memory/cpu handling problem ? (x86)

Hello Folks!

A small ansi C program which tries to get all cpu power and as much ram as possible got my system.

Background:
Discussing about NT and Linux with friends I decided to write a program to demonstrate how the os reacts if one process tries to get the whole system.

Please take a look into the attached program, before you continue reading.

1, NT Test
System keeps on beeing stable, no new programs can be started.

2. Linux Test (1 process)
System hangs. No keyboard nor network conection is possible. SysRq-Kill kills the process.

3. Linux Test (3 processes)
Nothing to do anymore. System is blocked. SysRq still reports what it does, but not even killall/SAK helps. s+u+b was the only possible solution.

The program ran under a standard user id on both operating systems. I was not able to ctrl+c/z the process under Linux.

Question: Isn't that some kind of security or at least DoS hole ? Could not anybody with SSH / Telnet access to a server stop this server running ?

Is it possible (via sysctl ?) to impede that ?

Please CC me when answering.

--
{Greetings,Gruss},
Nico Schottelius

I am some kind of busy -
Do not expect an answer within 24 hours.
Instead use the telephon: +49 (0) 173 - 750 7022.

/*******************************************************************************
* Test what the OS does, if there is no ram and cpu left.
* Author: Nico Schottelius <[email protected]>
* Date: 4th of July 2001
*
*
* 5th of Decembre 2001:

 *  added initialization of the new allocated ram. Without this 'feature'
 *  Linux 2.4 has no problems. It is even possible to allocate 2000 MB RAM, 
 *  where only 128 MB is! (Linux gives you 'SoftRam'...)

* ******************************************************************************/

#include <stdlib.h>
#include <stdio.h>

#define lint long int

lint add(lint last) {

	if(last == 0)
		return last;
	else if(last < 0)
		return -1;
	else
		return last + add(last -1);
}

int main ( void )
{

	char **uebervoll = NULL;
	int x = 1, a = 0; /* how many megs to allocate within one step */
	int one_k = 1024;
	int one_meg = one_k * 1024;
	lint i = 0;

         uebervoll = (char **) realloc(uebervoll, (i+1) * sizeof(char *) );

   for(i=0;;) {
      uebervoll = (char **) realloc(uebervoll, (i+1) * sizeof(char *));
      if(uebervoll != NULL) {
         uebervoll[i] = (char *) malloc(x * one_meg * sizeof(char) );
         /* init allocated ram */
         for(a=0;a<(x*one_meg*sizeof(char));a++) uebervoll[i][a] = 'a';
         if(uebervoll[i] != NULL) {
            i++;
            printf("%d MB RAM allocated\n",x * i);
         }
      }

}
}

$ man setrlimit

-Doug

Let us cross over the river, and rest under the shade of the trees.
   --T. J. Jackson, 1863

No problem at all:

cpuinfo:

processor : 0
vendor_id : GenuineIntel
cpu family : 6
model : 5
model name : Pentium II (Deschutes)
stepping : 3
cpu MHz : 399.323
cache size : 512 KB
fdiv_bug : no
hlt_bug : no
f00f_bug : no
coma_bug : no
fpu : yes
fpu_exception : yes
cpuid level : 2
wp : yes
flags : fpu vme de pse tsc msr pae mce cx8 sep mtrr pge mca
cmov pat pse36 mmx fxsr
bogomips : 796.26

mainboard: ??? (must be some asus or gigabyte...I think it was a gigabyte,
the one with the adaptec 7880 scsi controller on board.

RAM: 128 + 256 MB

Kernel 2.4.13

Does this help or do you need more informations ?

Sincerly,

Nico Schottelius

I am some kind of busy -
Do not expect an answer within 24 hours.
Instead use the telephon: +49 (0) 173 - 750 7022.

thanks for the answer!
I just thought linux could handle itself a process which tries to gain all cpu power.
Using setrlimit I can setup maximum cpu time seconds, but not maximum use of cpu
at a time, can't I ?

--
Nico Schottelius

Sorry for delayed messages. The backlog is getting smaller! It's less then 50 mails currently!

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