Re: [PATCH v3] fs: allow protected cross-uid sticky symlinks

[Kees Cook <kees.cook@canonical.com>]

On Tue, Jun 01, 2010 at 08:01:02PM +0100, Al Viro wrote:
> On Tue, Jun 01, 2010 at 11:52:48AM -0700, 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.
> > 
> > Some pointers to the history of earlier discussion that I could find:
> 
> I don't buy it.  If we are concerned about the symlinks in the middle of
> pathname, your checks are useless (mkdir /tmp/a, ln -s whatever /tmp/a/b,
> have victim open /tmp/a/b/something).  If we are not, then your checks are
> in the wrong place.

Well, that's not traditionally where the problems happen, but I have no
problem strengthening the protection to include a full examination of the
entire path looking for sticky/world-writable directories.

If not, what is the right place for the checks?

> "The more we prohibit, the safer we are" is best left to the likes of TSA;
> if we are really interested in security and not in security theatre or
> BDSM fetishism, let's make sure that heuristics we use make sense.

I'm not suggesting we remove symlinks.  :)  I don't feel that there is any
theatre here, since it only eliminates a strictly bad situation.  If there
are even more strictly bad situations, then we should eliminate those too.

-Kees

-- 
Kees Cook
Ubuntu Security Team