I just read this comment about Unix permissions in a whitepaper to do with securing databases. The comment is about Progress databases, which may not exist in your environment, but the comment is worth noting anyway:
Though counterintuitive, file modification is controlled by permissions on that file, but file creation and deletion are controlled by permissions on the parent directory. In other words, it is entirely legal to delete a file that you cannot modify if you have write access to the directory without having write access to the file itself.
One of the solutions suggested is this:
remove all read, write and execute permissions on all the database extent files and directories.
That came from this whitepaper: