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Date: Thu, 19 Feb 2004 07:24:43 +0300 (MSK)
From: Dan Yefimov <>
Cc:, <>,
Subject: Re: Second critical mremap() bug found in all Linux kernels

On Wed, 18 Feb 2004, Paul Starzetz wrote:

> Hash: SHA1
> 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:
> URL:
> CVE:       CAN-2004-0077
> Author:    Paul Starzetz <>
> 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)
> After mremap() but VMA limit:
> (ooooxxxxxxxxxxxxxxoooo)
> [---------VMA1---------]
> 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 <> has identified the vulnerability and 
> performed further research. COPYING, DISTRIBUTION, AND MODIFICATION OF 
> 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
> Version: GnuPG v1.0.7 (GNU/Linux)
> iD8DBQFAM1QzC+8U3Z5wpu4RAqXzAKCMOkFu1mXzzRgLyuFYp4ORpQCQDgCfe4M2
> 3IjbGvzniOjv/Hc7KKAzMtU=
> =GJds
Attached patch fixes this bug for kernel 2.2.25. It should also apply cleanly to 
kernels since at least 2.2.21.

    Sincerely Your, Dan.

View attachment "linux-2.2.25-mremap-security.patch" of type "TEXT/PLAIN" (4017 bytes)

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