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

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Date:	Tue, 29 Sep 2015 18:31:43 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	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>
Subject: Re: [PATCH] x86: Use entire page for the per-cpu GDT only if
 paravirt-enabled

On 09/29/2015 06:20 PM, Eric W. Biederman wrote:
> Linus Torvalds <torvalds@...ux-foundation.org> writes:
> 
>> 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.
> 
> I tested this by accident once when workinng on what has become known
> as coreboot.  Early in boot with your GDT in a EEPROM switching from
> real mode to 32bit protected mode causes a write and locks up the
> machine when the hardware declines the write to the GDT to set the
> accessed bit.  As I recall the write kept being retried and retried and
> retried...
> 
> Setting the access bit in the GDT cleared up the problem and I did not
> look back.
> 
> Way up in 64bit mode something might be different, but I don't know why
> cpu designeres would waste the silicon.
> 

This is totally different from a TLB violation.  In your case, the write
goes through as far as the CPU is concerned, but when the data is
fetched back, it hasn't changed.  A write to a TLB-protected location
will #PF.

	-hpa


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