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

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Message-ID: <CA+55aFxQfG09Hn33iTDt2L5iO5AiqTrXFODy-A+wf_xdpPoVKw@mail.gmail.com>
Date:	Tue, 29 Sep 2015 13:50:56 -0400
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andy Lutomirski <luto@...capital.net>
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 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.

We do something very similar for the old Pentium F0 0F bug - we mark
the IDT read-only, which causes the (bogus) locked read of the IDT
entry that the F00F bug resulted in to be caught as a page fault
instead.

              Linus
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