Date: Mon, 17 Aug 2020 23:37:50 +0200 From: Solar Designer <solar@...nwall.com> To: lkrg-users@...ts.openwall.com Cc: Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...e.de>, Kees Cook <keescook@...omium.org> 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 <tglx@...utronix.de> > 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 <tglx@...utronix.de> > Signed-off-by: Borislav Petkov <bp@...e.de> > Link: https://firstname.lastname@example.org > > 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: https://github.com/openwall/lkrg/issues/1 (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. Alexander
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