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Date: Thu, 5 Jan 2012 09:30:08 -0500
From: Nick Bowler <>
To: Kees Cook <>
Cc:, Alexander Viro <>,
	Ingo Molnar <>,
	Andrew Morton <>,
	Rik van Riel <>,
	Federica Teodori <>,
	Lucian Adrian Grijincu <>,
	Peter Zijlstra <>,
	Eric Paris <>, Randy Dunlap <>,
	Dan Rosenberg <>,,,
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:
> 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
> +
> +	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?

Nick Bowler, Elliptic Technologies (

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