From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pa0-f70.google.com (mail-pa0-f70.google.com [209.85.220.70]) by kanga.kvack.org (Postfix) with ESMTP id 1A77C6B0069 for ; Wed, 19 Oct 2016 13:58:03 -0400 (EDT) Received: by mail-pa0-f70.google.com with SMTP id tz10so14670211pab.3 for ; Wed, 19 Oct 2016 10:58:03 -0700 (PDT) Received: from out01.mta.xmission.com (out01.mta.xmission.com. [166.70.13.231]) by mx.google.com with ESMTPS id r85si24760843pfr.113.2016.10.19.10.58.01 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 19 Oct 2016 10:58:02 -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> Date: Wed, 19 Oct 2016 12:55:53 -0500 In-Reply-To: (Andy Lutomirski's message of "Wed, 19 Oct 2016 10:32:39 -0700") Message-ID: <87pomwi5p2.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 10:29 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. 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