linux-kernel - Re: [PATCH 1/4] x86: entry.S: tidy up several suboptimal insns

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Date:	Wed, 25 Feb 2015 01:27:25 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	Andy Lutomirski <luto@...capital.net>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] x86: entry.S: tidy up several suboptimal insns

On 02/25/2015 01:20 AM, Ingo Molnar wrote:
>
> I think the fundamental fragility is that we allow the high
> 32 bits to be nonzero.
>
> So could we just zap the high 32 bits of RAX early in the
> entry code, and then from that point on we could both use
> 32-bit ops and won't have to remember the possibility
> either?
>

We do that, but people keep "optimizing" the zero extend away.  We have 
had this cause a wide-open security hole twice already.  So the extra 
REX prefix is a cheap cost to avoid this happen again.

	-hpa
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