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

[Al Viro <viro@ZenIV.linux.org.uk>]

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.

"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.