From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1750925AbdE1H5D (ORCPT <rfc822;w@1wt.eu>);
        Sun, 28 May 2017 03:57:03 -0400
Received: from bombadil.infradead.org ([65.50.211.133]:36148 "EHLO
        bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1750820AbdE1H5B (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Sun, 28 May 2017 03:57:01 -0400
Date: Sun, 28 May 2017 00:56:54 -0700
From: Christoph Hellwig <hch@infradead.org>
To: Kees Cook <keescook@chromium.org>
Cc: kernel-hardening@lists.openwall.com,
        Hannes Frederic Sowa <hannes@stressinduktion.org>,
        "Signed-off-by : David S . Miller" <davem@davemloft.net>,
        Laura Abbott <labbott@redhat.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 06/20] randstruct: Whitelist UNIXCB cast
Message-ID: <20170528075654.GC22193@infradead.org>
References: <1495829844-69341-1-git-send-email-keescook@chromium.org>
 <1495829844-69341-7-git-send-email-keescook@chromium.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <1495829844-69341-7-git-send-email-keescook@chromium.org>
User-Agent: Mutt/1.8.0 (2017-02-23)
X-SRS-Rewrite: SMTP reverse-path rewritten from <hch@infradead.org> by bombadil.infradead.org. See http://www.infradead.org/rpr.html
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Fri, May 26, 2017 at 01:17:10PM -0700, Kees Cook wrote:
> This is another false positive in bad cast detection:
> 
> net/unix/af_unix.c: In function ‘unix_skb_scm_eq’:
> net/unix/af_unix.c:1621:31: note: found mismatched rhs struct pointer types: ‘struct unix_skb_parms’ and ‘char’
> 
>   const struct unix_skb_parms *u = &UNIXCB(skb);
>                                ^
> 
> UNIXCB is:
> 
> 	#define UNIXCB(skb)     (*(struct unix_skb_parms *)&((skb)->cb))
> 
> And ->cb is:
> 
> 	char                    cb[48] __aligned(8);
> 
> This is a rather crazy cast, but appears to be safe in the face of
> randomization, so whitelist it in the plugin.

We have a lot of network protocol that use the ->cb area, which makes me
wonder why this one would be so special.

It seems like everyone is just using a plain cast to a pointer without
doing the address taking trick that doesn't make sense for arrays
anyway.

Maybe we just need to fix up the af_unix code to work the same as all
other protocols?