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

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Date:	Wed, 10 Oct 2012 13:59:43 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Cyrill Gorcunov <gorcunov@...nvz.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Andrew Vagin <avagin@...nvz.org>,
	Oleg Nesterov <oleg@...hat.com>, Greg KH <greg@...ah.com>
Subject: Re: [PATCH] pidns: remove recursion from free_pid_ns() v5

Andrew Morton <akpm@...ux-foundation.org> writes:

> 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?

CAP_SYS_ADMIN is required to create a new pid namespace today.

With a little luck the user namespace bits that allow unprivelged
creation of pid namespaces will be ready for 3.8.

Eric

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