Re: SSE related security hole

Pavel Machek
2002-04-21T21:12:23Z

Hi!

> > Um, people here seem to be assuming that, in the absence of MMX,
> > fninit *doesn't* leak information.
> >
> > I thought it was well-known to just clear (set to all-ones) the
> > tag register and not alter the actual floating-point registers.
> >
> > Thus, it seems quite feasible to reset the tag word with FLDENV and
> > store out the FPU registers, even on an 80387.
> >
> > Isn't this the same security hole? Shouldn't there be 8 FLDZ instructions
> > (or equivalent) in the processor state initialization?
>
> Well, if what's on the internal stack of the FPU can actually leak
> information, I think the notion of "leak" has expanded just a bit
> too much.
>
> A rogue process could not even know what instruction was about to
> be executed, nor what the previous instruction was, nor when since
> boot it was executed, nor by whom. The 'data' associated with those

If fpu unit was used to memcpy your .ssh/identity, well, you might
change your mind.
Pavel
--
(about SSSCA) "I don't say this lightly. However, I really think that the U.S.
no longer is classifiable as a democracy, but rather as a plutocracy." --hpa