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UPDATED: NetBSD Security Advisory 2000-001
From: security-officer () NETBSD ORG (Daniel Carosone)
Date: Sat, 19 Feb 2000 15:27:24 +1100


An updated version of this security advisory has been issued; note
that the advisory is now applicable to a wider range of systems than
had previously been stated.

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                 NetBSD Security Advisory 2000-001
                 =================================

Topic:          procfs security hole
Version:        NetBSD 1.4.1 and prior; NetBSD-current until 20000126
Severity:       If the kernel has procfs configured, any user can become root

Abstract
========

The procfs filesystem makes the different resources of a process
available under the directory /proc/<pid>/. One of these resources
is the memory image of the process. Reading to and writing from this
special file is restricted. However, by tricking a setuid binary to
write into this file, this restriction can be circumvented, and the
memory image of another setuid binary can be manipulated in such a way
that it will execute a shell.

Systems which have procfs configured in the kernel, but not mounted
normally, are still vulnerable because user processes may mount
procfs. This includes most default NetBSD installations.

Technical Details
=================

Access to /proc/<pid>/mem is protected by the procfs_checkioperm()
function in sys/miscfs/procfs/procfs_mem.c. However, this function
does allow access if the effective uid of the writing process is 0.
If a setuid process can be manipulated in such a way that it writes to
a filedescriptor referring to an open /proc/<pid>/mem, this check
will not protect the memory written. One way to do this is to open
/proc/<pid>/mem, dup2() that filedescriptor onto filedescriptor 2,
do a seek on that filedescriptor to an appropriate offset (the right
stack address), execute a setuid binary, and trick it into writing an
error message that contains code to execute a shell. If the main
program, meanwhile, has executed another setuid binary, this will have
its stack overwritten, and execute a shell, giving the user root
access.

Solutions and Workarounds
=========================

A patch is available for NetBSD 1.4.1, that revokes all vnodes
referring to procfs files when a process is about to execute
a setuid or setgid binary. It is located at:

    ftp://ftp.NetBSD.ORG/pub/NetBSD/misc/security/patches/20000130-procfs

This patch will be included in the upcoming NetBSD 1.4.2 minor release.

NetBSD-current since 20000126 is not vulnerable.  Users of
NetBSD-current should upgrade to a source tree later than 20000126.

If this action cannot be taken, a workaround is to disable the use of
the proc filesystem. It is not mounted by default in NetBSD, and
nothing in the NetBSD base tree depends on it.

The procfs filesystem should be disabled by removing it from the
kernel config and rebuilding a new kernel. It is recommended that the
patch above be applied in this case anyway.

An earlier version of this advisory suggested removing any procfs
lines from /etc/fstab, however this is not sufficient. User processes
are able to mount filesystems (subject to some conditions) and the
procfs filesystem is compiled into default NetBSD kernels.  If a user
mounts the procfs filesystem, the system will be vulnerable as above.

In response to this issue, as of 20000216 NetBSD-current implements a
sysctl 'vfs.generic.usermount' to allow administrators to select
whether user mounts should be allowed; by default they are now
disallowed.

Thanks To
=========

Jason Thorpe and Charles Hannum for commenting on the fix, Chris Jones
for observing the user mount problem, and Frank van der Linden for
implementing both the fix to procfs and the usermount sysctl.

Revision History
================

        2000/01/29 - initial version
        2000/01/31 - corrected spelling of "onto"
        2000/02/13 - minor editorial changes for release.
        2000/02/16 - Noted user mount problem, corrected these dates

More Information
================

Information about NetBSD and NetBSD security can be found at
http://www.NetBSD.ORG/ and http://www.NetBSD.ORG/Security/.

Copyright 2000, The NetBSD Foundation, Inc.  All Rights Reserved.

$NetBSD: NetBSD-SA2000-001.txt,v 1.3 2000/02/19 04:02:43 dan Exp $

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