internal() create_dir() // in glue_dir sysfs_create_dir_ns() kernfs_create_dir_ns(sd)
sysfs_remove_dir() // glue_dir->sd=NULL sysfs_put() // free glue_dir->sd
// sd is freed kernfs_new_node(sd) kernfs_get(glue_dir) kernfs_add_one() kernfs_put()
Before CPU1 remove last child device under glue dir, if CPU2 add a new device under glue dir, the glue_dir kobject reference count will be increase to 2 via kobject_get() in get_device_parent(). And CPU2 has been called kernfs_create_dir_ns(), but not call kernfs_new_node(). Meanwhile, CPU1 call sysfs_remove_dir() and sysfs_put(). This result in glue_dir->sd is freed and it's reference count will be 0. Then CPU2 call kernfs_get(glue_dir) will trigger a warning in kernfs_get() and increase it's reference count to 1. Because glue_dir->sd is freed by CPU1, the next call kernfs_add_one() by CPU2 will fail(This is also use-after-free) and call kernfs_put() to decrease reference count. Because the reference count is decremented to 0, it will also call kmem_cache_free() to free the 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 CPU1 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@gmail.com Reviewed-by: Mukesh Ojha mojha@codeaurora.org Signed-off-by: Prateek Sood prsood@codeaurora.org Link: https://lore.kernel.org/r/20190727032122.24639-1-smuchun@gmail.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- drivers/base/core.c | 53 +++++++++++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 52 insertions(+), 1 deletion(-)
--- a/drivers/base/core.c +++ b/drivers/base/core.c @@ -1648,12 +1648,63 @@ static inline struct kobject *get_glue_d */ 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. + * + * CPU1: CPU2: + * + * device_add() + * get_device_parent() + * class_dir_create_and_add() + * kobject_add_internal() + * create_dir() // create glue_dir + * + * device_add() + * get_device_parent() + * kobject_get() // get glue_dir + * + * device_del() + * cleanup_glue_dir() + * kobject_del(glue_dir) + * + * kobject_add() + * kobject_add_internal() + * create_dir() // in glue_dir + * sysfs_create_dir_ns() + * kernfs_create_dir_ns(sd) + * + * sysfs_remove_dir() // glue_dir->sd=NULL + * sysfs_put() // free glue_dir->sd + * + * // sd is freed + * kernfs_new_node(sd) + * kernfs_get(glue_dir) + * kernfs_add_one() + * kernfs_put() + * + * Before CPU1 remove last child device under glue dir, if CPU2 add + * a new device under glue dir, the glue_dir kobject reference count + * will be increase to 2 in kobject_get(k). And CPU2 has been called + * kernfs_create_dir_ns(). Meanwhile, CPU1 call sysfs_remove_dir() + * and sysfs_put(). This result in glue_dir->sd is freed. + * + * Then the CPU2 will see a stale "empty" but still potentially used + * glue dir around in kernfs_new_node(). + * + * In order to avoid this happening, we also should make sure that + * kernfs_node for glue_dir is released in CPU1 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);