Re: SSE related security hole

["H. Peter Anvin" <hpa@zytor.com>]

Followup to:  <20020418183639.20946.qmail@science.horizon.com>
By author:    linux@horizon.com
In newsgroup: linux.dev.kernel
>
> 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?
> 

Perhaps the right thing to do is to have a description in data of the
desired initialization state and just F[NX]RSTOR it?

	-hpa
-- 
<hpa@transmeta.com> at work, <hpa@zytor.com> in private!
"Unix gives you enough rope to shoot yourself in the foot."
http://www.zytor.com/~hpa/puzzle.txt	<amsp@zytor.com>