The Early Unix History of Chown() Being Restricted to Root

It was one of those restrictions that seemed unjustified to me but I figured someone smarter than I had seen a reason.

Not being able to chown() caused us grief developing Frame Maker back in the 80s. The responsible way to handle "save" was to write the document into a new file mydoc.new, then rename mydoc.cur to mydoc.backup and then rename mydoc.new to mydoc.cur, so that failure never left you in the lurch. The only problem was that there was no way to create mydoc.new to have the same owner as mydoc.cur and customers complained that we'd keep changing the owner of their files. If only the semantics of the unix filesystem supported file generation numbers, like on Tops20 or VaxVMS, where the default for writing to a file isn't "yeah, sure, write over top of the old data, and let's hope nothing fails along the way" this would not have been a problem.

Wait. You can use chown as non root?

If you could chown files to an arbitrary other user, you could use that to evade disk quotas.

The protocol for changing ownership should be two step.

1. The file is put into an "offered" state, e.g. "offered to bob". Only the owner or superuser can make this state change.

2. Bob can take an "offered to bob" file and change ownership to bob.

Files can always be in an offered state; i.e. have an offered user which is normaly equal to their owner. So when ownership is taken, the two match again.

This reminds me of an interesting security incident that occurred on the undergrad.math.uwaterloo.ca unix cluster while I was an undergrad, circa 1996.

When I'd started, the cluster had three SunOS servers, named cayley, descartes, and napier; undergrad math students had their home directory allocated on a local disk on one of these three machines, which each cross-mounted the others' via NFS. At this time, however, the Math Faculty Computing Facility had just received a fancy new dedicated NFS file server from (IIRC) NetApp, and all our home directories had been moved there instead, presumably freeing up desperately-needed CPU cycles on the three compute servers so we could run the Modula-3 and μC++ compilers.

One evening I was in one of the XTerm labs in the Math and Computer centre working on a CS assignment (the only alternative being to do from my dorm room via 2400 buad dialup). As was tradition, I had left the assignment until the night before it was due to start work on. Indeed, it seems that we all must have, because after getting part way through I needed to access some input data files that were shared from the home directory of the course account—something like ~csXYZ/assignments/N/input—only to find I could not read them.

These files were of course owned by the csXYZ course account and should have been either world-readable or readable by the corresponding csXYZ group to which all students registered that term belonged. Unfortunately something had gone wrong, and although the files were rw-r-----, they belonged to the wrong group, so that I and the other students in the class were not able to access them.

It now being after 6pm there was no hope of tracking down one of the course professors or the tutor to rectify this before morning (and it's quite likley the assignment submission deadline was 9am).

Fortunately, I was a naive and ignorant undergrad student, and not knowing what should and should not have been possible I began to think about how I might obtain access to the needed files.

I knew about suid and sgid binaries, and knew that on these modern SunOS 4 machines you could also have suid and sgid script, so I created a script to cat the needed files, then changed its group to match that to which the files belonged, then tried to chmod g+s the script—but of course this (correctly) failed with a message informing that I could not make my file sgid if I didn't belong to the group in question. I then took a different tack: I chgrped the script back to a gropu I did belong to, ran chmod g+s, then chgrped the script back to the group that owned the files I wanted to read.

I now know that this should have resulted in the script losing its setgid bit, but at the time I was unaware of the expected behaviour—and it seemed that the computer was as ignorant as I was because it duly changed the group as requested without resetting the setgid bit, and I was able to run the script, obtain the files I needed, and finish the assignment.

I then headed over to the CS Club office to discuss what had happened, because I was somewhat surprised this had worked and I wanted to understand why, and I knew that despite the lateness of the hour the office would certainly be open and very likely contain someone more expert than I who would be able to explain.

The office was indeed open but no explanation was forthcoming; instead, I was admonished not to discuss this security hole with anyone until I had reported it, in person, to the system administrators.

Thus it was that bright and early the next morning I found myself in Bill Ince's office with a printout of the terminal history containing a demonstration of the exploit in hand. I informed him I had a security issue to report, and handed him the printout.

He scanned the paper for a moment or two, and then replied simply "ahh, you found it".

It seems I was not the first to report the issue, and he explained that it was due to a bug in the new NetApp file server. He then turned monitor of the terminal on his desk around to show me a long list of filenames scrolling by, and (in hindsight rather unwisely) informed me that it was displaying a list of files that were vulnerable to being WRITTEN to due to the same hole.

He duly swore me to secrecy until the issue could be resolved by NetApp (which it was a few days later), thanked me, and sent me on my way.

> Forbidden

> You don't have permission to access /~cks/space/blog/unix/ChownRestrictionEarlyHistory on this server.

I laughed out loud.

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