netbsd.noexec.txt

`Date: Thu, 18 Mar 1999 21:27:39 +1100  
From: matthew green <[email protected]>  
To: [email protected]  
Subject: NetBSD Security Advisory 1999-007  
  
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NetBSD Security Advisory 1999-007  
=================================  
  
Topic: noexec mount flag is not properly handled by non-root mount  
Version: NetBSD 1.3.3 and prior; NetBSD-current until 19990318  
Severity: Local users can execute binaries they're not allowed to  
  
  
Abstract  
========  
  
On a system where all partitions writable by regular users are mounted with  
the `noexec' option, a regular user should not be able to execute a binary  
which was not put on the system by the administrator. Insufficient checks  
in the mount system call may allow a regular user to mount a device,  
remote host or local directory without the `noexec' option, allowing them  
to execute arbitrary binaries.  
  
  
Technical Details  
=================  
  
The mount syscall does not require root privileges, it only requires that  
the user has read access to the target and is owner of the mount point.  
For such mounts, the `nosuid' and `nodev' flags, which disable set-id  
executables and device special files respectively, are automatically handled  
by the mount system call, but not the `noexec' flag, which disables the  
ability to execute binaries on this partition. This allows a regular  
user to perform a mount on a mount point he owns, and then execute binaries  
>from this mount point, even if the mount point was initially in a sub-tree  
of the global filesystem mounted with the `noexec' option. The easiest way  
to bypass a `noexec' restriction is to use a nullfs mount, but a NFS mount,  
or a mount from a readable block device can allow it as well.  
  
  
Solutions and Workarounds  
=========================  
  
A patch is available for the NetBSD 1.3.3 which makes the mount system call  
inherit the `noexec' flag from the mount point. You may find this patch on  
the NetBSD ftp server:  
  
ftp://ftp.NetBSD.ORG/pub/NetBSD/misc/security/patches/19990317-mount  
  
NetBSD-current since 19990318 is not vulnerable. Users of NetBSD-current  
should upgrade to a source tree later than 19990318.  
  
  
Thanks To  
=========  
  
Manuel Bouyer <[email protected]> for the solution.  
  
  
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-SA1999-007.txt,v 1.1 1999/03/18 07:35:55 mrg Exp $  
  
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