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

[Herton R. Krzesinski]

On Wed, Jan 13, 2016 at 09:54:03AM -0800, Peter Hurley wrote:
> 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.

Thanks for looking/reviewing the patches! :)

> 
> 
> > Signed-off-by: Herton R. Krzesinski <herton@xxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx # 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())

I renamed the functions and submitted a v2. Not sure if you also want the
move to devpts_new_index()/devpts_kill_index(), I left that as is for now.
I personally don't have much preference here, I left as separate functions
which are called at pty.c because that seemed to be "more logical", in that it's
a tty specific requirement and not tied to devpts fs itself, not sure if it makes
sense (and anyway devpts is only used by tty anyway). It's mostly a cosmetic/
different way of doing thigs, but if required or preferred I can do a v3 and
move/incorporate it into devpts_new_index and devpts_kill_index.

> 
> 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)
> >  {
> > 
> 

-- 
Regards,
Herton