NetBSD Security Advisory 2014-009: Multiple vulnerabilities in the execve system call

[NetBSD Security Officer <security-officer%NetBSD.org@localhost>]

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                NetBSD Security Advisory 2014-009
                =================================

Topic:          Multiple vulnerabilities in the execve system call


Version:        NetBSD-current:         source prior to Fri, Feb 14th 2014
                NetBSD 6.1 - 6.1.3:     affected
                NetBSD 6.1.4:           not affected
                NetBSD 6.0 - 6.0.4:     affected
                NetBSD 6.0.5:           not affected
                NetBSD 5.1 - 5.1.4:     not affected
                NetBSD 5.2 - 5.2.2:     not affected

Severity:       Local DoS

Fixed:          NetBSD-current:         Fri, Feb 14th 2014
                NetBSD-6-0 branch:      Fri, Feb 14th 2014
                NetBSD-6-1 branch:      Fri, Feb 14th 2014
                NetBSD-6 branch:        Fri, Feb 14th 2014

Teeny versions released later than the fix date will contain the fix.

Please note that NetBSD releases prior to 5.1 are no longer supported.
It is recommended that all users upgrade to a supported release.


Abstract
========

The execve system call is affected by two vulnerabilities:
 1) A memory leak in the kernel could cause a local (un)privileged user
to use up kernel memory via a bogus ELF binary, and thus to freeze - or
eventually panic - the system.
 2) A bug in the kernel could lead to a use-after-free condition when
loading a binary or a script, which would allow a local (un)privileged
user to crash the system.


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

 1) When trying to execute an ELF binary, the kernel looks up the
corresponding "interpreter" (in case of native dynamic ELF binaries: the
dynamic linker ld.elf_so). If this interpreter cannot be accessed
appropriately, or if it is bogus, a structure allocated to hold special
information on this interpreter was not freed.
    If a standard toolchain is installed, a local user can easily create
such broken binaries by passing the -dynamic-linker switch to the linker.

 2) When executing a binary via execve(), the kernel computes the new
user stack size, and returns an error if this size exceeds the maximum
architecture-defined stack size or the maximum stack size allowed by the
calling process through rlimit. However, the variable in charge of hold-
ing the error code returned was not properly initialised, causing the
kernel to keep setting up the new process environment and use data that
was already freed.
    Both the new stack size and the rlimit stack size are approximately
user-controllable, which makes it easy to trigger from a local user.


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

For all NetBSD versions, you need to obtain fixed kernel sources,
rebuild and install the new kernel, and reboot the system.
                                      
The fixed source may be obtained from the NetBSD CVS repository.        
The following instructions briefly summarise how to upgrade your        
kernel.  In these instructions, replace:

  ARCH     with your architecture (from uname -m),               
  KERNCONF with the name of your kernel configuration file and  
  VERSION  with the file version below

File versions containing the fixes:

FILE    HEAD            netbsd-6        netbsd-6-1      netbsd-6-0
- ----    ----            --------        ----------      ----------
sys/kern/exec_elf.c
        1.55            1.37.2.2        1.37.2.1.6.1    1.37.2.1.4.1
sys/kern/kern_exec.c
        1.403           1.339.2.9       1.339.2.6.2.2   1.339.2.5.4.3

To update from CVS, re-build, and re-install the kernel:

        # cd src
        # cvs update -d -P -r VERSION sys/kern/exec_elf.c
        # cvs update -d -P -r VERSION sys/kern/kern_exec.c
        # ./build.sh kernel=KERNCONF
        # mv /netbsd /netbsd.old
        # cp sys/arch/ARCH/compile/obj/KERNCONF/netbsd /netbsd 
        # shutdown -r now

For more information on how to do this, see:    

   http://www.NetBSD.org/guide/en/chap-kernel.html


Thanks To
=========

Thanks to Maxime Villard, who found the issues and provided fixes.


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

        2014-08-27      Initial release


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

Advisories may be updated as new information becomes available.
The most recent version of this advisory (PGP signed) can be found at 
  http://ftp.NetBSD.org/pub/NetBSD/security/advisories/NetBSD-SA2014-009.txt.asc

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


Copyright 2014, The NetBSD Foundation, Inc.  All Rights Reserved.
Redistribution permitted only in full, unmodified form.

$NetBSD: NetBSD-SA2014-009.txt,v 1.1 2014/08/27 00:19:19 tonnerre Exp $

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