Synopsis:  Linux kernel do_mremap local privilege escalation vulnerability
Product:   Linux kernel
Version:   2.2, 2.4 and 2.6 series

Vendor:    https://www.kernel.org/
URL:       https://isec.pl/vulnerabilities/isec-0013-mremap.txt
CVE:       https://cve.mitre.org/cgi-bin/cvename.cgi?name=CAN-2003-0985
Author:    Paul Starzetz <ihaquer@isec.pl>,
           Wojciech Purczynski <cliph@isec.pl>
Date:      January 5, 2004


Issue:
======

A critical security vulnerability has been found  in  the  Linux  kernel
memory  management  code in mremap(2) system call due to incorrect bound
checks.


Details:
========

The mremap system call provides functionality of resizing (shrinking  or
growing)  as well as moving across process's addressable space of existing
virtual memory areas (VMAs) or any of its parts.

A typical VMA covers at least one memory page (which is exactly  4kB  on
the i386  architecture). An incorrect bound check discovered inside the
do_mremap() kernel code performing remapping of a virtual memory  area
may lead to creation of a virtual memory area of 0 bytes length.

The problem bases on the general mremap flaw that remapping of 2 pages
from inside a VMA creates a memory hole of only one page in length but
an additional  VMA  of two pages. In the case of a zero sized remapping
request no VMA hole is created but an additional VMA descriptor of 0
bytes in length is created.

Such a malicious virtual memory area may disrupt the operation of other
parts of the kernel memory management subroutines finally leading to 
unexpected behavior.

A typical process's memory layout showing invalid VMA created with
mremap system call:

    08048000-0804c000 r-xp 00000000 03:05 959142     /tmp/test
    0804c000-0804d000 rw-p 00003000 03:05 959142     /tmp/test
    0804d000-0804e000 rwxp 00000000 00:00 0
    40000000-40014000 r-xp 00000000 03:05 1544523    /lib/ld-2.3.2.so
    40014000-40015000 rw-p 00013000 03:05 1544523    /lib/ld-2.3.2.so
    40015000-40016000 rw-p 00000000 00:00 0
    4002c000-40158000 r-xp 00000000 03:05 1544529    /lib/libc.so.6
    40158000-4015d000 rw-p 0012b000 03:05 1544529    /lib/libc.so.6
    4015d000-4015f000 rw-p 00000000 00:00 0
[*] 60000000-60000000 rwxp 00000000 00:00 0
    bfffe000-c0000000 rwxp fffff000 00:00 0

The broken VMA in the above example has been marked with a [*].


Impact:
=======

Since no special privileges are required to use the mremap(2) system
call any process may misuse its unexpected behavior to disrupt the kernel
memory management subsystem. Proper exploitation of this vulnerability may
lead to local privilege escalation including execution of  arbitrary  code
with kernel level access. Proof-of-concept exploit code has been created 
and successfully tested giving UID 0 shell on vulnerable systems.

The exploitability of the discovered vulnerability is possible, although
not a trivial one. We have identified at least two different attack 
vectors for the  2.4 kernel series. All users are encouraged to patch all
vulnerable systems as soon as appropriate vendor patches are released.


Credits:
========

Paul Starzetz <ihaquer@isec.pl> has identified the vulnerability and
performed further research. 


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.