Re: dcache_readdir NULL inode oops

["gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>]

Adding Eric as he touched this code last :)

On Thu, Nov 29, 2018 at 07:25:48PM +0000, Jan Glauber wrote:
> On Wed, Nov 28, 2018 at 08:08:06PM +0000, Will Deacon wrote:
> > I spent some more time looking at this today...
> > 
> > On Fri, Nov 23, 2018 at 06:05:25PM +0000, Will Deacon wrote:
> > > Doing some more debugging, it looks like the usual failure case is where
> > > one CPU clears the inode field in the dentry via:
> > >
> > >       devpts_pty_kill()
> > >               -> d_delete()   // dentry->d_lockref.count == 1
> > >                       -> dentry_unlink_inode()
> > >
> > > whilst another CPU gets a pointer to the dentry via:
> > >
> > >       sys_getdents64()
> > >               -> iterate_dir()
> > >                       -> dcache_readdir()
> > >                               -> next_positive()
> > >
> > > and explodes on the subsequent inode dereference when trying to pass the
> > > inode number to dir_emit():
> > >
> > >       if (!dir_emit(..., d_inode(next)->i_ino, ...))
> > >
> > > Indeed, the hack below triggers a warning, indicating that the inode
> > > is being cleared concurrently.
> > >
> > > I can't work out whether the getdents64() path should hold a refcount
> > > to stop d_delete() in its tracks, or whether devpts_pty_kill() shouldn't
> > > be calling d_delete() like this at all.
> > 
> > So the issue is that opening /dev/pts/ptmx creates a new pty in /dev/pts,
> > which disappears when you close /dev/pts/ptmx. Consequently, when we tear
> > down the dentry for the magic new file, we have to take the i_node rwsem of
> > the *parent* so that concurrent path walkers don't trip over it whilst its
> > being freed. I wrote a simple concurrent program to getdents(/dev/pts/) in
> > one thread, whilst another opens and closes /dev/pts/ptmx: it crashes the
> > kernel in seconds.
> 
> I also made a testcase and verified that your fix is fine. I also tried
> replacing open-close on /dev/ptmx with mkdir-rmdir but that does not
> trigger the error.
> 
> > Patch below, but I'd still like somebody else to look at this, please.
> 
> I wonder why no inode_lock on parent is needed for devpts_pty_new(), but
> I'm obviously not a VFS expert... So your patch looks good to me and
> clearly solves the issue.
> 
> thanks,
> Jan
> 
> > Will
> > 
> > --->8
> > 
> > diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c
> > index c53814539070..50ddb95ff84c 100644
> > --- a/fs/devpts/inode.c
> > +++ b/fs/devpts/inode.c
> > @@ -619,11 +619,17 @@ void *devpts_get_priv(struct dentry *dentry)
> >   */
> >  void devpts_pty_kill(struct dentry *dentry)
> >  {
> > -       WARN_ON_ONCE(dentry->d_sb->s_magic != DEVPTS_SUPER_MAGIC);
> > +       struct super_block *sb = dentry->d_sb;
> > +       struct dentry *parent = sb->s_root;
> > 
> > +       WARN_ON_ONCE(sb->s_magic != DEVPTS_SUPER_MAGIC);

Side note, I wonder if this is even needed anymore...

> > +
> > +       inode_lock(parent->d_inode);
> >         dentry->d_fsdata = NULL;
> >         drop_nlink(dentry->d_inode);
> >         d_delete(dentry);
> > +       inode_unlock(parent->d_inode);
> > +
> >         dput(dentry);   /* d_alloc_name() in devpts_pty_new() */
> >  }

This feels right but getting some feedback from others would be good.

thanks,

greg k-h