Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752773Ab2KTVuA (ORCPT ); Tue, 20 Nov 2012 16:50:00 -0500 Received: from mail-oa0-f46.google.com ([209.85.219.46]:53196 "EHLO mail-oa0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752646Ab2KTVt7 (ORCPT ); Tue, 20 Nov 2012 16:49:59 -0500 MIME-Version: 1.0 In-Reply-To: <20121120211312.57f1b63d@pyramind.ukuu.org.uk> References: <20121120204238.GA19554@www.outflux.net> <20121120211312.57f1b63d@pyramind.ukuu.org.uk> Date: Tue, 20 Nov 2012 13:49:58 -0800 X-Google-Sender-Auth: g6tbdS32FtZfyHse3AfmSSKFEbY Message-ID: Subject: Re: [PATCH v2] devtmpfs: mount with noexec and nosuid From: Kees Cook To: Alan Cox Cc: linux-kernel@vger.kernel.org, Greg Kroah-Hartman , ellyjones@chromium.org, Kay Sievers , Roland Eggner Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1905 Lines: 45 On Tue, Nov 20, 2012 at 1:13 PM, Alan Cox wrote: > On Tue, 20 Nov 2012 12:42:38 -0800 > Kees Cook wrote: > >> Since devtmpfs is writable, make the default noexec,nosuid as well. This >> protects from the case of a privileged process having an arbitrary file >> write flaw and an argumentless arbitrary execution (i.e. it would lack >> the ability to run "mount -o remount,exec,suid /dev"). > > Ok this looks crap on two levels. > > 1. Why not just have your userspace mount -o remount the file system this > way already in early boot. (and if you trojanned boot that early then any > supposed security gain is already lost) That's certainly possible, but I am hoping to avoid adding any extra boot time. The kernel is responsible for this mount, so its flags should be configurable, resulting in no time penalty anywhere. > 2. If you want to do this right then you need to work out what you are > trying to prevent. Your devtmpfs can force file permissions on the > underlying device nodes by having its own operation handling for chmod. > > At that point you can force permissions on anything that you want to > avoid floating around that filesystem with other rights, while not > touching it on device or directory nodes where the meaning is different. > > In its current form however it appears to be a kernel implementation of > "mount is too hard". This change also stops mmap() with PROT_EXEC which a chmod handler wouldn't be able to do. The noexec and nosuid mount options were designed for this sort of thing, so I think that's where it should be handled. -Kees -- Kees Cook Chrome OS Security -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/