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Date: Fri, 24 Feb 2012 14:35:57 -0500
From: Nick Bowler <nbowler@...iptictech.com>
To: Vasiliy Kulikov <segoon@...nwall.com>
Cc: Kees Cook <keescook@...omium.org>, Greg KH <gregkh@...uxfoundation.org>,
David Windsor <dwindsor@...il.com>,
Roland Dreier <roland@...estorage.com>,
Djalal Harouni <tixxdz@...ndz.org>,
kernel-hardening@...ts.openwall.com,
Ubuntu security discussion <ubuntu-hardened@...ts.ubuntu.com>,
linux-kernel@...r.kernel.org, pageexec@...email.hu,
spender@...ecurity.net
Subject: Re: Re: Add overflow protection to kref
On 2012-02-24 23:13 +0400, Vasiliy Kulikov wrote:
> On Fri, Feb 24, 2012 at 14:05 -0500, Nick Bowler wrote:
> > On 2012-02-24 10:52 -0800, Kees Cook wrote:
> > > On Fri, Feb 24, 2012 at 10:37 AM, Greg KH <gregkh@...uxfoundation.org> wrote:
> > > > On Fri, Feb 24, 2012 at 12:58:35PM -0500, David Windsor wrote:
> > [...]
> > > >> diff --git a/include/linux/kref.h b/include/linux/kref.h
> > > >> index 9c07dce..fc0756a 100644
> > > >> --- a/include/linux/kref.h
> > > >> +++ b/include/linux/kref.h
> > > >> @@ -38,8 +38,12 @@ static inline void kref_init(struct kref *kref)
> > > >> */
> > > >> static inline void kref_get(struct kref *kref)
> > > >> {
> > > >> + int rc = 0;
> > > >> WARN_ON(!atomic_read(&kref->refcount));
> > > >> - atomic_inc(&kref->refcount);
> > > >> + smp_mb__before_atomic_inc();
> > > >> + rc = atomic_add_unless(&kref->refcount, 1, INT_MAX);
> > > >> + smp_mb__after_atomic_inc();
> > > >> + BUG_ON(!rc);
> > > >
> > > > So you are guaranteeing to crash a machine here if this fails? And you
> > > > were trying to say this is a "security" based fix?
> > >
> > > This is the same principle as the stack protector. When something has
> > > gone horribly wrong and cannot be sensibly recovered from, crash the
> > > machine. Wrapping the refcount would cause all kinds of problems, so
> > > that certainly seems worthy of a BUG().
> >
> > But in this case, the principle does not apply because we can recover.
> > The reason we cannot recover from the stack protector case is because
> > the stack protector is reacting after the fact, which is not the case
> > here. Simply peg the reference count at the maximum value, neither
> > incrementing it nor decrementing it further.
>
> ...and simply loose one reference, which leads to use-after-free.
Please explain how a use-after-free could possibly occur if the
reference count is never incremented or decremented again?
Cheers,
--
Nick Bowler, Elliptic Technologies (http://www.elliptictech.com/)
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