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Date: Mon, 17 Aug 2020 23:37:50 +0200
From: Solar Designer <>
Cc: Thomas Gleixner <>, Borislav Petkov <>,
	Kees Cook <>
Subject: Re: lkrg: Fix build LKRG for kernel 5.8+

On Wed, Aug 05, 2020 at 08:48:00PM +0200, Solar Designer wrote:
> On Wed, Aug 05, 2020 at 07:46:48PM +0200, Mikhail Morfikov wrote:
> > DKMS make.log for lkrg-0.8.1+git20200805 for kernel 5.8.0-amd64 (x86_64)
> [...]
> > ERROR: modpost: "native_write_cr4" [/var/lib/dkms/lkrg/0.8.1+git20200805/build/p_lkrg.ko] undefined!
> Here's what this is about:
> commit 21953ee5013d6632bee90ec89f2df59c69050db0
> Author: Thomas Gleixner <>
> Date:   Sun Apr 26 18:55:15 2020 +0200
>     x86/cpu: Export native_write_cr4() only when CONFIG_LKTDM=m
>     Modules have no business poking into this but fixing this is for later.
>      [ bp: Carve out from an earlier patch. ]
>     Signed-off-by: Thomas Gleixner <>
>     Signed-off-by: Borislav Petkov <>
>     Link:
> where LKTDM is "Linux Kernel Dump Test Module".
> We use native_write_cr4() via its wrapper __write_cr4() (in a kernel
> header file) to restore SMEP and/or SMAP if these are unexpectedly
> disabled yet kernel panic on this event is not enabled in LKRG
> configuration (by default it is enabled).
> We'll need to find a way around this.

We believe we've worked around the issue:

(And yes, we're just starting to use GitHub issues now that we've moved
the project to GitHub.)

Mikhail (or/and anyone else), can you please test and confirm?

> Perhaps will just access the register directly from our own inline asm.

We ended up looking up the address of native_write_cr4 in a hackish way,
not to introduce extra CR4-writing gadgets.


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