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 726e410979 ("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: 726e410979 ("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>
ac43432cb1