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SubjectRe: [OFFTOPIC] Re: /dev/nvram on my Celebris
On Fri, 27 Nov 1998, Neil Conway wrote:

> Rafael Reilova wrote:
> >
> > On Thu, 26 Nov 1998, Neil Conway wrote:
> >
> > > On this (bios destruction by rogue root) thread, someone already
> > > suggested that a reboot wasn't necessary.
> > >

[...]
> Well, if we were *really* bothered about this, we could surely hardwire
> the kernel never to write to certain pages, in such a way as to require

This will be hard. You'll have to search the sources for all places where
*root* might trick the kernel into writing to the BIOS flashing registers.
Think of purposely misconfiguring a driver for instance.

> a reboot with a new kernel or at least a module to be loaded. Even root
> can only get certain permissions by either loading modules (which get
> priv mode in the chip) or by using device files for access to existing
> drivers. Or I could still be talking rubbish. Couldn't
> /dev/{mem|kmem|kcore} be hardwired to prevent BIOS flashing ?
>
> It's easy to prevent modules being loaded: don't compile the kernel with
> support. It's also easy to prevent rebooting with a new kernel...
>

Sure, you can start disabling all those "pesky features" that may be
misused by root... Let's see, you would need to disable the above /dev/
files, the proc FS, raw access to block devices (to really prevent booting
with a new kernel), module loading (as you said), the ioperm syscall...
did I miss anything? In the end, the resulting system will lose a lot of
functionality (i.e. no Xserver or even fsck).

> N
> ps: naturally, we probably don't give a damn anyway, as the machine
> would already be totally compromised, but it could save a few quid if
> the BIOS were still intact after the attack.

It might be possible, but because un*x assumes that root really knows what
he's doing (which BTW is a good thing TM), there are too many holes to
plug. You can of course try to make it really hard, with the
understanding that a sufficiently knowledgeable rogue root can circunvent
most (if not all) your security measures.

My recomendation for those losing sleep over some rogue root flashing his
production server BIOS into oblivion is to replace the EPROM with a
non-flashable version, or given the chip specs, physicaly cut out the
pin(s) used for flashing.

Cheers,

Rafael


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