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Date: Thu, 21 Feb 2019 09:37:52 -0800
From: Kees Cook <>
To: Sean Christopherson <>
Cc: Thomas Gleixner <>, Jann Horn <>, 
	Dominik Brodowski <>, LKML <>, 
	Kernel Hardening <>, X86 ML <>
Subject: Re: [PATCH v2] x86/asm: Pin sensitive CR4 bits

On Thu, Feb 21, 2019 at 9:33 AM Sean Christopherson
<> wrote:
> On Wed, Feb 20, 2019 at 10:09:34AM -0800, Kees Cook wrote:
> > +     if (WARN_ONCE((val & cr4_pin) != cr4_pin, "cr4 bypass attempt?!\n"))
> Printing what bits diverged would be helpful in the unlikely event that the
> WARN_ONCE triggers.  "cr4 bypass attempt" is probably only meaningful to
> people that are already familiar with the code, e.g.:
>         if (WARN_ONCE((val & cr4_pin) != cr4_pin,
>             "Attempt to unpin cr4 bits: %lx, cr4 bypass attack?!", ~val & cr4_pin))

Ah yeah, I like it. I'll send a v3. Thanks!

Kees Cook

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