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

[Mukesh Ojha <mojha@codeaurora.org>]

On 7/24/2019 7:11 PM, Mukesh Ojha wrote:
>
> On 7/18/2019 4:49 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@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)
> Instead of hardcoding "1 " , can't we check against zero Like Prateek 
> sood did in his patch.https://lkml.org/lkml/2019/5/1/3
>
> +ref = kref_read(&glue_dir->kref);
> +if  (!kobject_has_children(glue_dir) && --ref)
>
I meant if  (!kobject_has_children(glue_dir) && ! --ref)
>
>
> We have seen many pattern of this issue which is coming from different 
> driver e.g uinput as well in 4.14 
> kernel.(https://lkml.org/lkml/2019/4/10/146)
> The reason why it  started coming after
>
> 726e41097920 ("drivers: core: Remove glue dirs from sysfs earlier")
>
> Looks good to me
>
> Reviewed-by: Mukesh Ojha  <mojha@codeaurora.org>
>
>
> -Mukesh
>
>
>>           kobject_del(glue_dir);
>>       kobject_put(glue_dir);
>>       mutex_unlock(&gdp_mutex);