Date: Fri, 17 Feb 2012 09:53:22 -0800
From: Greg KH <gregkh@linuxfoundation.org>
To: Vasiliy Kulikov <segoon@openwall.com>
Cc: kernel-hardening@lists.openwall.com, Kees Cook <keescook@chromium.org>,
        Ubuntu security discussion <ubuntu-hardened@lists.ubuntu.com>,
        linux-kernel@vger.kernel.org, David Windsor <dwindsor@gmail.com>,
        pageexec@freemail.hu, spender@grsecurity.net
Subject: Re: [kernel-hardening] Re: Add overflow protection to kref
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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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