From owner-freebsd-hackers@FreeBSD.ORG Sat May 17 17:15:03 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 7AFA737B401; Sat, 17 May 2003 17:15:03 -0700 (PDT) Received: from smtp-relay1.barrysworld.com (ns1.barrysworld.com [213.221.172.238]) by mx1.FreeBSD.org (Postfix) with ESMTP id 89D0543F85; Sat, 17 May 2003 17:15:02 -0700 (PDT) (envelope-from killing@barrysworld.com) Received: from [213.221.181.50] (helo=barrysworld.com) by smtp-relay1.barrysworld.com with esmtp (Exim 4.12) id 19HBok-00027g-00; Sun, 18 May 2003 01:14:22 +0100 Received: from gromit [212.211.96.3] by barrysworld.com with ESMTP (SMTPD32-7.15) id A10740700254; Sun, 18 May 2003 01:17:11 +0100 Message-ID: <012201c31cda$b9dafa50$9f00a8c0@mshome.net> From: "Killing" To: "Robert Watson" , "Poul-Henning Kamp" References: <6557.1053197031@critter.freebsd.dk> Date: Sun, 18 May 2003 02:13:41 +0100 MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 5.50.4522.1200 X-MimeOLE: Produced By Microsoft MimeOLE V5.50.4522.1200 cc: freebsd-hackers@freebsd.org 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: Sun, 18 May 2003 00:15:03 -0000 I'll look at screen more carfully, hopefully on monday evening. As what its doing atm is succeding its access checks for the controlling terminal but then failing to actually reattach. I agree that there really should be no need for the wrapper I have which traps the screen tty dev access error and acts on it. Thanks for all the feedback guys. Steve ----- Original Message ----- From: "Poul-Henning Kamp" To: "Robert Watson" Cc: "Killing" ; ; Sent: Saturday, May 17, 2003 7:43 PM Subject: Re: open and euid security flaw in 5.0-Current? > 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. >