From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from second.openwall.net (second.openwall.net [193.110.157.125]) by smtp.lore.kernel.org (Postfix) with SMTP id E80A3C3271E for ; Mon, 8 Jul 2024 19:38:45 +0000 (UTC) Received: (qmail 31873 invoked by uid 550); 8 Jul 2024 19:38:33 -0000 Mailing-List: contact kernel-hardening-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Received: (qmail 31765 invoked from network); 8 Jul 2024 19:38:33 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1720467500; bh=e5oHiwwZ0DY7Dmv393o9ZUxAt+dMtNFWFPwbkMtnVJQ=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=Jfb5YOAvhFAWSlZd9EaSBneZ/YcVipzNxWs4j8tWUSij/ubtl3hRK2uYE+qVesZk8 L+ZMtxuh+PC5igYpPTqAUN1mef7sfB2u6n/PWF0O8s3cbQmc0OxIs49tnpe/fdX6Q7 P/yKUh0CNat8czIKYGHk/p/ioGlbNn8Dn4gMc15PHkuUQ/VC2/GkSGooGnAXzMVgpb 2rkWcXcpWR3oZIR4MGr5SJPDPwtdVVOLu+uft6AMPYOl0CGNmO0vg1F2Q9Kc/xWLCB REf//u5kAPiLeRho9Yfk8b8TcVfRACOr0RIAzQibnvhTJ03AjzgTNn2CDd9k86IOj2 5XOChLcu7orZQ== Date: Mon, 8 Jul 2024 12:38:19 -0700 From: Kees Cook To: =?iso-8859-1?Q?Micka=EBl_Sala=FCn?= Cc: Al Viro , Christian Brauner , Linus Torvalds , Paul Moore , Theodore Ts'o , Alejandro Colomar , Aleksa Sarai , Andrew Morton , Andy Lutomirski , Arnd Bergmann , Casey Schaufler , Christian Heimes , Dmitry Vyukov , Eric Biggers , Eric Chiang , Fan Wu , Florian Weimer , Geert Uytterhoeven , James Morris , Jan Kara , Jann Horn , Jeff Xu , Jonathan Corbet , Jordan R Abrahams , Lakshmi Ramasubramanian , Luca Boccassi , Luis Chamberlain , "Madhavan T . Venkataraman" , Matt Bobrowski , Matthew Garrett , Matthew Wilcox , Miklos Szeredi , Mimi Zohar , Nicolas Bouchinet , Scott Shell , Shuah Khan , Stephen Rothwell , Steve Dower , Steve Grubb , Thibaut Sautereau , Vincent Strubel , Xiaoming Ni , Yin Fengwei , kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-integrity@vger.kernel.org, linux-kernel@vger.kernel.org, linux-security-module@vger.kernel.org Subject: Re: [RFC PATCH v19 1/5] exec: Add a new AT_CHECK flag to execveat(2) Message-ID: <202407081237.42C50C2F7@keescook> References: <20240704190137.696169-1-mic@digikod.net> <20240704190137.696169-2-mic@digikod.net> <202407041656.3A05153@keescook> <20240705.uch1saeNi6mo@digikod.net> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20240705.uch1saeNi6mo@digikod.net> On Fri, Jul 05, 2024 at 07:53:10PM +0200, Mickaël Salaün wrote: > On Thu, Jul 04, 2024 at 05:04:03PM -0700, Kees Cook wrote: > > On Thu, Jul 04, 2024 at 09:01:33PM +0200, Mickaël Salaün wrote: > > > Add a new AT_CHECK flag to execveat(2) to check if a file would be > > > allowed for execution. The main use case is for script interpreters and > > > dynamic linkers to check execution permission according to the kernel's > > > security policy. Another use case is to add context to access logs e.g., > > > which script (instead of interpreter) accessed a file. As any > > > executable code, scripts could also use this check [1]. > > > > > > This is different than faccessat(2) which only checks file access > > > rights, but not the full context e.g. mount point's noexec, stack limit, > > > and all potential LSM extra checks (e.g. argv, envp, credentials). > > > Since the use of AT_CHECK follows the exact kernel semantic as for a > > > real execution, user space gets the same error codes. > > > > Nice! I much prefer this method of going through the exec machinery so > > we always have a single code path for these kinds of checks. > > > > > Because AT_CHECK is dedicated to user space interpreters, it doesn't > > > make sense for the kernel to parse the checked files, look for > > > interpreters known to the kernel (e.g. ELF, shebang), and return ENOEXEC > > > if the format is unknown. Because of that, security_bprm_check() is > > > never called when AT_CHECK is used. > > > > I'd like some additional comments in the code that reminds us that > > access control checks have finished past a certain point. > > Where in the code? Just before the bprm->is_check assignment? Yeah, that's what I was thinking. -- Kees Cook