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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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