linux-kernel - Re: [PATCH v2012.2] fs: symlink restrictions on sticky directories

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Date:	Sun, 19 Feb 2012 13:31:51 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Kees Cook <keescook@...omium.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Rik van Riel <riel@...hat.com>,
	Federica Teodori <federica.teodori@...glemail.com>,
	Lucian Adrian Grijincu <lucian.grijincu@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Eric Paris <eparis@...hat.com>,
	Randy Dunlap <rdunlap@...otime.net>,
	Dan Rosenberg <drosenberg@...curity.com>,
	linux-doc@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH v2012.2] fs: symlink restrictions on sticky directories


* Kees Cook <keescook@...omium.org> wrote:

> >> > I think I disagree with this. __If the person compiling 
> >> > the kernel includes the feature in his kernel via the 
> >> > time-honoured process of "wtf is that thing? __Yeah, 
> >> > whatev", it gets turned on by default. __This could 
> >> > easily result in weird failures which would take a *long* 
> >> > time for an unsuspecting person to debug.
> >> >
> >> > Would it not be kinder to our users to start this out as 
> >> > turned-off-at-runtime unless the kernel configurer has 
> >> > deliberately gone in and enabled it?
> >>
> >> There was a fair bit of back-and-forth discussion about it. 
> >> Originally, I had it disabled, but, IIRC, Ingo urged me to 
> >> have it be the default. I can sent a patch to disable it if 
> >> you want.
> >
> > What is the reasoning behind the current setting?
> 
> The logic is currently:
> 
> - from a security perspective, enabling the restriction is 
> safer
> - in the last many years, nothing has been found to be
>  broken by this restriction
> 
> The evidence for the second part mostly comes from people's 
> recollections using OpenWall, grsecurity, and lately Ubuntu. I 
> can speak from the Ubuntu history, which is that in the 1.5 
> years the symlink restriction has been enabled, no bugs about 
> it were reported that I'm aware of (and I was aware of, and 
> fixed, several of bugs in the other restrictions that are 
> carried in Ubuntu).

I'd say all this current evidence suggests that it should be on 
by default - having it off only helps attackers and hermite 
systems.

So at minimum we should wait until the first regression report 
before twiddling it off. I could be wrong though.

Thanks,

	Ingo
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