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 70E95C3DA45 for ; Wed, 10 Jul 2024 10:05:30 +0000 (UTC) Received: (qmail 28106 invoked by uid 550); 10 Jul 2024 10:05:19 -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 28086 invoked from network); 10 Jul 2024 10:05:19 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=digikod.net; s=20191114; t=1720605910; bh=58eL5zKFVrJFUT+TpbxjO2RgasvOa3v7FdFqIaHbAZ8=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=DNVbx2udibxVu+14OKvyFsM2tte1G7+GSl8Z+NOcHcVVvvBg1ydxpZsw6h/1RkTaN ejzvZofhsiG9dNul4ThaAxY28YALObLrSzblZ8Zjk8uwQdQKc/EuaduylSGnGw3pZ7 LEta7bCt/CqWCCKmMGDr9F+uKDtZtEAT6JChcVeQ= Date: Wed, 10 Jul 2024 12:05:05 +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, Eric Biederman , linux-mm@kvack.org Subject: Re: [PATCH] binfmt_elf: Fail execution of shared objects with ELIBEXEC (was: Re: [RFC PATCH v19 1/5] exec: Add a new AT_CHECK flag to execveat(2)) Message-ID: <20240710.Lu2thiemeil2@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> <20240708.zooj9Miaties@digikod.net> <878qybet6t.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: <878qybet6t.fsf_-_@oldenburg.str.redhat.com> X-Infomaniak-Routing: alpha On Mon, Jul 08, 2024 at 06:37:14PM +0200, Florian Weimer wrote: > * Mickaël Salaün: > > > 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? > > I'm not sure if this was ever reported upstream as an RFE to fail with > ELIBEXEC. We have downstream bug report: > > Prevent executed .so files with e_entry == 0 from attempting to become > a process. > Thanks for the info. > > I've put together a patch which seems to work, see below. > > I don't think there's any impact on AT_CHECK with execveat because that > mode will never get to this point. Correct, that is not an issue for AT_CHECK use cases.