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Synopsis:  Linux kernel do_mremap VMA limit local privilege escalation
           vulnerability
Product:   Linux kernel
Version:   2.2 up to 2.2.25, 2.4 up to 2.4.24, 2.6 up to 2.6.2
Vendor:    http://www.kernel.org/
URL:       http://isec.pl/vulnerabilities/isec-0014-mremap-unmap.txt
CVE:       CAN-2004-0077
Author:    Paul Starzetz <ihaquer@isec.pl>
Date:      February 18, 2004


Issue:
======

A critical security vulnerability has been found in the Linux kernel 
memory management code inside the mremap(2) system call due to missing 
function return value check. This bug is completely unrelated to the 
mremap bug disclosed on 05-01-2004 except concerning the same internal 
kernel function code.


Details:
========

The Linux kernel manages a list of user addressable valid memory 
locations on a per process basis. Every process owns a single linked 
list of so called virtual memory area descriptors (called from now on 
just VMAs). Every VMA describes the start of a valid memory region, its 
length and moreover various memory flags like page protection. 

Every VMA in the list corresponds to a part of the process's page table. 
The page table contains descriptors (in short page table entries PTEs) 
of physical memory pages seen by the process. The VMA descriptor can be 
thus understood as a high level description of a particular region of 
the process's page table storing PTE properties like page R/W flag and 
so on.

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

Moving a part of the virtual memory from inside a VMA area to a new 
location requires creation of a new VMA descriptor as well as copying 
the underlying page table entries described by the VMA from the old to 
the new location in the process's page table.

To accomplish this task the do_mremap code calls the do_munmap() 
internal kernel function to remove any potentially existing old memory 
mapping in the new location as well as to remove the old virtual memory 
mapping. Unfortunately the code doesn't test the return value of the 
do_munmap() function which may fail if the maximum number of available 
VMA descriptors has been exceeded. This happens if one tries to unmap 
middle part of an existing memory mapping and the process's limit on the 
number of VMAs has been reached (which is currently 65535).

One of the possible situations can be illustrated with the following 
picture. The corresponding page table entries (PTEs) have been marked 
with o and x:

Before mremap():

(oooooooooooooooooooooooo)     (xxxxxxxxxxxx)
[----------VMA1----------]     [----VMA2----]
      [REMAPPED-VMA] <---------------|


After mremap() without VMA limit:

(oooo)(xxxxxxxxxxxx)(oooo)
[VMA3][REMAPPED-VMA][VMA4]


After mremap() but VMA limit:

(ooooxxxxxxxxxxxxxxoooo)
[---------VMA1---------]
     [REMAPPED-VMA]


After the maximum number of VMAs in the process's VMA list has been 
reached do_munmap() will refuse to create the necessary VMA hole because 
it would split the original VMA in two disjoint VMA areas exceeding the 
VMA descriptor limit.

Due to the missing return value check after trying to unmap the middle 
of the VMA1 (this is the first invocation of do_munmap inside do_mremap 
code) the corresponding page table entries from VMA2 are still inserted 
into the page table location described by VMA1 thus being subject to 
VMA1 page protection flags. It must be also mentioned that the original 
PTEs in the VMA1 are lost thus leaving the corresponding page frames 
unusable for ever.

The kernel also tries to insert the overlapping VMA area into the VMA 
descriptor list but this fails due to further checks in the low level 
VMA manipulation code. The low level VMA list check in the 2.4 and 2.6 
kernel versions just call BUG() therefore terminating the malicious 
process.

There are also two other unchecked calls to do_munmap() inside the 
do_mremap() code and we believe that the second occurrence of unchecked 
do_munmap is also exploitable. The second occurrence takes place if the 
VMA to be remapped is beeing truncated in place. Note that do_munmap can 
also fail on an exceptional low memory condition while trying to 
allocate a VMA descriptor.

We were able to create a robust proof-of-concept exploit code giving 
full super-user privileges on all vulnerable kernel versions. The 
exploit code will be released next week.


Impact:
=======

Since no special privileges are required to use the mremap(2) system 
call any process may use its unexpected behavior to disrupt the kernel 
memory management subsystem.

Proper exploitation of this vulnerability leads to local privilege 
escalation giving an attacker full super-user privileges. The 
vulnerability may also lead to a denial-of-service attack on the 
available system memory.

Tested and known to be vulnerable kernel versions are all <= 2.2.25, <= 
2.4.24 and <= 2.6.1. The 2.2.25 version of Linux kernel does not 
recognize the MREMAP_FIXED flag but this does not prevent the bug from 
being successfully exploited. All users are encouraged to patch all 
vulnerable systems as soon as appropriate vendor patches are released. 
There is no hotfix for this vulnerablity. Limited per user virtual 
memory still permits do_munmap() to fail.


Credits:
========

Paul Starzetz <ihaquer@isec.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.

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

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