linux-kernel - Re: [PATCH] pidns: remove recursion from free_pid

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Date:	Wed, 10 Oct 2012 13:54:08 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Cyrill Gorcunov <gorcunov@...nvz.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Andrew Vagin <avagin@...nvz.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Oleg Nesterov <oleg@...hat.com>, Greg KH <greg@...ah.com>
Subject: Re: [PATCH] pidns: remove recursion from free_pid_ns() v5

On Thu, 11 Oct 2012 00:42:56 +0400
Cyrill Gorcunov <gorcunov@...nvz.org> wrote:

> The free_pid_ns function done in recursion fashion:
> 
> free_pid_ns(parent)
>   put_pid_ns(parent)
>     kref_put(&ns->kref, free_pid_ns);
>       free_pid_ns
> 
> thus if there was a huge nesting of namespaces the userspace
> may trigger avalanche calling of free_pid_ns leading to
> kernel stack exhausting and a panic eventually.
> 
> This patch turns the recursion into iterative loop.
> 
> v5 (from oleg@):
>  - Drop @ret variable
>  - Make put_pid_ns non-inline since it grows in size,
>    in turn make free_pid_ns static

OK, let's try that.  I'll sit on this until -rc2 to give it a bit of
time to cook.

A -stable backport might be needed.  What capabilities does userspace
need to be able to trigger the kernel stack overflow?
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