Subject: Re: LKM's shouldn't be allowed to be loaded in multiuser mode.
To: matthew green <>
From: Bakul Shah <>
List: tech-kern
Date: 03/18/1995 17:59:23
> you have to pay something for security.  to have the concept of
> securelevel, it means that you lose somethings that are otherwise
> doable.  this includes the ability to load extra modules as you
> want.  if you want to be able to do this, then you have to give
> up the extra security that securelevel gives you.  the ability to
> load *any* random code into the kernel means you've got the
> ability to do anything to the system.  securelevel is supposed to
> stop you from having that -- no write access to /dev/k?mem, or to
> the disk devices while securelevel > 0.

(I think)  I understand the *concept* of securelevel; but to
make your system really secure you also then have to
disallow auto reboot of any sort because otherwise the bad
guy can become root, change /etc/rc etc. and reboot such
that the next time around securelevel is -1.  You have to
make sure the bad guy does not set things up so that his LKM
is loaded the next time around because it can defeat
securelevel by directly reading/writing memory.  He can even
modify the kernel binary to change securelevel.  I am sure
there are many other nasty things one can find to do **if
you assume one can somehow become root**.

It seems to me that if you restrict things more and more to
plug holes as they are discovered, people will be tempted to
just keep the damn door open (ie. set securelevel to -1) all
the time just so they can get in without too much
inconvenience; thereby losing whatever extra security was

> you may as well not have securelevel if you are going to allow
> lkm's with securelevel > 0.

The only other way I can think of is use of capabilities,
where root access does not really matter.  You have to have
a capability that lets you load/unload LKMs and getting
that capability can be made arbitrarily difficult.

Note that unix file-descriptors are sort of like
capabilities....  A careful extension would not be all that
weird and give you a very powerful tool that can be
refined/customized to your heart's content.  I admit this
is not an easy project but certainly worth looking into.

Bakul Shah