linux-kernel - Re: [PATCH v2 11/39] x86/ibt,kvm: Add ENDBR to fastops

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Date:   Fri, 25 Feb 2022 11:24:36 +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, mbenes@...e.cz, rostedt@...dmis.org,
        mhiramat@...nel.org, alexei.starovoitov@...il.com
Subject: Re: [PATCH v2 11/39] x86/ibt,kvm: Add ENDBR to fastops

On Thu, Feb 24, 2022 at 04:54:04PM -0800, Kees Cook wrote:
> On Thu, Feb 24, 2022 at 03:51:49PM +0100, Peter Zijlstra wrote:
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> >  arch/x86/kvm/emulate.c |    6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > --- a/arch/x86/kvm/emulate.c
> > +++ b/arch/x86/kvm/emulate.c
> > @@ -189,7 +189,7 @@
> >  #define X16(x...) X8(x), X8(x)
> >  
> >  #define NR_FASTOP (ilog2(sizeof(ulong)) + 1)
> > -#define FASTOP_SIZE 8
> > +#define FASTOP_SIZE (8 * (1 + HAS_KERNEL_IBT))
> 
> Err, is this right? FASTOP_SIZE is used both as a size and an alignment.
> But the ENDBR instruction is 4 bytes? Commit log maybe needed to
> describe this.

Note how that comes out as 8*1 or 8*2, iow 8 or 16. Does that clarify?
That is, 8+4 being 12 is ovbiuosly a fail for alignment.