linux-kernel - Re: [PATCH] x86: entry_32.S: change ESPFIX test to not touch PT

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Message-ID: <CA+55aFyVh=bLtC+vVvZ4id09gn0tvGPSXwEyNdzGwLNTsDKJQA@mail.gmail.com>
Date:	Mon, 9 Mar 2015 11:16:03 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.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>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: entry_32.S: change ESPFIX test to not touch PT_OLDSS(%esp)

On Mon, Mar 9, 2015 at 11:04 AM, Andy Lutomirski <luto@...capital.net> wrote:
>
> One option would be to change the NMI entry code to move itself down 8
> bytes if this happens (came from kernel mode or sp == sp0 - 12,
> perhaps).

Hmm. That whole code currently depends on the stack setup being just a
single instruction (the move to esp). And that simplifies things, I'd
like to keep it that way.

I'd *much* rather just keep the 8-byte padding. What was so
problematic with that? It worked. It's been around forever. Removing
it is the bug.

                        Linus
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