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Date: Thu, 5 Jan 2017 18:34:49 -0800
From: Andy Lutomirski <>
To: Linus Torvalds <>
Cc: Andy Lutomirski <>, Thomas Garnier <>, 
	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

On Thu, Jan 5, 2017 at 3:05 PM, Linus Torvalds
<> wrote:
> On Thu, Jan 5, 2017 at 12:18 PM, Andy Lutomirski <> wrote:
>> Hmm.  I bet that if we preset the accessed bits in all the segments
>> then we don't need it to be writable in general.
> I'm not sure that this is architecturally safe.

Hmm.  Last time I looked, I couldn't find *anything* in the SDM
explaining what happened if a GDT access resulted in a page fault.  I
did discover that Xen intentionally (!) lazily populates and maps LDT
pages.  An attempt to access a not-present page results in #PF with
the error cod e indicating kernel access even if the access came from
user mode.

SDM volume 3 7.2.2 says "Pages corresponding to the previous task’s
TSS, the current task’s TSS, and the descriptor table entries for
each all should be marked as read/write."  But I don't see how a CPU
implementation could possibly care what the page table for the TSS
descriptor table entries says after LTR is done because the CPU isn't
even supposed to *read* that memory.

OTOH a valid implementation could easily require that the page table
says that the page is writable merely to load a segment, especially in
weird cases (IRET?).  That being said, this is all quite easy to test.

Also, Thomas, why are you creating a new memory region?  I don't see
any benefit to randomizing the GDT address.  How about just putting it
in the fixmap?  This  would be NR_CPUS * 4 pages if do my limit=0xffff
idea.  I'm not sure if the fixmap code knows how to handle this much

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