Re: [PATCH] fsnotify: Fix possible use-after-free in inode iteration on umount

[Jan Kara <jack@suse.cz>]

Forgot to CC Al directly. Added. Thanks to Eric for the suggestion.

								Honza

On Mon 12-12-16 17:30:05, Jan Kara wrote:
> fsnotify_unmount_inodes() plays complex tricks to pin next inode in the
> sb->s_inodes list when iterating over all inodes. Furthermore the code has a
> bug that if the current inode is the last on i_sb_list that does not have e.g.
> I_FREEING set, then we leave next_i pointing to inode which may get removed
> from the i_sb_list once we drop s_inode_list_lock thus resulting in
> use-after-free issues (usually manifesting as infinite looping in
> fsnotify_unmount_inodes()).
> 
> Fix the problem by keeping current inode pinned somewhat longer. Then we can
> make the code much simpler and standard.
> 
> CC: stable@vger.kernel.org
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  fs/notify/inode_mark.c | 45 +++++++++------------------------------------
>  1 file changed, 9 insertions(+), 36 deletions(-)
> 
> I have found this bug during my testing. If nobody objects, I'll push this
> patch to Linus through my tree.
> 
> 								Honza
> 
> diff --git a/fs/notify/inode_mark.c b/fs/notify/inode_mark.c
> index 741077deef3b..a3645249f7ec 100644
> --- a/fs/notify/inode_mark.c
> +++ b/fs/notify/inode_mark.c
> @@ -150,12 +150,10 @@ int fsnotify_add_inode_mark(struct fsnotify_mark *mark,
>   */
>  void fsnotify_unmount_inodes(struct super_block *sb)
>  {
> -	struct inode *inode, *next_i, *need_iput = NULL;
> +	struct inode *inode, *iput_inode = NULL;
>  
>  	spin_lock(&sb->s_inode_list_lock);
> -	list_for_each_entry_safe(inode, next_i, &sb->s_inodes, i_sb_list) {
> -		struct inode *need_iput_tmp;
> -
> +	list_for_each_entry(inode, &sb->s_inodes, i_sb_list) {
>  		/*
>  		 * We cannot __iget() an inode in state I_FREEING,
>  		 * I_WILL_FREE, or I_NEW which is fine because by that point
> @@ -178,49 +176,24 @@ void fsnotify_unmount_inodes(struct super_block *sb)
>  			continue;
>  		}
>  
> -		need_iput_tmp = need_iput;
> -		need_iput = NULL;
> -
> -		/* In case fsnotify_inode_delete() drops a reference. */
> -		if (inode != need_iput_tmp)
> -			__iget(inode);
> -		else
> -			need_iput_tmp = NULL;
> +		__iget(inode);
>  		spin_unlock(&inode->i_lock);
> -
> -		/* In case the dropping of a reference would nuke next_i. */
> -		while (&next_i->i_sb_list != &sb->s_inodes) {
> -			spin_lock(&next_i->i_lock);
> -			if (!(next_i->i_state & (I_FREEING | I_WILL_FREE)) &&
> -						atomic_read(&next_i->i_count)) {
> -				__iget(next_i);
> -				need_iput = next_i;
> -				spin_unlock(&next_i->i_lock);
> -				break;
> -			}
> -			spin_unlock(&next_i->i_lock);
> -			next_i = list_next_entry(next_i, i_sb_list);
> -		}
> -
> -		/*
> -		 * We can safely drop s_inode_list_lock here because either
> -		 * we actually hold references on both inode and next_i or
> -		 * end of list.  Also no new inodes will be added since the
> -		 * umount has begun.
> -		 */
>  		spin_unlock(&sb->s_inode_list_lock);
>  
> -		if (need_iput_tmp)
> -			iput(need_iput_tmp);
> +		if (iput_inode)
> +			iput(iput_inode);
>  
>  		/* for each watch, send FS_UNMOUNT and then remove it */
>  		fsnotify(inode, FS_UNMOUNT, inode, FSNOTIFY_EVENT_INODE, NULL, 0);
>  
>  		fsnotify_inode_delete(inode);
>  
> -		iput(inode);
> +		iput_inode = inode;
>  
>  		spin_lock(&sb->s_inode_list_lock);
>  	}
>  	spin_unlock(&sb->s_inode_list_lock);
> +
> +	if (iput_inode)
> +		iput(iput_inode);
>  }
> -- 
> 2.10.2
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR