Prevent deletion or moving of files

You need to let users create files in a common directory, but you don't want them to be able to delete other's files. Or you've put certain files, directories or symlinks into a user's home directory and don't want them to be able to mess with any of those. What can you do?

"t" bit

If you create /foo and do "chmod 1777 /foo", you'll have a world-writeable directory with the "text bit" set. Any user can create files here, but they can only delete files that they own (root can still rm anything). That's ownership as listed in the "owner" column of an "ls -l". Group ownership doesn't come into play here although it does change responses a bit.

Let's see what happens when Sam tries to remove Pete's files in a directory with the text bit set:

[sam@localhost foo]$ ls -ld .
drwxrwxrwt 2 root root 4096 Sep 18 06:00 .
[sam@localhost foo]$ ls -l
total 12
-rw-rw-r-- 1 pete pete  29 Sep 18 05:52 pete
-rw-rw-r-- 1 pete apl   29 Sep 18 06:00 peteapl
-rw-rw-r-- 1 pete wheel 29 Sep 18 06:00 petewheel
[sam@localhost foo]$ id
uid=502(sam) gid=502(sam) groups=502(sam)
[sam@localhost foo]$ rm *
rm: remove write-protected regular file `pete'? y
rm: cannot remove `pete': Operation not permitted
rm: remove write-protected regular file `peteapl'? y
rm: cannot remove `peteapl': Operation not permitted
rm: remove write-protected regular file `petewheel'? y
rm: cannot remove `petewheel': Operation not permitted
 

Now watch what happens when a user in the "wheel" group does the same thing:

[apl@localhost ~]$ cd /foo
[apl@localhost foo]$ ls -l
total 12
-rw-rw-r-- 1 pete pete  29 Sep 18 05:52 pete
-rw-rw-r-- 1 pete apl   29 Sep 18 06:00 peteapl
-rw-rw-r-- 1 pete wheel 29 Sep 18 06:00 petewheel
[apl@localhost foo]$ id
uid=500(apl) gid=500(apl) groups=10(wheel),500(apl)
[apl@localhost foo]$ rm *
rm: remove write-protected regular file `pete'? y
rm: cannot remove `pete': Operation not permitted
rm: cannot remove `peteapl': Operation not permitted
rm: cannot remove `petewheel': Operation not permitted
[apl@localhost foo]$ 
[apl@localhost foo]$ 
[apl@localhost foo]$ rm peteapl
rm: cannot remove `peteapl': Operation not permitted
[apl@localhost foo]$ rm petewheel
rm: cannot remove `petewheel': Operation not permitted
[apl@localhost foo]$ rm pete
rm: remove write-protected regular file `pete'? y
rm: cannot remove `pete': Operation not permitted

Having write permission makes rm proceed without caution, only to be brought up short by the restrictions of the "t" bit.

mount --bind

If the problem is removal of a directory and it is not terribly inconvenient for you to have that directory actually be on a separate filesystem, then "mount" can make the directory safe from removal. You can read more at mount --bind, but it's not very complicated. Let's say we have /dev/foo mounted at /foo and I want a "link" to that under /home/fred. All I have to do is:

Fred can have full write permissions on /foo if he needs it, but he will not be able to remove /home/fred/foo. Not even root can:

# rm -rf /home/fred/foo
rm: cannot remove directory '/home/fred/foo': Device or resource busy

Now THAT is removal protection!

ACL's

Typically, ACL's let you avoid complicated groups by setting specific permissions for specific users. Other than setting a file as "immutable" (chattr +i filename on some Linuxes), you really can't prevent removal of a file. Of course setting it that way may also make it useless, as even the owner can't modify or remove it either without doing "chattr -i" first.. See ACL's for more on that.

[pete@localhost foo]$ id
uid=501(pete) gid=501(pete) groups=501(pete)
[pete@localhost foo]$ chattr +i pete
[pete@localhost foo]$ rm -f pete
rm: cannot remove `pete': Operation not permitted
[pete@localhost foo]$ mv pete /tmp/
mv: cannot move `pete' to `/tmp/pete': Operation not permitted
[pete@localhost foo]$ 
 

Macworld Mac Security Superguide $9.99

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