linux-kernel - Re: [PATCH v2 18/39] x86/ibt: Add IBT feature, MSR and #CP handling

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Date:   Tue, 1 Mar 2022 15:13:03 -0800
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "hjl.tools@...il.com" <hjl.tools@...il.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "joao@...rdrivepizza.com" <joao@...rdrivepizza.com>,
        "Cooper, Andrew" <andrew.cooper3@...rix.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "samitolvanen@...gle.com" <samitolvanen@...gle.com>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "alexei.starovoitov@...il.com" <alexei.starovoitov@...il.com>,
        "Milburn, Alyssa" <alyssa.milburn@...el.com>,
        "mhiramat@...nel.org" <mhiramat@...nel.org>,
        "mbenes@...e.cz" <mbenes@...e.cz>,
        "ndesaulniers@...gle.com" <ndesaulniers@...gle.com>
Subject: Re: [PATCH v2 18/39] x86/ibt: Add IBT feature, MSR and #CP handling

On Tue, Mar 01, 2022 at 10:02:45PM +0100, Peter Zijlstra wrote:
> On Tue, Mar 01, 2022 at 04:14:42PM +0100, Peter Zijlstra wrote:
> 
> > Something like so?
> > 
> > diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
> > index 399f075ccdc4..5b65f6ec5ee6 100644
> > --- a/arch/x86/kernel/relocate_kernel_64.S
> > +++ b/arch/x86/kernel/relocate_kernel_64.S
> > @@ -114,6 +114,14 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
> >  	/* store the start address on the stack */
> >  	pushq   %rdx
> >  
> > +	/*
> > +	 * Clear X86_CR4_CET (if it was set) such that we can clear CR0_WP
> > +	 * below.
> > +	 */
> > +	movq	%cr4, %rax
> > +	andq	$~(X86_CR4_CET), %rax
> > +	movq	%rax, %cr4
> > +
> >  	/*
> >  	 * Set cr0 to a known state:
> >  	 *  - Paging enabled
> 
> I *think* it worked, I 'apt install kexec-tools' and copied the magic
> commands Josh gave over IRC and the machine went and came back real
> quick.
> 
> Lacking useful console I can't say much more.
> 
> I pushed out a version with these things on.

I just used your latest git tree, kexec into a non-IBT kernel worked for
me as well.

-- 
Josh