NetBSD Security Advisory 1999-006

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                 NetBSD Security Advisory 1999-006
                 =================================

Topic:		Security hole in umapfs
Version:	NetBSD 1.3.3 and prior; NetBSD-current until 19990312
Severity:	Local users can gain root privileges


Abstract
========

Insufficient kernel checking in the umapfs virtual file system allows
local users to remap their user id to any other user including the root
user.

umapfs is enabled in the default (GENERIC) kernel for the following ports:
amiga, arm32, atari, bebox, i386, mac68k, macppc, newsmips, next68k,
next68k, ofppc, pmax, sparc, sparc64, vax, x68k.

The alpha, hp300, mvme68k, pc532 and sun3 ports do not include umapfs
by default.

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

umapfs creates a null layer, duplicating a sub-tree of the file system
name space under another part of the global file system, with uid/gid
remapping.  The uid and gid mappings are described in two files supplied
by the user to mount_umap(8).

When a umapfs mount is attempted, no additional checks are done in the
kernel other than the usual checks: the user must be root, or have read
access of the target and be owner of the mount point.  The only
permission checks made were erroneously placed in the mount_umap(8)
command.  A malicious user can compile their own mount_umap binary that
does not include these checks.  With this modified mount_umap a user
can mount any directory on another directory they have write access to
with their uid mapped to 0.  They will then have be able to create and
modify root owned files in the source directory, including the ability
to create setuid root binaries.


Solution and Workarounds
=========================

A patch is available for the NetBSD 1.3.3 which restricts umapfs mounts
to root and fixes the above problem.  You may find this patch on the
NetBSD ftp server:

    ftp://ftp.NetBSD.ORG/pub/NetBSD/misc/security/patches/19990311-umapfs

NetBSD-current since 19990312 is not vulnerable.  Users of NetBSD-current
should upgrade to a source tree later than 19990312.

If neither of the above can be performed, a simple work around is to
remove umapfs from your kernel configuration and rebuild a kernel.
For this you need to remove or comment out the line:

	file-system     UMAPFS          # NULLFS + uid and gid remapping

in the configuration file.  See these URL's for documentation building
a NetBSD kernel:

    http://www.NetBSD.ORG/Documentation/kernel/index.html#downloading_kernel_source
    http://www.NetBSD.ORG/Documentation/kernel/index.html#building_a_kernel

Thanks To
=========

Thanks go to Manuel Bouyer <bouyerat_private> for the discovery
and solution for this problem.


Revision History
================
	1999/03/17 - initial version


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

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


Copyright 1999, The NetBSD Foundation, Inc.  All Rights Reserved.

$NetBSD: NetBSD-SA1999-006.txt,v 1.5 1999/03/17 12:15:13 mrg Exp $

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