linux-kernel - Re: namespaces?: bug at mm/slub.c:2750

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Message-ID: <20090211172416.GA30756@us.ibm.com>
Date:	Wed, 11 Feb 2009 11:24:16 -0600
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	David Howells <dhowells@...hat.com>
Cc:	Vegard Nossum <vegard.nossum@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Eric Sesterhenn <snakebyte@....de>, containers@...ts.osdl.org,
	linux-kernel@...r.kernel.org,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: namespaces?: bug at mm/slub.c:2750

Quoting David Howells (dhowells@...hat.com):
> Serge E. Hallyn <serue@...ibm.com> wrote:
> 
> >  static void uid_hash_remove(struct user_struct *up)
> >  {
> > +	put_user_ns(up->user_ns);
> >  	hlist_del_init(&up->uidhash_node);
> >  }
> 
> Don't you need to do the hlist_del_init() first?  Otherwise, mightn't the
> put_user_ns() cause the namespace to be freed before hlist_del_init() removes
> the user_struct from it?

It's called under uidhash_lock spinlock so should be ok, but in
principle you're right so it's probably a good idea.

The main point is that without this patch, put_user_ns is done before
the hlist_del_init and *not* atomically under uidhash_lock.

thanks,
-serge
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