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From: Andy Lutomirski <luto@amacapital.net>
Date: Mon, 9 Mar 2015 08:09:56 -0700
Message-ID: <CALCETrWE4xfHuhtQ4K=dhevgrY+ThHiARZX0gg2X8nw3ngCT_w@mail.gmail.com>
Subject: Re: [PATCH] x86: entry_32.S: change ESPFIX test to not touch PT_OLDSS(%esp)
To: Denys Vlasenko <vda.linux@googlemail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@kernel.org>,
        Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Alexei Starovoitov <ast@plumgrid.com>, Will Drewry <wad@chromium.org>,
        Kees Cook <keescook@chromium.org>, X86 ML <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On Mon, Mar 9, 2015 at 8:00 AM, Denys Vlasenko <vda.linux@googlemail.com> wrote:
> On Mon, Mar 9, 2015 at 3:18 PM, Andy Lutomirski <luto@amacapital.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