BSD tty security, part 3: How to Fix It

[Mark Crispin]

In article <kre.672866069 at mundamutti.cs.mu.OZ.AU> kre at cs.mu.oz.au (Robert Elz) writes:
>The real problem is that the whole model used by "write" is simply
>wrong - sure its simple, and in a sense, elegant, but its simply
>not the way that things should be done.
>
>The whole concept of some other random process, owned by some other
>random user, being able to open a direct channel onto my terminal,
>however the output is massaged, or identified, or restricted, is
>simply silly.

Hopefully this is well understood by now.

Fortunately I never worry about it, since I rarely use multi-user Unix
systems.  The only thing I use our departmental Unix engine for is as
a mail drop, and even then I read the mail remotely.

>The "modern" way where /dev/ttyX is mode 620 and "write" is setgid "tty"
>is safer, but now means that I can't create my own "write" like program
>with my own preferred user interface, I can only use one of the standard
>ones.
>
>What's worse, the appearance of the message that I get to see depends
>on the interface that the sending user has decided to select.

On TOPS-20, you could create your own "write" like program, but it had
to go through either a system call or daemon that enforced various
restrictions on the contents and length of the text.  The call/daemon
also wrapped the message with header/trailer text so there was only
one way for text to display.

>This is all absurd - clearly the intelligent way to run things is for
>messages that come from another user to be displayed the way I want them
>using a process I control - then I can have the message displayed using
>whatever interface I desire

This is exactly how ITS worked.  You opened a core-link (pipe) to the
destination HACTRN (shell) and sent it a string.  The receiving HACTRN
could decide what, if anything, it wanted to do with the string
including refuse to open its end of the core-link if the user didn't
want to receive sends (writes).

It is so wonderful to find that in the 1990's we are rediscovering
technology of the late 60's/early 70's.  1/2 ;-)