linux-kernel - Re: [GIT PULL] x86 fixes

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Date:	Fri, 14 Aug 2015 12:18:11 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...nel.org>, Juergen Gross <jgross@...e.com>,
	Andy Lutomirski <luto@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Brian Gerst <brgerst@...il.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] x86 fixes

On Fri, Aug 14, 2015 at 12:06 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Fri, Aug 14, 2015 at 11:57 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>>
>> That code seems fine to me (and explicitly errors out when it's not in
>> the LDT). FPU_CS is actually the CS selector value.
>>
>> So testing that for being in the LDT by checking bit #2, and then
>> using FPU_get_ldt_descriptor() on it actually seems *correct*.
>>
>> It's the actual instruction data segment handling that looks entirely
>> broken, and was explicitly made *more* broken by that commit.
>
> Note that in practice, it's *probably* true that if CS ends up being
> in the LDT (so we're running something odd like Wine), then *probably*
> the data segments are going to be in the LDT too. So the old code that
> unconditionally looked things up in the LDT probably worked in
> practice, even if it was wrong.
>
> The new code cannot *possibly* work at all, because even if the data
> segment register is in the LDT, it uses the wrong thing to look up the
> LDT entry, so it will get the wrong base.
>
> But as mentioned, it will only *matter* on something like a 486SX, and
> only when the whole "CS/DS didn't match the default flat segments"
> case triggers, so not only do you have to run on a 486SX, you will
> have to run something like Wine on it. So it sounds very very unlikely
> that this bug matters in practice.

Unless I'm missing something, it's literally a one-line fix -- just
put the missing PM_REG_(segment) back in.

--Andy
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