linux-kernel - Re: [kernel-hardening] Re: Add overflow protection to kref

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Message-ID: <20120218161849.GA4176@kroah.com>
Date:	Sat, 18 Feb 2012 08:18:49 -0800
From:	Greg KH <gregkh@...uxfoundation.org>
To:	Roland Dreier <roland@...estorage.com>
Cc:	Djalal Harouni <tixxdz@...ndz.org>,
	Vasiliy Kulikov <segoon@...nwall.com>,
	kernel-hardening@...ts.openwall.com,
	Kees Cook <keescook@...omium.org>,
	Ubuntu security discussion <ubuntu-hardened@...ts.ubuntu.com>,
	linux-kernel@...r.kernel.org, David Windsor <dwindsor@...il.com>,
	pageexec@...email.hu, spender@...ecurity.net
Subject: Re: [kernel-hardening] Re: Add overflow protection to kref

On Fri, Feb 17, 2012 at 05:44:57PM -0800, Roland Dreier wrote:
> On Fri, Feb 17, 2012 at 3:39 PM, Djalal Harouni <tixxdz@...ndz.org> wrote:
> >> 2) what to do with architectures-loosers?
> > There is lib/atomic64.c but with a static hashed array of raw_spinlocks.
> 
> Even leaving aside performance impact of atomic64_t (and probably
> in most cases the performance of kref is not important at all), it is
> unfortunate to bloat the size from 4 bytes to 8 bytes.
> 
> It seems much better to have some out-of-line code for overflow
> checking rather than increasing the size of every data structure
> that embeds a kref.

Please realize that kref is an in-line structure now.

> Greg, I'm not sure why you're opposed to adding this checking...
> it's pretty clear that buggy error paths that forget to do a put are
> pretty common and will continue to be common in new code, and
> making them harder to exploit seems pretty sane to me.
> 
> What's the downside?

The downside is that there has not even been a patch sent for any of
this.  Combine that with a lack of understanding about reference
counting and atomic_t usages in the kernel, and the whole thing is ripe
for misunderstanding and confusion.

greg k-h
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