Stressing kernel for fun

Author: akpalanaza

Just playing with bash and memory allocation.

Summary

• What is this?
• Why?
• How?
• Remediation
• Usecases
• References

What-is-this?

Do you remember :(){ :|:& };: forkbomb? funny indeed but not as much as what we could do.

This forkbomb defines a shell function that recursively calls itself creating an exponential number of child processes until system resources are exhausted.

What if we used the following malloc bomb instead?

while true; do echo $(</dev/zero) & done

If you check your cpu and memory usage (memory usage, not allocation) you should see that they are not much affected by the malloc bomb.

However if you check your load you should see it rise to infinity, and beyond!

But what does it mean ?

Linux load is calculated as the average number of processes in a runnable or uninterruptible state over a certain period of time.

Creating an enormous malloc forces the kernel to allocate all available memory, thus, causing a stall in the creation of any new tasks. (see How ? for more explanation)

Why?

Cause it's way funnier like this.

How?

This malloc bomb looks simple, and it is.

First the while loop.

while true; do ... & done

This is a simple infinite loop, but, the & done means that the loop is not waiting for the function (content of ...) to finish, instead, it simply ends the loop (with the function still running in a background process) and repeat while true is ...true.

Then the interesting part.

echo $(</dev/zero)

First, some syntax ;

Now that we are good with what each individual command does we can now try to understand the malloc bomb and why it's a malloc bomb.

When the <... try to read the /dev/zero content to return it to the $(...) statement it first needs to allocate memory before reading it.

When the kernel tries to allocate memory it comes to a point where no more memory can be allocated, therefore, stalling any new process attempting to be created.

If you are a player you can still kill the process that initiate the bomb, it will also stop every child process thus ending the denial of service.

Note: Technical explanation could be either partially wrong or not fully accurate, if you want to rephrase and/or improve it, feel free to PR.

Remediation

Blocking this bomb on your system is pretty easy and straight forward.

Simply use ulimit command to block the maximum virtual memory allocated by a process and the maximum number of processes that a user can create.

ulimit -v 1048576 # Limit virtual memory alloation for each process to 1GB
ulimit -u 10000 # Limit user created processes to 10,000

Or

Removing read permission on /dev/zero (not advisable). But, if something other than root needs \00 to be returned, it WILL cause issues. It should be funny to see.

Warning: This is a really bad fix. Do this only if you are dumb.

Usecases

Whenever you have access to linux shell

Just slap the command in the prompt no permission needed, no weird bin needed, only linux shell syntax abuse.

Boot?

You could modify the init script of a system to execute this malloc bomb during the boot sequence, thus, stalling the system without easy/verbose debug axis (beside viewing diff in init script...) and never breaking a the same moment.

References :

• https://docs.kernel.org/admin-guide/cpu-load.html
• https://docs.kernel.org/admin-guide/device-mapper/zero.html

Discovered with fun by akpalanaza