From owner-freebsd-hackers@FreeBSD.ORG Sat May 17 11:43:55 2003 Return-Path: Delivered-To: freebsd-hackers@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id E924F37B401; Sat, 17 May 2003 11:43:54 -0700 (PDT) Received: from critter.freebsd.dk (critter.freebsd.dk [212.242.86.163]) by mx1.FreeBSD.org (Postfix) with ESMTP id CCD1243F3F; Sat, 17 May 2003 11:43:53 -0700 (PDT) (envelope-from phk@phk.freebsd.dk) Received: from critter.freebsd.dk (localhost [127.0.0.1]) by critter.freebsd.dk (8.12.9/8.12.9) with ESMTP id h4HIhpC2006558; Sat, 17 May 2003 20:43:52 +0200 (CEST) (envelope-from phk@phk.freebsd.dk) To: Robert Watson From: "Poul-Henning Kamp" In-Reply-To: Your message of "Sat, 17 May 2003 10:28:21 EDT." Date: Sat, 17 May 2003 20:43:51 +0200 Message-ID: <6557.1053197031@critter.freebsd.dk> cc: freebsd-hackers@freebsd.org cc: Killing cc: freebsd-security@freebsd.org Subject: Re: open and euid security flaw in 5.0-Current? X-BeenThere: freebsd-hackers@freebsd.org X-Mailman-Version: 2.1.1 Precedence: list List-Id: Technical Discussions relating to FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sat, 17 May 2003 18:43:55 -0000 In message , Robe rt Watson writes: > >On Sat, 17 May 2003, Killing wrote: > >> Thanks for that Robert will do some more investigation as it does break >> screen :( > >Try replacing the devfs_access() contents with solely a call to: > > return (vaccess(vp->v_type, de->de_mode, de->de_uid, de->de_gid, > ap->a_mode ,ap->a_cred, NULL)); > >This should restore the traditional access controls for the controlling >terminal. Again, I'm not sure what the rationale is for the new access >controls, and want to find out before we make any changes to the base >system, but it does strike me that screen breaking is gratuitous :-). This is one of those areas, where the hackish way (ie: /dev/tty) which something were implemented, leaves us with the problem of guessing what the underlying intent actually was/is. It used to be that /dev/tty had its own pseudo device driver, which would do weird stunts to act on the applicable real tty device driver for the controlling terminal of the current process. The resulting semantics of this is that a process can always open its controlling terminal, by opening "/dev/tty", but inconsistently, is not guaranteed to be able open it by name: ssh machine -l user1 ... user1% date > /dev/tty # works user1% date > `tty` # works user1% ls -l `tty` crw--w---- 1 user1 tty 5, 1 May 17 20:24 /dev/ttyp1 user1% su - user2 user2% date > /dev/tty # works user2% date > `tty` # doesn't work. The change I did, was to use the "on demand device creation" feature of DEVFS, to make /dev/tty a sort of "variant symlink" to the current process' controlling terminal device, and thereby getting rid of a lot of hackish code, which amongst other things, complicated locking. critter phk> ls -l /dev/tty `tty` crw--w---- 1 phk tty 5, 3 May 17 20:40 /dev/tty crw--w---- 1 phk tty 5, 3 May 17 20:40 /dev/ttyp3 This means that VOP_OPEN checked against the _real_ permissions of the tty breaking the the following scenario: ssh machine -l user1 ... user1% ls -l `tty` crw--w---- 1 user1 tty 5, 1 May 17 20:24 /dev/ttyp1 user1% su - user2 # user2 has no access to /dev/ttyp1, so /dev/tty cannot # be opened. Therefore, the access check was changed to always allowing the controlling terminal to be opened resulting in the following much simpler semantics: % date > /dev/tty # always works. % date > `tty` # always works. This IMO, reflects the intentions of the original /dev/tty, and since it is simpler and contains no exceptions, I also think it correctly reflects the "UNIX[*] philosophy" much better than the previous behaviour. I have no idea why or what screen(1) is doing, but from your description it seems to rely on the undocumented fact that in certain specific situations user2% date > `/dev/tty' would fail. In my eyes, that is a clear bug in screen(1). Poul-Henning -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 phk@FreeBSD.ORG | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence.