linux-kernel - Re: [PATCH] x86: Use entire page for the per-cpu GDT only if paravirt-enabled

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Message-ID: <CALCETrUqscPpX4oRt1+_3uco22oGOagySSjV3QZMJCrxiw_khQ@mail.gmail.com>
Date:	Tue, 29 Sep 2015 11:02:24 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Vrabel <david.vrabel@...rix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Brian Gerst <brgerst@...il.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Gleb Natapov <gleb@...nel.org>,
	Kees Cook <keescook@...omium.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Joerg Roedel <joro@...tes.org>, X86 ML <x86@...nel.org>,
	kvm list <kvm@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86: Use entire page for the per-cpu GDT only if paravirt-enabled

On Tue, Sep 29, 2015 at 10:50 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Tue, Sep 29, 2015 at 1:35 PM, Andy Lutomirski <luto@...capital.net> wrote:
>>
>> Does anyone know what happens if you stick a non-accessed segment in
>> the GDT, map the GDT RO, and access it?
>
> You should get a #PF, as you guess, but go ahead and test it if you
> want to make sure.
>

Then I think that, if we do this, the patch series should first make
it percpu and fixmapped but RW and then flip it RO as a separate patch
in case we need to revert the actual RO bit.  I don't want to break
Wine or The Witcher 2 because of this, and we might need various
fixups.  I really hope that no one uses get_thread_area to check
whether TLS has been accessed.

--Andy
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