Re: [PATCH 1/1] threads_max: Simple lockout prevention patch

[Al Boldi <a1426z@gawab.com>]

Nick Piggin wrote:
> Al Boldi wrote:
> > Nick Piggin wrote:
> >>Al Boldi wrote:
> >>>Could do that by:
> >>>
> >>>	# echo 1 > /proc/sys/kernel/su-pid
> >>>
> >>>which would imply nr-threads = 1
> >>>
> >>>So maybe introduce /proc/sys/kernel/nr-threads to allow that to be
> >>>variable, but this isn't really critical.
> >>
> >>Why not just have su-nr-threads?
> >
> > Unless I am misunderstanding you, even root/root-proc can be hit by a
> > runaway, so the threads-max limits this globally which is great, but
> > this may lock-you out of being able to control the situation based on
> > uid only.
> >
> > Thus this patch gives root the ability to allow a certain pid to exceed
> > the threads-max limit, while all other pids are still limited.
>
> But the point is that root is able to get their pids under control,
> and can't be DoSed by unpriv users. Right?

Or even by root.

> Nothing is going to be perfect, I mean the su-pid pid could get "hit
> bya runaway" and is arguably worse than nr-threads-su, because it has
> no upper limit and coult take down the whole system.

The su-pid is a temporary measure set by root after evaluating the current 
situation, and additionally limiting this by su-nr(pid)-threads may probably 
be a good idea.  Maybe something like this:

	if (nr_threads >= max_threads)
		if ((p->pid != su_pid) || (nr_threads >= (max_threads + su_pid_threads)))
			goto bad_fork_cleanup_count;

But I really don't think this is critical.

Thanks!

--
Al