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Aucbvax.2806 fa.unix-wizards utzoo!decvax!ucbvax!unix-wizards Tue Aug 25 19:25:28 1981 Re: On the Correctness of Set-User-ID programs >From Eps@UCLA-SECURITY Mon Aug 24 17:06:31 1981 Random comments:

Locking: As I recall, passwd(1) links /etc/passwd to /etc/ptmp as a lock in V7 Unix.

General: Unix/TS and its derivatives set the uid in the proc table from RUID, so a user can kill(1) a SUID program.

Unix-V7: as for /tmp being writable, I've seen quite a few Unix systems that have the following lines in /etc/rc: cd /tmp rm -f * Needless to say, PATH was set to something like ":/bin:/usr/bin:/etc"

Unix-V7: cithep's solution to the at(run) problem is to create the files in /usr/spool/at with mode 6744. atrun will refuse to run a file without both SUID and SGID. This is especially important since the owner of a file (as well as the superuser) can chown(chgrp) files (although set bits get reset if the user isn't super and the uid/gid don't match the euid/egid) on that system. This way at(1) is left unprivileged, and /usr/spool/at is left writable.

I've found that set-gid is sufficient for most "magic" programs, and that set-uid-root is only needed when the full capabilities of the Superuser are essential. The authors also failed to mention the tremendously useful access(2) call, which is not easily available to SUID shell scripts. access() is fairly cheap insurance when checking permissions on an existing file, it does get a little trickier when you want to see if you can "safely" do a creat() call on behalf of the RUID. One of the things I've found annoying is that there is no way to "temporarily disable your privileges"--you can only throw them away. This case crops up when a set-uid-not-root program needs to open a file that only the RUID can. One possible solution to this problem might be the addition of xchuid() and xchgid() calls, to swap Rs and Es. Comments?

--Eric -------

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