linux-kernel - Re: [PATCH v2012.1] fs: symlink restrictions on sticky directories

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Message-ID: <20120105143008.GA31728@elliptictech.com>
Date:	Thu, 5 Jan 2012 09:30:08 -0500
From:	Nick Bowler <nbowler@...iptictech.com>
To:	Kees Cook <keescook@...omium.org>
Cc:	linux-kernel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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 v2012.1] fs: symlink restrictions on sticky directories

On 2012-01-04 12:18 -0800, Kees Cook wrote:
> A long-standing class of security issues is the symlink-based
> time-of-check-time-of-use race, most commonly seen in world-writable
> directories like /tmp. The common method of exploitation of this flaw
> is to cross privilege boundaries when following a given symlink (i.e. a
> root process follows a symlink belonging to another user). For a likely
> incomplete list of hundreds of examples across the years, please see:
> http://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=/tmp
> 
> The solution is to permit symlinks to only be followed when outside
> a sticky world-writable directory, or when the uid of the symlink and
> follower match, or when the directory owner matches the symlink's owner.
[...]
> diff --git a/fs/Kconfig b/fs/Kconfig
> index 5f4c45d..26ede24 100644
> --- a/fs/Kconfig
> +++ b/fs/Kconfig
> @@ -278,3 +278,19 @@ source "fs/nls/Kconfig"
>  source "fs/dlm/Kconfig"
>  
>  endmenu
> +
> +config PROTECTED_STICKY_SYMLINKS
> +	bool "Protect symlink following in sticky world-writable directories"
> +	default y
[...]

Why do we need a config option for this?  What's wrong with just using
the sysctl?

Why have you made this option "default y", when enabling it clearly
makes user-visible changes to kernel behaviour?

Cheers,
-- 
Nick Bowler, Elliptic Technologies (http://www.elliptictech.com/)

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