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 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
