netbsd.umapfs.txt

`Date: Thu, 18 Mar 1999 00:03:40 +1100  
From: matthew green <[email protected]>  
To: [email protected]  
Subject: 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 <[email protected]> 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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