Re: [PATCH] fsnotify: Fix oops in fsnotify_clear_marks_by_group_flags()

[Lino Sanfilippo <LinoSanfilippo@gmx.de>]

Hi,

On 26.06.2015 16:50, Jan Kara wrote:
> From: Jan Kara <jack@suse.cz>
> 
> fsnotify_clear_marks_by_group_flags() can race with
> fsnotify_destroy_marks() so when fsnotify_destroy_mark_locked() drops
> mark_mutex, a mark from the list iterated by
> fsnotify_clear_marks_by_group_flags() can be freed and we dereference
> free memory in the loop there.
> 
> Fix the problem by keeping mark_mutex held in
> fsnotify_destroy_mark_locked(). The reason why we drop that mutex is
> that we need to call a ->freeing_mark() callback which may acquire
> mark_mutex again. To avoid this and similar lock inversion issues, we
> move the call to ->freeing_mark() callback to the kthread destroying the
> mark.
> 
> Reported-by: Ashish Sangwan <a.sangwan@samsung.com>
> Suggested-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  fs/notify/mark.c | 38 ++++++++++++++++----------------------
>  1 file changed, 16 insertions(+), 22 deletions(-)
> 
> Ashish, can you verify whether this patch fixes the problem you have hit?
> Thanks!
> 
> diff --git a/fs/notify/mark.c b/fs/notify/mark.c
> index 92e48c70f0f0..11e433c2bf1c 100644
> --- a/fs/notify/mark.c
> +++ b/fs/notify/mark.c
> @@ -152,31 +152,15 @@ void fsnotify_destroy_mark_locked(struct fsnotify_mark *mark,
>  		BUG();
>  
>  	list_del_init(&mark->g_list);
> -
>  	spin_unlock(&mark->lock);
>  
> -	if (inode && (mark->flags & FSNOTIFY_MARK_FLAG_OBJECT_PINNED))
> -		iput(inode);
> -	/* release lock temporarily */
> -	mutex_unlock(&group->mark_mutex);
> -
>  	spin_lock(&destroy_lock);
>  	list_add(&mark->g_list, &destroy_list);
>  	spin_unlock(&destroy_lock);
>  	wake_up(&destroy_waitq);
> -	/*
> -	 * We don't necessarily have a ref on mark from caller so the above destroy
> -	 * may have actually freed it, unless this group provides a 'freeing_mark'
> -	 * function which must be holding a reference.
> -	 */
>  
> -	/*
> -	 * Some groups like to know that marks are being freed.  This is a
> -	 * callback to the group function to let it know that this mark
> -	 * is being freed.
> -	 */
> -	if (group->ops->freeing_mark)
> -		group->ops->freeing_mark(mark, group);
> +	if (inode && (mark->flags & FSNOTIFY_MARK_FLAG_OBJECT_PINNED))
> +		iput(inode);
>  
>  	/*
>  	 * __fsnotify_update_child_dentry_flags(inode);
> @@ -191,8 +175,6 @@ void fsnotify_destroy_mark_locked(struct fsnotify_mark *mark,
>  	 */
>  
>  	atomic_dec(&group->num_marks);
> -
> -	mutex_lock_nested(&group->mark_mutex, SINGLE_DEPTH_NESTING);
>  }
>  
>  void fsnotify_destroy_mark(struct fsnotify_mark *mark,
> @@ -205,7 +187,10 @@ void fsnotify_destroy_mark(struct fsnotify_mark *mark,
>  
>  /*
>   * Destroy all marks in the given list. The marks must be already detached from
> - * the original inode / vfsmount.
> + * the original inode / vfsmount. Note that we can race with
> + * fsnotify_clear_marks_by_group_flags(). However we hold a reference to each
> + * mark so they won't get freed from under us and nobody else touches our
> + * free_list list_head.
>   */
>  void fsnotify_destroy_marks(struct list_head *to_free)
>  {
> @@ -406,7 +391,7 @@ struct fsnotify_mark *fsnotify_find_mark(struct hlist_head *head,
>  }
>  
>  /*
> - * clear any marks in a group in which mark->flags & flags is true
> + * Clear any marks in a group in which mark->flags & flags is true.
>   */
>  void fsnotify_clear_marks_by_group_flags(struct fsnotify_group *group,
>  					 unsigned int flags)
> @@ -460,6 +445,7 @@ static int fsnotify_mark_destroy(void *ignored)
>  {
>  	struct fsnotify_mark *mark, *next;
>  	struct list_head private_destroy_list;
> +	struct fsnotify_group *group;
>  
>  	for (;;) {
>  		spin_lock(&destroy_lock);
> @@ -471,6 +457,14 @@ static int fsnotify_mark_destroy(void *ignored)
>  
>  		list_for_each_entry_safe(mark, next, &private_destroy_list, g_list) {
>  			list_del_init(&mark->g_list);
> +			group = mark->group;
> +			/*
> +			 * Some groups like to know that marks are being freed.
> +			 * This is a callback to the group function to let it
> +			 * know that this mark is being freed.
> +			 */
> +			if (group && group->ops->freeing_mark)
> +				group->ops->freeing_mark(mark, group);
>  			fsnotify_put_mark(mark);
>  		}
>  
> 

looks good, thanks for fixing this Jan!

Regards,
Lino