Common Permission Error

One extremely important part of using any multi-user Operating System is correctly understanding permissions. Over the past couple years, I have been administering a Linux server with a lot of users, and there is one error that has popped up over and over again.  A lot of users forget or don’t realize that permission to delete a file from a directory is determined by the write flag of the directory, not the write flag of the file.

Take this scenario: Bob is an unprivileged user on a LAMP box and he is hosting a dynamic website.  He wants to allow changes to folders within his public_html directory so he can do things like upload templates, media, etc.  However, the apache user is not Bob, it is something like www-data.  As an unprivileged user, Bob cannot chown or chgrp a directory to a group he doesn’t belong to.  He might think about contacting the sysadmin, but more likely he chmods the directories to be a+w, and is careful to make the files not have the write flag (assuming that this is what determines if a file can be deleted or not). This is an incorrect assumption, and he is leaving his files to be deleted by whoever else has an account on the server.

For example:

bob@lamp:~$ ls -l     #note the test directory has o+w
total 4
drwxrwxrwx 2 bob stupid 4096 2008-07-01 10:27 test

bob@lamp:~$ cd test/

bob@lamp:~/test$ ls -l myfile     #note that myfile does not have o+w
-rw-r–r– 1 lundeen2 stupid 5 2008-07-01 10:28 myfile

bob@lamp:~/test$ su otheruser

otheruser@lamp:~/test$ rm myfile     #other random users are able to delete this file
rm: remove write-protected regular file `myfile’? y

otheruser@lamp:~/test$ ls -l
total 0

There are several ways to handle Bob’s situation.  Bob could ask a privileged user to add him to the www-data group (though this won’t work very well if all the users are part of this group) or he could ask the admin to setattr +i the file to make it undeletable (though he himself could not delete it afterward). A better way would probably be for Bob to use acls (eg setfacl) or to set the sticky bit on the directory (chmod +t).  The sticky bit might be good enough, as it is probably what Bob wanted in the first place – for other users besides himself to be able to write to his directory, but not giving those users a chance to delete his or www-data’s files. Using acls is probably best, but can also be slightly more complicated.

While the behavior of permissions may be obvious to a system administrator, to an average user, it seems it is not.  In my weekly cron scripts I have a  “find / -type d  ( -perm -o+w  -perm 1000 )” to search for all files with this permission.