Re: hard links on Linux create local DoS vulnerability and security problems

[Casper Dik <casper () holland sun com>]

> > on Linux it is possible for any user to create a hard link to a file belonging
> > to another user.
>
> Only if they can write to some directory on the same partition.
>
> > Furthermore, users can even create links to a setuid binary.
>
> Only if it's on the same partition.  This is just one of a huge number of
> reasons you shouldn't allow users to write to your root or /usr partitions.
>
> I think that your observation of the ability to keep a security hole open is
> very interesting, but, fortunately, it should be moot.

Many (older) recipies for replacing set-uid binaries consisted of:

        mv /usr/bin/buggy /usr/bin/buggy.old
        chmod 0 /usr/bin/buggy.old

so it's certainly possible to work around that problem.

> I think that this is too drastic a change to the semantics of the unix
> filesystem.  Except for the kludge around the sticky bit, nothing about
> creation and deletion of files (links) depends upon the permissions on the
> file itself, just on the enclosing directory.

But it can't be denied that hard links to files one doesn't
own pose some interesting challenges; not just in the scope
of quotas but also when it comes to actions users cannot
undo (such as making many links to files belonging to other
users in /tmp)

> > If you can check whether this problem also exists on other unix-like
> > operating systems, please post the results.
>
> It certainly does.  This is part of the original design of the unix
> filesystem.  Creating a link requires write access to the directory you're
> creating it in, not to the file you're linking to.
>
>
> I think that if a user creates a bunch of hard links to someone else's
> temporary files, the evidence should point to the original user, on a typical
> well-maintained unix system.  The link to setuid programs is more of concern
> except that it won't be able to happen unless you have setuid-root programs
> in a home directory partition, which sounds bad anyway.


And you can easily work around it when replacing executables.

Casper

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