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SubjectRe: immutable flag on ext2fs


On Mon, 21 Jun 1999, Mike A. Harris wrote:

> >FreeBSD does it through single user mode, which I beleive is inappropriate
> >since you may want to make system file changes on the fly.
>
> Single user mode doesn't have networking up, so there should only
> be local consoles to begin with. Or is FreeBSD's single user
> mode different? Granted, one can edit the initscripts to do

It's not single-user. Quoth the Daemon Book:

>>>Security levels are defined as follows:

-1. Permanently insecure mode: Always run system in level 0 mode (must be
compiled into the kernel).

0. Insecure mode: Immutable and append-only flags may be turned off. All
devices can be read or written, subject to their permissions.

1. Secure mode: The superuser-settable immutable and append-only flags cannot
be cleared; disks for mounted filesystems and kernel memory (/dev/mem and
/dev/kmem) are read-only.

2. Highly secure mode: This mode is the same as secure mode, except that
disks are always read-only whether mounted or not. This level precludes
even a superuser from tampering with filesystems by unmounting them, but
also inhibits formatting of new filesystems.

Normally the system runs with level 0 security while in single-user mode,
and with level 1 security while in multiuser mode. If level 2 security is
desired while the system is running in multiuser mode, it should be set in
/etc/rc startup script<<<

superuser-settable flags are not supported in Linux right now. The idea
being: there are two immutable flags - user and root. For VFS they are the
same (i.e. presense of any makes file immutable). user flag can be
set/reset by owner of file and by root. root flag can be changed *only* by
root and only if securelevel < 1. In other words, immutable is available
for users, but root can additionally set stronger variant that can't be
revoked without rebooting into insecure mode. The same goes for
append-only. *Good* idea, but unfortunately ext2 doesn't support it.

> whatever, but I can't see it making a machine more secure. Spend
> the time putting up a proper firewall, and using "good" security
> techniques instead. Security through obscurity is none at all.

It is not a security through obscurity. It doesn't close all holes (as any
other measure), but it's a useful additional tool.


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