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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