Date: Fri, 7 Jan 2005 12:46:18 +0100 (CET)
From: Paul Starzetz <ihaquer@...c.pl>
To: bugtraq@...urityfocus.com, <full-disclosure@...ts.netsys.com>,
	<vulnwatch@...nwatch.org>
Cc: vendor-sec <vendor-sec@....de>
Subject: Linux kernel sys_uselib local root vulnerability

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

Hi all,

first of all I must comply about the handling of this vulnerability that I
reported to vendorsec. Obviously my code posted there has been stolen and
plagiated by Stefan Esser from Ematters. The posting containing the
plagiate will follow. Now I have been forced to release the full advisory
however another disclosure timeline have been agreed on vendorsec. Sorry
for the inconvenience.

- ---------------------------------------------------------------------------

Synopsis:  Linux kernel uselib() privilege elevation
Product:   Linux kernel
Version:   2.4 up to and including 2.4.29-rc2, 2.6 up to and including 2.6.10
Vendor:    http://www.kernel.org/
URL:       http://isec.pl/vulnerabilities/isec-0021-uselib.txt
CVE:       CAN-2004-1235
Author:    Paul Starzetz <ihaquer@...c.pl>
Date:      Jan 07, 2005


Issue:
======

Locally  exploitable  flaws  have  been found in the Linux binary format
loaders'  uselib()  functions  that  allow  local  users  to  gain  root
privileges.


Details:
========

The Linux kernel provides a binary format loader layer to load (execute)
programs of different binary formats like ELF or  a.out  and  more.  The
kernel   also   provides   a  function  named  sys_uselib()  to  load  a
corresponding library.  This  function  is  dispatched  to  the  current
process's binary format handler and is basicaly a simplified mmap() code
coupled with some header parsing code.

An analyse of the uselib function load_elf_library()  from  binfmt_elf.c
revealed a flaw in the handling of the library's brk segment (VMA). That
segment is created with the  current->mm->mmap_sem  semaphore  NOT  held
while  modyfying  the  memory layout of the calling process. This can be
used to disturb the memory management and gain elevated privileges. Also
the binfmt_aout binary format loader code is affected in the same way.


Discussion:
=============

The  vulnerable  code  resides  for  example  in fs/binfmt_elf.c in your
kernel source code tree:

static int load_elf_library(struct file *file)
{
[904]  down_write(&current->mm->mmap_sem);
       error = do_mmap(file,
                     ELF_PAGESTART(elf_phdata->p_vaddr),
                     (elf_phdata->p_filesz +
                      ELF_PAGEOFFSET(elf_phdata->p_vaddr)),
                     PROT_READ | PROT_WRITE | PROT_EXEC,
                     MAP_FIXED | MAP_PRIVATE | MAP_DENYWRITE,
                     (elf_phdata->p_offset -
                      ELF_PAGEOFFSET(elf_phdata->p_vaddr)));
       up_write(&current->mm->mmap_sem);
       if (error != ELF_PAGESTART(elf_phdata->p_vaddr))
              goto out_free_ph;

       elf_bss = elf_phdata->p_vaddr + elf_phdata->p_filesz;
       padzero(elf_bss);

       len = ELF_PAGESTART(elf_phdata->p_filesz + elf_phdata->p_vaddr + ELF_MIN_ALIGN - 1);
       bss = elf_phdata->p_memsz + elf_phdata->p_vaddr;
       if (bss > len)
              do_brk(len, bss - len);

The line numbers are all valid for the 2.4.28 kernel version. As can  be
seen  the  mmap_sem  is  released  prior to calling do_brk() in order to
create the data section of the ELF library. On the other  hand,  looking
into  the code of sys_brk() from mm/mmap.c reveals that do_brk() must be
called with the semaphore held.

A short look into the code of do_brk() shows that:

[1094] vma = kmem_cache_alloc(vm_area_cachep, SLAB_KERNEL);
       if (!vma)
              return -ENOMEM;

       vma->vm_mm = mm;
       vma->vm_start = addr;
       vma->vm_end = addr + len;
       vma->vm_flags = flags;
       vma->vm_page_prot = protection_map[flags & 0x0f];
       vma->vm_ops = NULL;
       vma->vm_pgoff = 0;
       vma->vm_file = NULL;
       vma->vm_private_data = NULL;

       vma_link(mm, vma, prev, rb_link, rb_parent);

where   rb_link   and   rb_parent   were   both   found    by    calling
find_vma_prepare().  Obviously,  if  the kmem_cache_alloc() call sleeps,
the newly created VMA descriptor  may  be  inserted  at  wrong  position
because the process's VMA list and the VMA RB-tree may have been changed
by another thread. This is absolutely enough to gain root privileges.

We  have  found  at  least  three  different  ways   to   exploit   this
vulnerability.  The  race condition can be easily won by consuming a big
amount of memory. A proof  of  concept  code  exists  but  will  not  be
released yet.


Impact:
=======

Unprivileged local users can gain elevated (root) privileges.


Credits:
========

Paul  Starzetz  <ihaquer@...c.pl>  has  identified the vulnerability and
performed further research. COPYING, DISTRIBUTION, AND  MODIFICATION  OF
INFORMATION  PRESENTED  HERE  IS ALLOWED ONLY WITH EXPRESS PERMISSION OF
ONE OF THE AUTHORS.


Disclaimer:
===========

This document and all the information it contains are provided "as  is",
for  educational  purposes  only,  without warranty of any kind, whether
express or implied.

The authors reserve the right not to be responsible for the  topicality,
correctness,  completeness  or  quality  of the information  provided in
this document. Liability claims regarding damage caused by  the  use  of
any  information  provided,  including  any kind of information which is
incomplete or incorrect, will therefore be rejected.


Appendix:
=========

Code attached.

- ------------------------------------------------------------------------

- -- 
Paul Starzetz
iSEC Security Research
http://isec.pl/

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.0.7 (GNU/Linux)

iD8DBQFB3naPC+8U3Z5wpu4RAqi6AKCmSe50fEBcKB5TVygGEVuy3Gz1LwCeNPr5
+lKciODPNWQvg829jcx3Lvk=
=CRPn
-----END PGP SIGNATURE-----


View attachment "elflbl_v108.c" of type "TEXT/PLAIN" (18970 bytes)

Powered by blists - more mailing lists