linux-kernel - Re: [PATCH 15/29] x86: Disable IBT around firmware

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Date:   Mon, 21 Feb 2022 14:22:37 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     x86@...nel.org, joao@...rdrivepizza.com, hjl.tools@...il.com,
        jpoimboe@...hat.com, andrew.cooper3@...rix.com,
        linux-kernel@...r.kernel.org, ndesaulniers@...gle.com,
        samitolvanen@...gle.com, mark.rutland@....com,
        alyssa.milburn@...el.com
Subject: Re: [PATCH 15/29] x86: Disable IBT around firmware

On Mon, Feb 21, 2022 at 11:06:15AM +0100, Peter Zijlstra wrote:
> 
> Could you trim replies so that I can actually find what you write?
> 
> On Mon, Feb 21, 2022 at 12:27:20AM -0800, Kees Cook wrote:
> 
> > >+#ifdef CONFIG_X86_IBT
> > >+
> > >+u64 ibt_save(void)
> > >+{
> > >+	u64 msr = 0;
> > >+
> > >+	if (cpu_feature_enabled(X86_FEATURE_IBT)) {
> > >+		rdmsrl(MSR_IA32_S_CET, msr);
> > >+		wrmsrl(MSR_IA32_S_CET, msr & ~CET_ENDBR_EN);
> > >+	}
> > >+
> > >+	return msr;
> > >+}
> > >+
> > >+void ibt_restore(u64 save)
> > 
> > Please make these both __always_inline so there no risk of them ever gaining ENDBRs and being used by ROP to disable IBT...
> 
> Either that or mark them __noendbr. The below seems to work.
> 
> Do we have a preference?

The inline thing runs into header hell..