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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED autolearn=ham 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 653B8C43441 for ; Sun, 18 Nov 2018 19:05:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2779020815 for ; Sun, 18 Nov 2018 19:05:23 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 2779020815 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=cyphar.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726320AbeKSF0Z (ORCPT ); Mon, 19 Nov 2018 00:26:25 -0500 Received: from mx1.mailbox.org ([80.241.60.212]:51252 "EHLO mx1.mailbox.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725800AbeKSF0Z (ORCPT ); Mon, 19 Nov 2018 00:26:25 -0500 Received: from smtp2.mailbox.org (unknown [IPv6:2001:67c:2050:105:465:1:2:0]) (using TLSv1.2 with cipher ECDHE-RSA-CHACHA20-POLY1305 (256/256 bits)) (No client certificate requested) by mx1.mailbox.org (Postfix) with ESMTPS id D886D4A2BB; Sun, 18 Nov 2018 20:05:18 +0100 (CET) X-Virus-Scanned: amavisd-new at heinlein-support.de Received: from smtp2.mailbox.org ([80.241.60.241]) by spamfilter01.heinlein-hosting.de (spamfilter01.heinlein-hosting.de [80.241.56.115]) (amavisd-new, port 10030) with ESMTP id 4YqSOEW_gerF; Sun, 18 Nov 2018 20:05:16 +0100 (CET) Date: Mon, 19 Nov 2018 06:05:04 +1100 From: Aleksa Sarai To: Daniel Colascione Cc: Andy Lutomirski , Randy Dunlap , Christian Brauner , "Eric W. Biederman" , LKML , "Serge E. Hallyn" , Jann Horn , Andrew Morton , Oleg Nesterov , Al Viro , Linux FS Devel , Linux API , Tim Murray , Kees Cook , Jan Engelhardt Subject: Re: [PATCH] proc: allow killing processes via file descriptors Message-ID: <20181118190504.ixglsqbn6mxkcdzu@yavin> References: MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="n33er6mvlsotrfdd" Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --n33er6mvlsotrfdd Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On 2018-11-18, Daniel Colascione wrote: > > Here's my point: if we're really going to make a new API to manipulate > > processes by their fd, I think we should have at least a decent idea > > of how that API will get extended in the future. Right now, we have > > an extremely awkward situation where opening an fd in /proc requires > > certain capabilities or uids, and using those fds often also checks > > current's capabilities, and the target process may have changed its > > own security context, including gaining privilege via SUID, SGID, or > > LSM transition rules in the mean time. This has been a huge source of > > security bugs. It would be nice to have a model for future APIs that > > avoids these problems. > > > > And I didn't say in my proposal that a process's identity should > > fundamentally change when it calls execve(). I'm suggesting that > > certain operations that could cause a process to gain privilege or > > otherwise require greater permission to introspect (mainly execve) > > could be handled by invalidating the new process management fds. > > Sure, if init re-execs itself, it's still PID 1, but that doesn't > > necessarily mean that: > > > > fd =3D process_open_management_fd(1); > > [init reexecs] > > process_do_something(fd); > > > > needs to work. >=20 > PID 1 is a bad example here, because it doesn't get recycled. Other > PIDs do. The snippet you gave *does* need to work, in general, because > if exec invalidates the handle, and you need to reopen by PID to > re-establish your right to do something with the process, that process > may in fact have died between the invalidation and your reopen, and > your reopened FD may refer to some other random process. I imagine the error would be -EPERM rather than -ESRCH in this case, which would be incredibly trivial for userspace to differentiate between. If you wish to re-open the path that is also trivial by re-opening through /proc/self/fd/$fd -- which will re-do any permission checks and will guarantee that you are re-opening the same 'struct file' and thus the same 'struct pid'. > The only way around this problem is to have two separate FDs --- one > to represent process identity, which *must* be continuous across > execve, and the other to represent some specific capability, some > ability to do something to that process. It's reasonable to invalidate > capability after execve, but it's not reasonable to invalidate > identity. In concrete terms, I don't see a big advantage to this > separation, and I think a single identity FD combined with > per-operation capability checks is sufficient. And much simpler. I think that the error separation above would trivially allow user-space to know whether the identity or capability of a process being monitored has changed. Currently, all operations on a '/proc/$pid' which you've previously opened and has died will give you -ESRCH. So the above separation I mentioned is entirely consistent with how users are using '/proc/$pid' to check for PID death today. > > I think you're overstating your case. To a pretty good approximation, > > setresuid() allows the caller to remove elements from the set {ruid, > > suid, euid}, unless the caller has CAP_SETUID. If you could ptrace a > > process before it calls setresuid(), you might as well be able to > > ptrace() it after, since you could have just ptraced it and made it > > call setresuid() while still ptracing it. >=20 > What about a child that execs a setuid binary? Yeah, for this reason I think that using -EPERM on operations that we think are not reasonable to allow possibly-less-privileged processes to do -- probably the most reasonable choice would be ptrace_may_access(). > > Similarly, it seems like > > it's probably safe to be able to open an fd that lets you watch the > > exit status of a process, have the process call setresuid(), and still > > see the exit status. >=20 > Is it? That's an open question. Well, if we consider wait4(2) it seems that this is already the case. If you fork+exec a setuid binary you can definitely see its exit code. > > My POLLERR hack, aside from being ugly, > > avoids this particular issue because it merely lets you wait for > > something you already could have observed using readdir(). >=20 > Yes. I mentioned this same issue-punting as the motivation behind > exithand, initially, just reading EOF on exit. One question I have about EOF-on-exit is that if we wish to extend it to allow providing the exit status (which is something we discussed in the original thread), how will multiple-readers be handled in such a scenario? Would we be storing the exit status or siginfo in the equivalent of a locked memfd? --=20 Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH --n33er6mvlsotrfdd Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAABCAAdFiEEb6Gz4/mhjNy+aiz1Snvnv3Dem58FAlvxt90ACgkQSnvnv3De m5907w/+Izk6UkNPV6dusUBlHUV56jUtHKpSc4dYRGSGmZu5WY6oiWap0GD5baVg 3nRrnRnAQh+8eJtzi5X/k5QJPpGAzigcg/b20JBPvsCUgfG/ZbvnDRKqObYREWRL R3RrgiKprp9pUUWOrn5DzET/bqbN96jsEQd9KCn44+XeZXwaEy0c5fk82+0Oz4OW FG2g8K5Y4kayg8yw8WjXoGssAcUWGsxUxcmkmiOoEx+rDiAXhKlCbQnnOtKKDdF3 HRpVCcJPvnGfVxDbuWcrXqmWmZFsMWs1ohnVKd6AeTSsvNS/qHfci66xi/8aG+iQ T3NZI0pqNveQLqCQ5D4yCoxavPcZ0K2M8p0cgTV87q7IKqc2HLwc7AhP4x5b34r4 CWqwQ7k4Rj57qq+Hpd1SjgrlmW2nq7FyoETDX6oYOo9wXyE4vN2SKMwzF9wgCENB K9x/yoe0EB4LPb5Ue0mbY5yUxQBoj1NY0y+ruKn+emNsUkd0+HmCcRCVe0J+TveR B9xhnVKS2t7NlmzsKZ+ZS7UikAf0Vdj0vEtzpD2mtXSTxkgx02AQHI6+EG3svAQD osbX/Ne3f0jBGX1+Nw2V34lFHsD2a6cLVb5WzDFswzU+fZk5T3lBl7RXsVSw4Rkm 7C5BhJAuUJriF5+1yHax30j+DCCpf7oszI8Wp0x7R3Yj5QxtcRQ= =rhny -----END PGP SIGNATURE----- --n33er6mvlsotrfdd--