dd if=/dev/mem can hang a machine?

[Andriy Gapon]

on 30/11/2008 22:14 Julian Stacey said the following:
> Gary Jennejohn wrote:
>> On Fri, 28 Nov 2008 15:28:35 +0200
>> Andriy Gapon <avg at icyb.net.ua> wrote:
>>
>>> I have a new machine with DG33TL mainboard (ICH9/G33).
>>> In a course of some hacking I ran dd if=/dev/mem ... to scan all memory,
>>> this caused the machine to hang.
>>> I tried to reproduce and this is 100% reproducible.
>>>
>>> I am not used to such behavior. In older days I could scan all the
>>> memory without any issues.
>>>
>>> Could this be related to some modern form of memory-mapped IO? Or to
>>> Intel Management Engine (that seems t bite into DRAM)?
>>> Or something else?
>>>
>>> Just wondering.
>>>
>> That's what I would assume.  With some hardware just reading a register
>> can be harmful.
> 
> I just crashed 3 normally stable machines trying that.
> I only tried for casual interest. I acknowledge Gary's comment above :-)
> 
> 	dd if=/dev/mem of=/dev/null
> 
> 	FreeBSD lapa.js.berklix.net 4.11-RELEASE FreeBSD 4.11-RELEASE
> 	#0: Fri Jul  7 17:56:30 CEST 2006
> 	jhs at lapa.jhs.private:/usr/src/sys/compile/LAPA.small  i386
> 
> 	FreeBSD laps.js.berklix.net 7.0-RELEASE FreeBSD 7.0-RELEASE
> 	#2: Mon Sep  8 15:39:53 CEST 2008
> 	jhs at laps.js.berklix.net:/usr1/ftp/pri/FreeBSD/releases/7.0-RELEASE/src/sys/i386/compile/LAPS.small
> 	i386
> 
> 	FreeBSD john.js.berklix.net 7.1-BETA2 FreeBSD 7.1-BETA2 #0:
> 	Sun Oct 12 20:59:28 UTC 2008
> 	root at driscoll.cse.buffalo.edu:/usr/obj/usr/src/sys/GENERIC  amd64

Thank you for going to all the trouble.
I see now that I have to be more careful in this respect.

-- 
Andriy Gapon