Re: DoS using tmpfs

[Christoph Rohland]

Hi Pavel,

On Fri, 8 Jun 2001, Pavel Roskin wrote:
> Hello!
> 
> It appears that a system with tmpfs mounted with the default (!!!)
> parameters can be used by ordinary users to make the system
> non-functional.

...

> 1) tmpfs, as opposed to ramfs doesn't limit the usage by
>    default. It's not a good default for a filesystem designed for
>    temporary files.

Yes, use the size parameter. And no, ramfs has no resource limits in
the stock kernel at all. In -ac it limits to half the size of the
physical RAM unconditionally. But that's not useful for tmpfs simce
this uses swap also. So it is the admins task to add a size
parameter. I would love to add a size paramater in percent of virtual
memory but this would need some changes in the swapon/off coding.

> 2) Not delivering SIGINT to processes is probably not the best
>    behavior if the memory if low. However, one could argue that some
>    processes would use even more resources if they get control with
>    SIGINT.
> 
> 3) All swap in the system was exhausted and yet tmpfs didn't return
>    ENOSPC to "dd".

That the kernel locks up is IMHO a mm fault. tmpfs allocates its pages
with GFP_USER and will return an error if this fails. Apparently it
never fails but locks up.

Greetings
		Christoph