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 X-Spam-Level: X-Spam-Status: No, score=-6.0 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 97B75C4361B for ; Tue, 15 Dec 2020 04:28:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 61CA72073C for ; Tue, 15 Dec 2020 04:28:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725306AbgLOE2R (ORCPT ); Mon, 14 Dec 2020 23:28:17 -0500 Received: from mail.kernel.org ([198.145.29.99]:53878 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725763AbgLOE2I (ORCPT ); Mon, 14 Dec 2020 23:28:08 -0500 Date: Tue, 15 Dec 2020 06:27:19 +0200 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1608006445; bh=2s/8/NZ46KXadcfTblsrfGQ7prr2OPLEfK8cAWsiAfs=; h=From:To:Cc:Subject:References:In-Reply-To:From; b=bXfpVKnlhdsdjRLRYViug7B+H9HWzWgSieBqjr0vFkvrMSJ/1zMhEuHGpy/KgI5RE 7STwGAy2VoaviaSbVi9r2PBY3I+Or4oKvj6Q5VTVm1eOdk8obsS4kwQKh2M6r+KPVN GOu5TwgCmwY/QbFiP7G+/JRvuLhvFhFo1sQ8d8Dk0rYQ8waswJwOL1x3a6UC4ifszP xtmhGhtrH+s3nkf6sIoMGtYq6PnvDhAoZTy3JIXXtKX4ZIGtjBXxjDgOli8y+97IYD hz+SkbfjTcChIyHzNHDFGAS6SBVELH8Tf6Ev604wlSMVa8le17WHFNUU2t6ED/bzPQ DTNk12Nbjds/w== From: Jarkko Sakkinen To: Ulrich Windl Cc: toiwoton@gmail.com, luto@amacapital.net, Ben Hutchings , jethro@fortanix.com, bruce.schlobohm@intel.com, kai.svahn@intel.com, luto@kernel.org, haitao.huang@linux.intel.com, jarkko.sakkinen@linux.intel.com, "systemd-devel@lists.freedesktop.org" , casey@schaufler-ca.com, sds@tycho.nsa.gov, linux-hotplug@vger.kernel.org, linux-sgx@vger.kernel.org Subject: Re: Antw: [EXT] Re: [systemd-devel] Creating executable device nodes in /dev? Message-ID: <20201215042719.GA23937@kernel.org> References: <0f17eade-5e99-be29-fd09-2d0a1949ac7f@gmail.com> <9DF5C88B-5156-455A-BA3F-EB19CAA0411B@amacapital.net> <20201209001521.GA64007@kernel.org> <20201211104635.GD12091@kernel.org> <27796c04-249e-6cf0-c3e1-0fd657a82f9c@gmail.com> <5FD7137E020000A10003D81B@gwsmtp.uni-regensburg.de> <20201215041903.GA21875@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20201215041903.GA21875@kernel.org> Precedence: bulk List-ID: X-Mailing-List: linux-sgx@vger.kernel.org On Tue, Dec 15, 2020 at 06:19:09AM +0200, Jarkko Sakkinen wrote: > On Mon, Dec 14, 2020 at 08:25:50AM +0100, Ulrich Windl wrote: > > >>> Topi Miettinen schrieb am 11.12.2020 um 12:46 in > > Nachricht > > <27796c04-249e-6cf0-c3e1-0fd657a82f9c@gmail.com>: > > > On 11.12.2020 12.46, Jarkko Sakkinen wrote: > > >> On Wed, Dec 09, 2020 at 10:35:21AM +0200, Topi Miettinen wrote: > > >>> On 9.12.2020 2.15, Jarkko Sakkinen wrote: > > >>>> On Wed, Dec 09, 2020 at 01:15:27AM +0200, Topi Miettinen wrote: > > >>>>>>>> As a further argument, I just did this on a Fedora system: > > >>>>>>>> $ find /dev ‑perm /ugo+x ‑a \! ‑type d ‑a \! ‑type l > > >>>>>>>> No results. So making /dev noexec doesn't seem to have any benefit. > > >>>>>>> > > >>>>>>> It's no surprise that there aren't any executables in /dev since > > >>>>>>> removing MAKEDEV ages ago. That's not the issue, which is that > > >>>>>>> /dev is a writable directory (for UID=0 but no capabilities are > > >>>>>>> needed) and thus a potential location for constructing unapproved > > >>>>>>> executables if it is also mounted exec (W^X). > > >>>>>> > > >>>>>> UID 0 can just change mount options, though, unless SELinux or similar > > is > > > used. And SELinux can protect /dev just fine without noexec. > > >>>>> > > >>>>> Well, mounting would need CAP_SYS_ADMIN in addition to UID 0. Also > > SELinux > > >>>>> is not universal and the policies might not contain all users or > > services. > > >>>>> > > >>>>> ‑Topi > > >>>> > > >>>> What's the data that supports having noexec /dev anyway? With root > > >>>> access I can then just use something else like /dev/shm mount. > > >>>> > > >>>> Has there been out in the wild real world cases that noexec mount > > >>>> of would have prevented? > > >>>> > > >>>> For me this sounds a lot just something that "feels more secure" > > >>>> without any measurable benefit. Can you prove me wrong? > > >>> > > >>> I don't think security works that way. An attacker has various methods to > > >>> choose from, some are more interesting than others. The case where > > rw,exec > > >>> /dev would be interesting would imply that easier or more common avenues > > >>> would be blocked, for example rw,exec /dev/shm, /tmp, /var/tmp, or > > >>> /run/user/$UID/ for user. Also fileless malware with pure ROP/JOP > > approach > > >>> with no need for any file system access is getting more common. It does > > not > > >>> mean that it would not be prudent to block the relatively easy approaches > > >>> too, including /dev. > > >> > > >> What if we add a new mount option "chrexec", which allows exec > > >> for character devices (S_IFCHR). > > > > > > I think devices are a bad match for SGX because devices haven't been > > > executable and SGX is actually an operation for memory. So something > > > like memfd_create(, MFD_SGX) or mmap(,, PROT_READ|PROT_EXEC|PROT_SGX) > > > would be much more natural. Even better would be something that > > > conceptully also works for AMD version (either with the same flags or > > > MFD_SGX / MFD_whatever_the_AMD_version_is). > > > > +1 > > SGX reserved memory from kernel's point of view is IO memory. > > Mapping SGX to memfd would not be a great idea, as it does not map > into concept of anonymous file backed by regular memory. > > A device file is very natural match actually. We have ioctl API for > uploading enclave pages during the build procedure to the enclave and > custom #PF handler. Conceptually it's a lot like video memory or such > special device specific memory area. > > There's no AMD equivalent of this technology. Anyway, I take a not on "PROT_SGX" as one of the ways sort this out in the future. That would at least fit what we have. Thanks for all the feedback. /Jarkko