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