linux-kernel - Re: [PATCH/RFC] user_ns: fix missing limiting of user

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Date:	Fri, 28 Dec 2012 19:21:52 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Vasily Kulikov <segoon@...nwall.com>
Cc:	kernel-hardening@...ts.openwall.com,
	Containers <containers@...ts.linux-foundation.org>,
	"Serge E. Hallyn" <serge.hallyn@...ntu.com>,
	linux-kernel@...r.kernel.org,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH/RFC] user_ns: fix missing limiting of user_ns counts

On Fri, Dec 28, 2012 at 11:04:35PM +0400, Vasily Kulikov wrote:

> > I'm sorry, but this is not a solution.  Kernel is not x86-only; there are
> > architectures with far bigger minimal stack frame size.  E.g. on sparc64
> > every fucking stack frame is at least 176 bytes.  So your 100 calls deep
> > call chain will happily overflow the damn stack all by itself - kernel
> > stack on sparc64 is 16Kb total, including struct thread_info living there.
> 
> Understood.  How to properly fix it then?  Looks like there are quite
> many kernel structures which may reference other structures which
> indirectly reference each other via kref, IOW it is not user_ns specific
> issue.  With unprivileged user_ns the way it should be freed must be
> somehow changed.

	There are many damn good reasons why kref should *not* be used without
thinking.  It's been oversold as easy solution to all refcounting problems;
it isn't one.  Don't use it here.
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