[PATCH v5] driver core: Fix use-after-free and double free on glue directory

From: Muchun Song
Date: Thu Jul 18 2019 - 07:20:25 EST

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>
---

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 | 54 ++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 53 insertions(+), 1 deletion(-)

diff --git a/drivers/base/core.c b/drivers/base/core.c
index 4aeaa0c92bda..1a67ee325584 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -1825,7 +1825,59 @@ static void cleanup_glue_dir(struct device *dev, struct kobject *glue_dir)
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.
+ */
+ if (!kobject_has_children(glue_dir) && kref_read(&glue_dir->kref) == 1)
kobject_del(glue_dir);
kobject_put(glue_dir);
mutex_unlock(&gdp_mutex);
--
2.17.1