Date: Sun, 18 May 2003 02:13:41 +0100 From: "Killing" <killing@barrysworld.com> To: "Robert Watson" <rwatson@freebsd.org>, "Poul-Henning Kamp" <phk@phk.freebsd.dk> Cc: freebsd-security@freebsd.org Subject: Re: open and euid security flaw in 5.0-Current? Message-ID: <012201c31cda$b9dafa50$9f00a8c0@mshome.net> References: <6557.1053197031@critter.freebsd.dk>
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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" <phk@phk.freebsd.dk>
To: "Robert Watson" <rwatson@freebsd.org>
Cc: "Killing" <killing@barrysworld.com>; <freebsd-hackers@freebsd.org>;
<freebsd-security@freebsd.org>
Sent: Saturday, May 17, 2003 7:43 PM
Subject: Re: open and euid security flaw in 5.0-Current?
> In message <Pine.NEB.3.96L.1030517102700.42953A-100000@fledge.watson.org>,
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.
>
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