Robin Hood And Friar Tuck
The following story was posted in news.sysadmin recently.
The more things change, the more they stay the same...
Back in the mid-1970s, several of the system support staff
at Motorola (I believe it was) discovered a relatively
simple way to crack system security on the Xerox CP-V
timesharing system (or it may have been CP-V's predecessor
UTS). Through a simple programming strategy, it was
possible for a user program to trick the system into running
a portion of the program in "master mode" (supervisor
state), in which memory protection does not apply. The
program could then poke a large value into its "privilege
level" byte (normally write-protected) and could then
proceed to bypass all levels of security within the
file-management system, patch the system monitor, and do
numerous other interesting things. In short, the barn door
was wide open.
Motorola quite properly reported this problem to XEROX via
an official "level 1 SIDR" (a bug report with a perceived
urgency of "needs to be fixed yesterday"). Because the text
of each SIDR was entered into a database that could be
viewed by quite a number of people, Motorola followed the
approved procedure: they simply reported the problem as
"Security SIDR", and attached all of the necessary
documentation, ways-to-reproduce, etc. separately.
Xerox apparently sat on the problem... they either didn't
acknowledge the severity of the problem, or didn't assign
the necessary operating-system-staff resources to develop
and distribute an official patch.
Time passed (months, as I recall). The Motorola guys
pestered their Xerox field-support rep, to no avail.
Finally they decided to take Direct Action, to demonstrate
to Xerox management just how easily the system could be
cracked, and just how thoroughly the system security systems
could be subverted.
They dug around through the operating-system listings, and
devised a thoroughly devilish set of patches. These patches
were then incorporated into a pair of programs called Robin
Hood and Friar Tuck. Robin Hood and Friar Tuck were
designed to run as "ghost jobs" (daemons, in Unix
terminology); they would use the existing loophole to
subvert system security, install the necessary patches, and
then keep an eye on one another's statuses in order to keep
the system operator (in effect, the superuser) from aborting
So... one day, the system operator on the main CP-V
software-development system in El Segundo was surprised by a
number of unusual phenomena. These included the following
(as I recall... it's been a while since I heard the story):
- Tape drives would rewind and dismount their tapes in the
middle of a job.
- Disk drives would seek back&forth so rapidly that they'd
attempt to walk across the floor.
- The card-punch output device would occasionally start up
of itself and punch a "lace card" (every hole punched).
These would usually jam in the punch.
- The console would print snide and insulting messages from
Robin Hood to Friar Tuck, or vice versa.
- The Xerox card reader had two output stackers; it could be
instructed to stack into A, stack into B, or stack into A
unless a card was unreadable, in which case the bad card
was placed into stacker B. One of the patches installed
by the ghosts added some code to the card-reader driver...
after reading a card, it would flip over to the opposite
stacker. As a result, card decks would divide themselves
in half when they were read, leaving the operator to
recollate them manually.
I believe that there were some other effects produced, as
Naturally, the operator called in the operating-system
developers. They found the bandit ghost jobs running, and
X'ed them... and were once again surprised. When Robin Hood
was X'ed, the following sequence of events took place:
id1: Friar Tuck... I am under attack! Pray save me! (Robin Hood)
id1: Off (aborted)
id2: Fear not, friend Robin! I shall rout the Sheriff of Nottingham's men!
id3: Thank you, my good fellow! (Robin)
Each ghost-job would detect the fact that the other had been
killed, and would start a new copy of the recently-slain
program within a few milliseconds. The only way to kill
both ghosts was to kill them simultaneously (very difficult)
or to deliberately crash the system.
Finally, the system programmers did the latter... only to
find that the bandits appeared once again when the system
rebooted! It turned out that these two programs had patched
the boot-time image (the /vmunix file, in Unix terms) and
had added themselves to the list of programs that were to be
started at boot time...
The Robin Hood and Friar Tuck ghosts were finally eradicated
when the system staff rebooted the system from a clean
boot-tape and reinstalled the monitor. Not long thereafter,
Xerox released a patch for this problem.
I believe that Xerox filed a complaint with Motorola's
management about the merry-prankster actions of the two
employees in question. To the best of my knowledge, no
serious disciplinary action was taken against either of
Several years later, both of the perpetrators were hired by
Honeywell, which had purchased the rights to CP-V after
Xerox pulled out of the mainframe business. Both of them
made serious and substantial contributions to the Honeywell
CP-6 operating system development effort. Robin Hood (Dan
Holle) did much of the development of the PL-6
system-programming language compiler; Friar Tuck (John
Gabler) was one of the chief communications-software gurus
for several years. They're both alive and well, and living
in LA (Dan) and Orange County (John). Both are among the
more brilliant people I've had the pleasure of working with.
Disclaimers: it has been quite a while since I heard the
details of how this all went down, so some of the details
above are almost certainly wrong. I shared an apartment
with John Gabler for several years, and he was my Best Man
when I married back in '86... so I'm somewhat predisposed to
believe his version of the events that occurred.
Coherent Thought Inc. 3350 West Bayshore #205 Palo Alto CA 94303