Re: hard links create local DoS vulnerability and security problems

[Jamie Lokier <jamie@shareable.org>]

Richard B. Johnson wrote:
> To prevent this, a user can set his default permissions so that
> neither group nor world can read the files. This is usually done
> by setting the attributes in the user's top directory.

Correct, but the quota problem is genuine: what if I want to create a
lot of files in /home/jamie that are readable by other users, but I
want to be able to delete them at some later time and reuse my quota
for something else?

This is quite a normal scenario on multi-user systems with quotas.

You seem to be suggesting that the only method is to have a separate
partition for each user, which is absurd.

Another method is "tree quotas" which have come up on this list
before.  Hopefully they will be included one day; tree quotas seem
like they would solve this problem and some others.

> A setuid binary created with a hard-link will only work as a setuid
> binary if the directory it's in is owned by root.

That isn't true.

> If you have users that can create files or hard-links within such
> directories, you have users who either know the root password
> already or have used some exploit to become root. In any case, it's
> not a hard-link problem

/tmp is owned by root and anyone can create a hard link in /tmp to
other files, on a system where /tmp doesn't have its own filesystem.

> No. Users must be able to create hard links to files that belong
> to somebody else if they are readable. It's a requirement.

I disagree.  The ability to create files and declare that someone else
can't hard link to them would be useful in a multi-user environment.

-- Jamie