Re: inode_permission NULL pointer dereference in 3.13-rc1

Al Viro
2013-11-29T04:07:08Z

On Fri, Nov 29, 2013 at 03:59:39AM +0000, Al Viro wrote:
> On Fri, Nov 29, 2013 at 02:41:21AM +0000, Al Viro wrote:
> > 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...
>
> OK, we have a winner. generic/234 drops refcount of root dentry by about
> 20 (and yes, I should've started with that one, what with Ted's report).
> Run it several times (4 should suffice nicely) and the damn thing triggers
> right there. Uff... At least that takes under a minute instead of a couple
> of hours, which makes debugging that shite much more tolerable...

I think I see what's going on; it *is* unlazy_walk(), but not nd->path.
It's nd->root. IOW, the relevant fix to fs/namei.c is

@@ -513,8 +513,7 @@ static int unlazy_walk(struct nameidata *nd, struct dentry *dentry)

if (!lockref_get_not_dead(&parent->d_lockref)) {
nd->path.dentry = NULL;
- rcu_read_unlock();
- return -ECHILD;
+ goto out;
}

/*

(in addition to other pieces of fun found in process). I'll test and post
results in a few...