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SubjectRe: Intel microcode fixes [OFF-TOPIC]
On 20 Nov 1998, H. Peter Anvin wrote:

> Followup to: <Pine.LNX.4.05.9811192328200.6229-100000@tahallah.demon.co.uk>
> By author: Alex Buell <alex.buell@tahallah.demon.co.uk>
> In newsgroup: linux.dev.kernel
> >
> > > EGADS! Is there NO security on the processor microcode? Could this be
> > > exploited to do evil narsty things?
> >
> > Yeah. #1 Exploit: Resurrect the F00F bug. #2 Exploit: Infamous Pentium
> > floating point bug. The list is endless.
> >
>
> Actually, it is *believed* that the microcode upload contains some
> kind of security checking, in that it will reject code that isn't
> properly "blessed." If that is so is hard to say for certain.
>
> -hpa
>

I hear that the "writable control store" was the patent issue that
Intel had with Digital. Digital had an early patent and, in fact,
many VAXEN (730, 750, 785) were very dumb after a complete power-failure.
The microcode necessary to make the CPU Board into a CPU was uploaded
during startup by the console 8085 via the console tape that always
had to be present.

Intel and Digital kissed and made up with some technology swap.

The rest of the story says that, upon power-up, in real mode, it
is possible to update the new Pentium's WCS by writing the microcode
word-by-word to a register (don't know which), while manipulating
a bit in CR0. The bit (don't know which) acts like a transfer pump
and has to be set/reset for each word transferred.

Since it is possible to change the mode of the CPU from 32-bit
protected mode, to 16-bit real-mode, then back without screwing
anything up (you need a 16-bit descriptor), it is possible that
if Intel publishes the procedure and supplies the microcode, any
machine can have the capability of upgrading the CPU's microcode.

But but not to worry, such a transfer can only occur in kernel mode.
Real mode is not the same as Virtual 86 mode which is used to
emulate the DOS environment.

This is a GoodThing(tm). This means that, in the future, if a bad
CPU bug is found, it can be fixed.

Cheers,
Dick Johnson
***** FILE SYSTEM WAS MODIFIED *****
Penguin : Linux version 2.1.128 on an i686 machine (400.59 BogoMips).
Warning : It's hard to remain at the trailing edge of technology.


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