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Re: linux 2.0 PTE bug
From: pedward () WEBCOM COM (pedward () WEBCOM COM)
Date: Tue, 26 May 1998 14:11:00 -0700



Hi all,

there is the program :

[SNIP of wonderful reason to set rlimits]


The idea is to take a lot of memory. So, we map all our virtual pages,
to force the system to allow all the pte (am talking about PC box).
So, the process will have allocated 768 pages that will never be
swapped (that's the crucial point).

So, that's simple, you run this program as much time as you need to take
all the memory, and the PC won't be usable anymore (for my 64Mb box,
i need to run it about 20 times).

And then, you can have a wonderful light-show with your HD-led :) (PC will
spend its time in swapping).

How to solve it ?
Well, we could swap the pgd / pmd / pte, but i really don't know
if it is possible or what.

The default TASK_SIZE is set in /usr/src/linux/include/asm/processor.h it is the
virtual memory maximum map size.  This in itself is not dynamically configurable
(it really has nothing to do with resources).  The main factor is RLIMIT_AS,
defined in /usr/src/linux/include/asm/resource.h, it controls how much virtual
memory that a process can map, mmap utilizes virtual memory.  You can
safely throttle people by running a program which calls setrlimit(2) with
RLIMIT_AS as the resource.  By far, 3GB is too much.

This is only a DoS if you LET it be a DoS.


If this bug is already well know, am sorry to disturb with it.

It applies only on the 2.0 kernel, for the 2.1 (soon 2.2) i don't know
if it works, will have to read the source.

Sed.
p6mip300 () infop6 cicrp jussieu fr 



--Perry

--
Perry Harrington        System Software Engineer    zelur xuniL  ()
http://www.webcom.com  perry.harrington () webcom com  Think Blue.  /\



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