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Re: Announcement: Solaris loadable kernel module backdoor
From: kaos () OCS COM AU (Keith Owens)
Date: Thu, 23 Dec 1999 11:36:22 +1100


On Tue, 21 Dec 1999 14:33:50 -0800,
pedward () WEBCOM COM wrote:
At boot, compile the list of modules that are 'known good' (for the sake
of argument, it's the /lib/modules/x.y.z), then write the list, with
MD5 checksums, to a write once /proc interface to kmod.

kmod would check the MD5 sum before loading the requested module, if it didn't
match the in-kernel list, don't allow it.

kmod does not load modules.  It starts a kernel thread and invokes
modprobe.  modprobe runs /etc/modules.conf and the the dependency chain
then loads anywhere between zero and n modules.  All of this work is in
user space and it is all outside kernel control.

For the really paranoid, at compile time you could tar up all the modules and
create the MD5 sum of that, store it in the kernel at some spot, and modify
the module utils to read tarfiles.  If the MD5 sum of the tarfile doesn't
match the compiled in list, you can't load the module.

At which point crackers just trojan the modprobe and insmod binaries.
A chain is only as secure as its weakest link.

Any other ideas on preventing untrusted modules from being loaded or replaced
and loaded as an existing 'trusted' module?

As long as any OS allows root to run user space utilities to modify the
OS, the weak link is the lack of kernel support for checking that *all*
binaries run by a privileged user are secure.  There is no point
building checks into modutils when any cracker running as root can run
their own binary or LD_PRELOAD an existing binary.

The day that you can boot a Linux kernel and stop root from changing
anything and stop them using tricks like LD_PRELOAD, that is the day
you can start trusting your user space utilities.  And when you have a
system like that, modutils will be changed to verify modules before
loading.  Of course by then it is probably irrelevant, a system which
is that secure will not let anyone create rogue modules in the first
place because they cannot update the filesystem.

Adding a check to modutils without securing root's environment only
raises the bar 0.5 cm.  Any half competent cracker will switch to
trojan modutil binaries and bypass the check.  IMHO, building checksum
support into modutils is fake security.

Keith Owens, modutils maintainer.


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