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

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Message-ID: <20120217175322.GB29902@kroah.com>
Date:	Fri, 17 Feb 2012 09:53:22 -0800
From:	Greg KH <gregkh@...uxfoundation.org>
To:	Vasiliy Kulikov <segoon@...nwall.com>
Cc:	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 11:59:45AM +0400, Vasiliy Kulikov wrote:
> Hi,
> 
> > And in all that time, I've never seen an instance where you can overflow
> > the reference count,
> 
> Do you mean that the overflow is theoretically impossible or that this
> type of programmer error is rare?
> 
> If the former, it is only 2**32 incs - if you can find open() implementation
> with a missing atomic_dec() in error path and you can call open() faster than
> 10000 times per second, you can overflow the counter in ~4 days.
> 
> If the latter, it is just a question of finding missing put() in some triggerable
> error path.  Kees has already posted a link to a bug with a missing fput().
> 
> 
> BTW, moving from atomic_t to 64 bit refcounter would kill the possibility of
> overflow.  Unfortunately, AFAIU, 64 bit operations are not atomic on some 64 bit
> archs.

Can we switch it on those arches where it is an atomic operation?  That
would be a nice simple solution.

thanks,

greg k-h
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