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

From: Peter Zijlstra
Date: Mon Feb 21 2022 - 08:23:04 EST


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..