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 5C3E2C3271E for ; Mon, 8 Jul 2024 08:57:30 +0000 (UTC) Received: (qmail 23776 invoked by uid 550); 8 Jul 2024 08:57:17 -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 23752 invoked from network); 8 Jul 2024 08:57:17 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=digikod.net; s=20191114; t=1720429028; bh=7bLt7WxO3ulEYXhk1rhXT6KKMcWMaVDQidRXNgAZAWs=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=0MfGmOLJXnmucGesfHgTcGIrV1AuYeLm7OPKPfCmEW0e1L+aQfqE3m5322nbHkmbp o/g25CO5nyDvvCCX0mNrl+9Vxi2omD86s/oTmd2DYsR4YvIMPmAftfijfVaZPvAxYt 1uPPfRCpd1M7aQnk90+oYq6XmUBltSw7ahYrQLDs= Date: Mon, 8 Jul 2024 10:56:59 +0200 From: =?utf-8?Q?Micka=C3=ABl_Sala=C3=BCn?= To: Florian Weimer Cc: Al Viro , Christian Brauner , Kees Cook , 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 , 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: <20240708.zooj9Miaties@digikod.net> References: <20240704190137.696169-1-mic@digikod.net> <20240704190137.696169-2-mic@digikod.net> <87bk3bvhr1.fsf@oldenburg.str.redhat.com> <20240706.poo9ahd3La9b@digikod.net> <871q46bkoz.fsf@oldenburg.str.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <871q46bkoz.fsf@oldenburg.str.redhat.com> X-Infomaniak-Routing: alpha On Sat, Jul 06, 2024 at 05:32:12PM +0200, Florian Weimer wrote: > * Mickaël Salaün: > > > On Fri, Jul 05, 2024 at 08:03:14PM +0200, Florian Weimer wrote: > >> * Mickaël Salaün: > >> > >> > 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]. > >> > >> Some distributions no longer set executable bits on most shared objects, > >> which I assume would interfere with AT_CHECK probing for shared objects. > > > > A file without the execute permission is not considered as executable by > > the kernel. The AT_CHECK flag doesn't change this semantic. Please > > note that this is just a check, not a restriction. See the next patch > > for the optional policy enforcement. > > > > Anyway, we need to define the policy, and for Linux this is done with > > the file permission bits. So for systems willing to have a consistent > > execution policy, we need to rely on the same bits. > > Yes, that makes complete sense. I just wanted to point out the odd > interaction with the old binutils bug and the (sadly still current) > kernel bug. > > >> Removing the executable bit is attractive because of a combination of > >> two bugs: a binutils wart which until recently always set the entry > >> point address in the ELF header to zero, and the kernel not checking for > >> a zero entry point (maybe in combination with an absent program > >> interpreter) and failing the execve with ELIBEXEC, instead of doing the > >> execve and then faulting at virtual address zero. Removing the > >> executable bit is currently the only way to avoid these confusing > >> crashes, so I understand the temptation. > > > > Interesting. Can you please point to the bug report and the fix? I > > don't see any ELIBEXEC in the kernel. > > The kernel hasn't been fixed yet. I do think this should be fixed, so > that distributions can bring back the executable bit. Can you please point to the mailing list discussion or the bug report?