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 <smuc...@gmail.com> --- 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