inode_permission NULL pointer dereference in 3.13-rc1

[Al Viro]

On Thu, Nov 28, 2013 at 06:07:27PM -0800, Linus Torvalds wrote:

> HOWEVER. It's certainly *not* valid if "current->fs->root/pwd" points
> to it. So yeah, there must have been an extra dput() somewhere. Or,
> more likely, I think, we don't get the refcount to some dentry
> properly any more.
> 
> I don't see where, though. You did change where "LOOKUP_RCU" is
> cleared in unlazy_walk() but you did add that
> 
>         nd->path.dentry = NULL;
> 
> and that looks like it should be ok. And I don't see what else would care.

*nod*

BTW, vfsmount refcount is 12, so we *definitely* nowhere near the
final mntput(), etc. and mnt->mnt_root itself should also have
contributed.

I'm going to try to find out _which_ test buggers the refcount - at
least that way I'll have something resembling a usable reproducer...