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Subject: Re: [PATCH v6] driver core: Fix use-after-free and double free on
 glue directory
To:     Muchun Song <smuchun@gmail.com>, gregkh@linuxfoundation.org,
        rafael@kernel.org, mojha@codeaurora.org
Cc:     benh@kernel.crashing.org, gkohli@codeaurora.org,
        linux-kernel@vger.kernel.org
References: <20190724160006.21013-1-smuchun@gmail.com>
From:   Prateek Sood <prsood@codeaurora.org>
Message-ID: <8cf0bd10-5ae4-b1aa-abf3-c5d2a0f3a214@codeaurora.org>
Date:   Thu, 25 Jul 2019 11:52:24 +0530
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On 7/24/19 9:30 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>
> Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
> ---
> 
> Change in v6:
>        1. Remove hardcoding "1 "
> 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 | 57 ++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 56 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/base/core.c b/drivers/base/core.c
> index 4aeaa0c92bda..49bcb01e44cd 100644
> --- a/drivers/base/core.c
> +++ b/drivers/base/core.c
> @@ -1820,12 +1820,67 @@ static inline struct kobject *get_glue_dir(struct device *dev)
>   */
>  static void cleanup_glue_dir(struct device *dev, struct kobject *glue_dir)
>  {
> +	unsigned int ref;
> +
>  	/* see if we live in a "glue" directory */
>  	if (!live_in_glue_dir(glue_dir, dev))
>  		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.
> +	 */
> +	ref = kref_read(&glue_dir->kref);
> +	if (!kobject_has_children(glue_dir) && !--ref)
>  		kobject_del(glue_dir);
>  	kobject_put(glue_dir);
>  	mutex_unlock(&gdp_mutex);
>

I would recommend to describe the issue as done in patch
https://lkml.org/lkml/2019/5/1/3

It helps in visualizing the race condition.


Thanks
Prateek


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