Subject: kern/2426: pererse union fs mounts can causes system crash, other miscfs dangers
To: None <gnats-bugs@NetBSD.ORG>
From: Greg Stark <>
List: netbsd-bugs
Date: 05/17/1996 04:51:44
>Number:         2426
>Category:       kern
>Synopsis:       mount_union a/ b/; mount_union -b a/ b/
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people (Kernel Bug People)
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri May 17 05:20:01 1996
>Originator:     Greg Stark
The Student Information Processing Board
>Release:        1.1
System: NetBSD limekiller 1.1B NetBSD 1.1B (LIMEKILLER) #13: Sat May 11 17:43:31 EDT 1996 ghudson@zygorthian-space-raiders:/afs/ i386
from strings mount_union
$NetBSD: mount_union.c,v 1.2 1995/03/18 14:58:24 cgd Exp $
$NetBSD: getmntopts.c,v 1.3 1995/03/18 14:56:58 cgd Exp $


Some combinations of circular or otherwise confusing union mounts and
presumably nullfs mounts cause the machine to crash.

I verified this remotely so i didn't see exactly how the failure
occured. The machine seemed ok for a while; it responded to finger,
but a second login froze and it eventually crashed.

I fear this isn't the only danger, what would happen if you nullfs
mount a over b and b over a?  Or the same with union mounts?

This was recently reported as a "security hole" in FreeBSD 2.1 by
"Krzysztof Labanowski" <> and Adam Kubicki.

cd /var/tmp
mkdir a b
mount_union    a/ b/
mount_union -b a/ b/
# for me this second command never returned.

<synopsis of the problem (one line)>