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From: Andy Lutomirski <luto@amacapital.net>
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>
Subject: Re: [PATCH] x86: entry_32.S: change ESPFIX test to not touch PT_OLDSS(%esp)
Date: Mon, 9 Mar 2015 08:09:56 -0700	[thread overview]
Message-ID: <CALCETrWE4xfHuhtQ4K=dhevgrY+ThHiARZX0gg2X8nw3ngCT_w@mail.gmail.com> (raw)
In-Reply-To: <CAK1hOcOtCuuFJoCmR_T=K9=g-i0cqmu4QYoQJ6C2Utg6wbNf4A@mail.gmail.com>

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

  reply	other threads:[~2015-03-09 15:10 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-03-09 14:05 Denys Vlasenko
2015-03-09 14:18 ` Andy Lutomirski
2015-03-09 15:00   ` Denys Vlasenko
2015-03-09 15:09     ` Andy Lutomirski [this message]
2015-03-09 19:31       ` Denys Vlasenko
2015-03-09 15:13     ` Ingo Molnar
2015-03-09 15:18       ` Andy Lutomirski
2015-03-09 15:47       ` Steven Rostedt
2015-03-09 15:54         ` Ingo Molnar
2015-03-09 16:08 ` Linus Torvalds
2015-03-09 16:28   ` Denys Vlasenko
2015-03-09 16:44     ` Linus Torvalds
2015-03-09 17:44       ` H. Peter Anvin
2015-03-09 19:13         ` Andy Lutomirski
2015-03-09 19:26           ` H. Peter Anvin
2015-03-09 19:51             ` Andy Lutomirski
2015-03-09 17:42   ` H. Peter Anvin
2015-03-09 17:45     ` Andy Lutomirski
2015-03-09 17:59       ` Linus Torvalds
2015-03-09 18:04         ` Andy Lutomirski
2015-03-09 18:16           ` Linus Torvalds
2015-03-09 18:32             ` Denys Vlasenko
2015-03-09 18:36             ` Andy Lutomirski
2015-03-10  6:25               ` Ingo Molnar

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