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Date: Fri, 6 Jan 2017 07:49:00 +0100
From: Ingo Molnar <>
To: Thomas Garnier <>
Cc: Andy Lutomirski <>,
	Andy Lutomirski <>,
	Arjan van de Ven <>,
	Thomas Gleixner <>,
	Ingo Molnar <>, "H . Peter Anvin" <>,
	Kees Cook <>, Borislav Petkov <>,
	Dave Hansen <>, Chen Yucong <>,
	Paul Gortmaker <>,
	Andrew Morton <>,
	Masahiro Yamada <>,
	Sebastian Andrzej Siewior <>,
	Anna-Maria Gleixner <>,
	Boris Ostrovsky <>,
	Rasmus Villemoes <>,
	Michael Ellerman <>,
	Juergen Gross <>,
	Richard Weinberger <>, X86 ML <>,
	"" <>,
	"" <>
Subject: Re: [RFC] x86/mm/KASLR: Remap GDTs at fixed location

* Thomas Garnier <> wrote:

> >> Not sure I fully understood and I don't want to miss an important point. Do 
> >> you mean making GDT (remapping and per-cpu) read-only and switch the 
> >> writeable flag only when we write to the per-cpu entry?
> >
> > What I mean is: write to the GDT through normal percpu access (or whatever the 
> > normal mapping is) but load a read-only alias into the GDT register.  As long 
> > as nothing ever tries to write through the GDTR alias, no page faults will be 
> > generated.  So we just need to make sure that nothing ever writes to it 
> > through GDTR.  AFAIK the only reason the CPU ever writes to the address in 
> > GDTR is to set an accessed bit.
> A write is made when we use load_TR_desc (ltr). I didn't see any other yet.

Is this write to the GDT, generated by the LTR instruction, done unconditionally 
by the hardware?



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