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

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Message-ID: <CALCETrVLSgayUzP4nrjXVjGwcFQ=Ue6OvQkKHRio_y8Y0oWLVA@mail.gmail.com>
Date:	Mon, 9 Mar 2015 11:36:14 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
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 11:16 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> 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.

Let's at least fix it, then.  processor.h has:

#define INIT_TSS  {                              \
    .x86_tss = {                              \
        .sp0        = sizeof(init_stack) + (long)&init_stack, \

(moved in -tip)

That's bogus, and this bogosity is why I broke 32-bit -next in the
first place: I assumed it was correct.

I'll get it if no one beats me to it.

--Andy

>
>                         Linus



-- 
Andy Lutomirski
AMA Capital Management, LLC
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