Re: [PATCH 2/2] pty: make sure super_block is still valid in final /dev/tty close

[Peter Hurley <peter@hurleysoftware.com>]

Hi Herton,

On 01/11/2016 06:07 AM, Herton R. Krzesinski wrote:
> Considering current pty code and multiple devpts instances, it's possible
> to umount a devpts file system while a program still has /dev/tty opened
> pointing to a previosuly closed pty pair in that instance. In the case all
> ptmx and pts/N files are closed, umount can be done. If the program closes
> /dev/tty after umount is done, devpts_kill_index will use now an invalid
> super_block, which was already destroyed in the umount operation after
> running ->kill_sb. This is another "use after free" type of issue, but now
> related to the allocated super_block instance.
> 
> To avoid the problem (warning at ida_remove and potential crashes) for
> this specific case, I added two functions in devpts which grabs additional
> references to the super_block, which pty code now uses so it makes sure
> the super block structure is still valid until pty shutdown is done.
> I also moved the additional inode references to the same functions, which
> also covered similar case with inode being freed before /dev/tty final
> close/shutdown.

Thanks for discovering and working this problem.
Comments below.


> Signed-off-by: Herton R. Krzesinski <herton@redhat.com>
> Cc: stable@vger.kernel.org # 2.6.29+
> ---
>  drivers/tty/pty.c         |  9 ++++++---
>  fs/devpts/inode.c         | 20 ++++++++++++++++++++
>  include/linux/devpts_fs.h |  4 ++++
>  3 files changed, 30 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
> index 96016e5..7fc1b3e 100644
> --- a/drivers/tty/pty.c
> +++ b/drivers/tty/pty.c
> @@ -688,7 +688,7 @@ static void pty_unix98_shutdown(struct tty_struct *tty)
>  	else
>  		ptmx_inode = tty->link->driver_data;
>  	devpts_kill_index(ptmx_inode, tty->index);
> -	iput(ptmx_inode); /* drop reference we acquired at ptmx_open */
> +	devpts_iput_sb_deactive(ptmx_inode);
>  }
>  
>  static const struct tty_operations ptm_unix98_ops = {
> @@ -785,9 +785,12 @@ static int ptmx_open(struct inode *inode, struct file *filp)
>  	 * still have /dev/tty opened pointing to the master/slave pair (ptmx
>  	 * is closed/released before /dev/tty), we must make sure that the inode
>  	 * is still valid when we call the final pty_unix98_shutdown, thus we
> -	 * hold an additional reference to the ptmx inode
> +	 * hold an additional reference to the ptmx inode. For the same /dev/tty
> +	 * last close case, we also need to make sure the super_block isn't
> +	 * destroyed (devpts instance unmounted), before /dev/tty is closed and
> +	 * on its release devpts_kill_index is called.
>  	 */
> -	ihold(inode);
> +	devpts_ihold_sb_active(inode);
>  
>  	tty_add_file(tty, filp);
>  
> diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c
> index c35ffdc..66a5421 100644
> --- a/fs/devpts/inode.c
> +++ b/fs/devpts/inode.c
> @@ -575,6 +575,26 @@ void devpts_kill_index(struct inode *ptmx_inode, int idx)
>  	mutex_unlock(&allocated_ptys_lock);
>  }
>  
> +/*
> + * pty code needs to hold extra references in case of last /dev/tty close
> + */
> +
> +void devpts_ihold_sb_active(struct inode *ptmx_inode)
> +{
> +	struct super_block *sb = pts_sb_from_inode(ptmx_inode);
> +
> +	atomic_inc(&sb->s_active);
> +	ihold(ptmx_inode);
> +}
> +
> +void devpts_iput_sb_deactive(struct inode *ptmx_inode)
> +{
> +	struct super_block *sb = pts_sb_from_inode(ptmx_inode);
> +
> +	iput(ptmx_inode);
> +	deactivate_super(sb);
> +}

We might as well roll in this functionality into
devpts_new_index() and devpts_kill_index().

I realize that's muddying the separation of concern.

Alternatively, name the functions for the logical operation
rather than specifically for what they do (eg. devpts_add_ref())

Regards,
Peter Hurley


> +
>  /**
>   * devpts_pty_new -- create a new inode in /dev/pts/
>   * @ptmx_inode: inode of the master
> diff --git a/include/linux/devpts_fs.h b/include/linux/devpts_fs.h
> index 251a209..f73ef49 100644
> --- a/include/linux/devpts_fs.h
> +++ b/include/linux/devpts_fs.h
> @@ -19,6 +19,8 @@
>  
>  int devpts_new_index(struct inode *ptmx_inode);
>  void devpts_kill_index(struct inode *ptmx_inode, int idx);
> +void devpts_ihold_sb_active(struct inode *ptmx_inode);
> +void devpts_iput_sb_deactive(struct inode *ptmx_inode);
>  /* mknod in devpts */
>  struct inode *devpts_pty_new(struct inode *ptmx_inode, dev_t device, int index,
>  		void *priv);
> @@ -32,6 +34,8 @@ void devpts_pty_kill(struct inode *inode);
>  /* Dummy stubs in the no-pty case */
>  static inline int devpts_new_index(struct inode *ptmx_inode) { return -EINVAL; }
>  static inline void devpts_kill_index(struct inode *ptmx_inode, int idx) { }
> +static inline void devpts_ihold_sb_active(struct inode *ptmx_inode) { }
> +static inline void devpts_iput_sb_deactive(struct inode *ptmx_inode) { }
>  static inline struct inode *devpts_pty_new(struct inode *ptmx_inode,
>  		dev_t device, int index, void *priv)
>  {
>