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

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Date:	Mon, 9 Mar 2015 08:09:56 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Denys Vlasenko <vda.linux@...glemail.com>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>, 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] x86: entry_32.S: change ESPFIX test to not touch PT_OLDSS(%esp)

On Mon, Mar 9, 2015 at 8:00 AM, Denys Vlasenko <vda.linux@...glemail.com> wrote:
> On Mon, Mar 9, 2015 at 3:18 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> Do we actually need espfix on all returns to vm86 mode?
>
> No, the current code (and my new version) does *not* do
> espfix for vm86. It's not needed (apparently).
>
>>> +       btl     $X86_EFLAGS_VM_BIT,PT_EFLAGS(%esp)
>>> +       jc      restore_nocheck         # VM set, not it
>>
>> This seems useless.  In vm86 mode, espfix should work fine (even if
>> pointlessly), CS won't have the two low bits set, and SS won't
>> reference the LDT because it's not a selector at all.
>
> You seem to suggest we can drop VM flag test.
>
> If we do that, the tests for CS and SS will work on bogus data.
> I.e. they will semi-randomly rouse execution through espfix.
>

Mmm, right.  My bad, that test is needed.

> Which will probably work correctly, but IIRC espfix does crazy stuff
> which is likely to be slow.
>
> What we definitely should do here is at least frame this check with
> "#ifdef CONFIG_VM86".
>
>> That being said, what ends up in the high bits of esp when we iret to
>> vm86 mode?
>
> I don't know. I guess it's time to write an actual vm86 testcase :)

Ick.  I can try...

Anyway, you've convinced me that your patch is good.  I queued it up.

-- 
Andy Lutomirski
AMA Capital Management, LLC
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