From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf0-f198.google.com (mail-pf0-f198.google.com [209.85.192.198]) by kanga.kvack.org (Postfix) with ESMTP id 7E5656B0069 for ; Wed, 19 Oct 2016 17:28:50 -0400 (EDT) Received: by mail-pf0-f198.google.com with SMTP id e6so6002411pfk.2 for ; Wed, 19 Oct 2016 14:28:50 -0700 (PDT) Received: from out03.mta.xmission.com (out03.mta.xmission.com. [166.70.13.233]) by mx.google.com with ESMTPS id y7si35133432par.279.2016.10.19.14.28.49 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 19 Oct 2016 14:28:49 -0700 (PDT) From: ebiederm@xmission.com (Eric W. Biederman) References: <87twcbq696.fsf@x220.int.ebiederm.org> <20161018135031.GB13117@dhcp22.suse.cz> <8737jt903u.fsf@xmission.com> <20161018150507.GP14666@pc.thejh.net> <87twc9656s.fsf@xmission.com> <20161018191206.GA1210@laptop.thejh.net> <87r37dnz74.fsf@xmission.com> <87k2d5nytz.fsf_-_@xmission.com> <87y41kjn6l.fsf@xmission.com> <20161019172917.GE1210@laptop.thejh.net> <87pomwi5p2.fsf@xmission.com> Date: Wed, 19 Oct 2016 16:26:41 -0500 In-Reply-To: (Andy Lutomirski's message of "Wed, 19 Oct 2016 11:38:18 -0700") Message-ID: <87pomwghda.fsf@xmission.com> MIME-Version: 1.0 Content-Type: text/plain Subject: Re: [REVIEW][PATCH] exec: Don't exec files the userns root can not read. Sender: owner-linux-mm@kvack.org List-ID: To: Andy Lutomirski Cc: Jann Horn , Michal Hocko , "linux-kernel@vger.kernel.org" , Linux Containers , Oleg Nesterov , "linux-mm@kvack.org" , Linux FS Devel Andy Lutomirski writes: > On Wed, Oct 19, 2016 at 10:55 AM, Eric W. Biederman > wrote: >> Andy Lutomirski writes: >> >>> On Wed, Oct 19, 2016 at 10:29 AM, Jann Horn wrote: >>>> On Wed, Oct 19, 2016 at 11:52:50AM -0500, Eric W. Biederman wrote: >>>>> Andy Lutomirski writes: >>>>> > Simply ptrace yourself, exec the >>>>> > program, and then dump the program out. A program that really wants >>>>> > to be unreadable should have a stub: the stub is setuid and readable, >>>>> > but all the stub does is to exec the real program, and the real >>>>> > program should have mode 0500 or similar. >>>>> > >>>>> > ISTM the "right" check would be to enforce that the program's new >>>>> > creds can read the program, but that will break backwards >>>>> > compatibility. >>>>> >>>>> Last I looked I had the impression that exec of a setuid program kills >>>>> the ptrace. >>>>> >>>>> If we are talking about a exec of a simple unreadable executable (aka >>>>> something that sets undumpable but is not setuid or setgid). Then I >>>>> agree it should break the ptrace as well and since those programs are as >>>>> rare as hens teeth I don't see any problem with changing the ptrace behavior >>>>> in that case. >>>> >>>> Nope. check_unsafe_exec() sets LSM_UNSAFE_* flags in bprm->unsafe, and then >>>> the flags are checked by the LSMs and cap_bprm_set_creds() in commoncap.c. >>>> cap_bprm_set_creds() just degrades the execution to a non-setuid-ish one, >>>> and e.g. ptracers stay attached. >>> >>> I think you're right. I ought to be completely sure because I rewrote >>> that code back in 2005 or so back when I thought kernel programming >>> was only for the cool kids. It was probably my first kernel patch >>> ever and it closed an awkward-to-exploit root hole. But it's been a >>> while. (Too bad my second (IIRC) kernel patch was more mundane and >>> fixed the mute button on "new" Lenovo X60-era laptops and spend >>> several years in limbo...) >> >> Ah yes and this is only a problem if the ptracer does not have >> CAP_SYS_PTRACE. >> >> If the tracer does not have sufficient permissions any opinions on >> failing the exec or kicking out the ptracer? I am leaning towards failing >> the exec as it is more obvious if someone cares. Dropping the ptracer >> could be a major mystery. > > I would suggest leaving it alone. Changing it could break enough > things that a sysctl would be needed, and I just don't see how this is > a significant issue, especially since it's been insecure forever. > Anyone who cares should do the stub executable trick: > > /sbin/foo: 04755, literally just does execve("/sbin/foo-helper"); > > /sbin/foo-helper: 0500. I can't imagine what non-malware would depend on being able to circumvent file permissions and ptrace a read-only executable. Is there something you are thinking of? I know I saw someone depending on read-only executables being read-only earlier this week on the security list, and it could definitely act as part of a counter measure to make binaries harder to exploit. So given that people actually expect no-read permissions to be honored on executables (with what seem valid and sensible use cases), that I can't see any valid reason not to honor no-read permissions, that it takes a really convoluted setup to bypass the current no-read permissions, and that I can't believe anyone cares about the current behavior of ptrace I think this is worth fixing. Eric -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org