From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: MIME-Version: 1.0 In-Reply-To: <20161019172917.GE1210@laptop.thejh.net> 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> From: Andy Lutomirski Date: Wed, 19 Oct 2016 10:32:39 -0700 Message-ID: Subject: Re: [REVIEW][PATCH] exec: Don't exec files the userns root can not read. To: Jann Horn Cc: "Eric W. Biederman" , Michal Hocko , "linux-kernel@vger.kernel.org" , Linux Containers , Oleg Nesterov , "linux-mm@kvack.org" , Linux FS Devel Content-Type: text/plain; charset=UTF-8 Sender: owner-linux-mm@kvack.org List-ID: 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...) --Andy -- 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