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Date: Thu, 5 Jan 2017 10:01:21 -0800
From: Andy Lutomirski <>
To: Thomas Garnier <>, Paolo Bonzini <>
Cc: Thomas Gleixner <>, Ingo Molnar <>, 
	"H . Peter Anvin" <>, Kees Cook <>, Borislav Petkov <>, 
	Andy Lutomirski <>, Dave Hansen <>, Chen Yucong <>, 
	Arjan van de Ven <>, 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 9:54 AM, Thomas Garnier <> wrote:
> On Thu, Jan 5, 2017 at 9:51 AM, Andy Lutomirski <> wrote:
>> On Wed, Jan 4, 2017 at 2:16 PM, Thomas Garnier <> wrote:
>>> Each processor holds a GDT in its per-cpu structure. The sgdt
>>> instruction gives the base address of the current GDT. This address can
>>> be used to bypass KASLR memory randomization. With another bug, an
>>> attacker could target other per-cpu structures or deduce the base of the
>>> main memory section (PAGE_OFFSET).
>>> In this change, a space is reserved at the end of the memory range
>>> available for KASLR memory randomization. The space is big enough to hold
>>> the maximum number of CPUs (as defined by setup_max_cpus). Each GDT is
>>> mapped at specific offset based on the target CPU. Note that if there is
>>> not enough space available, the GDTs are not remapped.
>> Can we remap it read-only?  I.e. use PAGE_KERNEL_RO instead of
>> PAGE_KERNEL.  After all, the ability to modify the GDT is instant
>> root.
> That's my goal too. I started by doing a RO remap and got couple
> problems with hibernation. I can try again for the next iteration or
> delay it for another patch. I also need to look at KVM GDT usage, I am
> not familiar with it yet.

If you want a small adventure, I think a significant KVM-related
performance improvement is available.  Specifically, on VMX exits, the
GDT limit is hardwired to 0xffff (IIRC -- I could be remembering the
actual vaue wrong).  KVM does LGDT to fix it.

If we actually made the GDT have limit 0xffff (presumably by mapping
the zero page a few times to pad it out without wasting memory), then
we would avoid the LGDT.  LGDT is incredibly slow, so this would be a
big win. Want to see if you can make this work with your patch set?

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