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

From: "H. Peter Anvin" <hpa@zytor.com>
To: Andy Lutomirski <luto@amacapital.net>,
	Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ingo Molnar <mingo@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Denys Vlasenko <dvlasenk@redhat.com>,
	Borislav Petkov <bp@alien8.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	David Vrabel <david.vrabel@citrix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	Brian Gerst <brgerst@gmail.com>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Gleb Natapov <gleb@kernel.org>, Kees Cook <keescook@chromium.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Joerg Roedel <joro@8bytes.org>, X86 ML <x86@kernel.org>,
	kvm list <kvm@vger.kernel.org>
Subject: Re: [PATCH] x86: Use entire page for the per-cpu GDT only if paravirt-enabled
Date: Tue, 29 Sep 2015 13:30:52 -0700	[thread overview]
Message-ID: <560AF4FC.3000306@zytor.com> (raw)
In-Reply-To: <CALCETrUqscPpX4oRt1+_3uco22oGOagySSjV3QZMJCrxiw_khQ@mail.gmail.com>

On 09/29/2015 11:02 AM, Andy Lutomirski wrote:
> On Tue, Sep 29, 2015 at 10:50 AM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
>> On Tue, Sep 29, 2015 at 1:35 PM, Andy Lutomirski <luto@amacapital.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.
> 

Of course.

	-hpa