Re: [PATCH v1 OPT1] driver core: Fix use-after-free and double free on glue directory

[Greg KH]

On Wed, Jun 26, 2019 at 10:38:23PM +0800, 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_device_parent()
>         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 should not call kobject_del() on
> path2 when the reference count of glue_dir is greater than 1. So we add a
> conditional statement to fix it.
> 
> 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>
> ---
> 
> Hi Greg,
> 
> This fix patch is based on the refcount. It is also the original fix method.
> Another fix patch is based on the mutex lock.
> 
> It can reference to:
>         Subject: [PATCH v4 OPT2] driver core: Fix use-after-free and double free on glue directory
> 
> I am hoping you would chose which solution you prefered here. Thanks.
> 
>  drivers/base/core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/base/core.c b/drivers/base/core.c
> index 4aeaa0c92bda..5ac5376ae9af 100644
> --- a/drivers/base/core.c
> +++ b/drivers/base/core.c
> @@ -1825,7 +1825,7 @@ static void cleanup_glue_dir(struct device *dev, struct kobject *glue_dir)
>  		return;
>  
>  	mutex_lock(&gdp_mutex);
> -	if (!kobject_has_children(glue_dir))
> +	if (!kobject_has_children(glue_dir) && kref_read(&glue_dir->kref) == 1)
>  		kobject_del(glue_dir);

Ok, I guess I have to take this patch, as the other one is so bad :)

But, I need a very large comment here saying why we are poking around in
a kref and why we need to do this, at the expense of anything else.

So can you respin this patch with a comment here to help explain it so
we have a chance to understand it when we run across this line in 10
years?

thanks,

greg k-h