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

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Date:	Fri, 17 Feb 2012 17:44:57 -0800
From:	Roland Dreier <roland@...estorage.com>
To:	Djalal Harouni <tixxdz@...ndz.org>
Cc:	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,
	gregkh@...uxfoundation.org
Subject: Re: [kernel-hardening] Re: Add overflow protection to kref

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.

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?

 - R.
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