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Date:   Tue, 18 Oct 2022 16:31:48 -0700
From:   Josh Poimboeuf <jpoimboe@kernel.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Kees Cook <keescook@chromium.org>, x86@kernel.org,
        Sami Tolvanen <samitolvanen@google.com>,
        Joao Moreira <joao@overdrivepizza.com>,
        linux-kernel@vger.kernel.org, Mark Rutland <mark.rutland@arm.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>
Subject: Re: [PATCH] x86/ibt: Implement FineIBT
Message-ID: <20221018233148.lgoiis2tws7caw3l@treble>
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On Tue, Oct 18, 2022 at 09:56:36PM +0200, Peter Zijlstra wrote:
> On Tue, Oct 18, 2022 at 11:09:13AM -0700, Kees Cook wrote:
> > > +config FINEIBT
> > > +	def_bool y
> > > +	depends on X86_KERNEL_IBT && CFI_CLANG
> > > +	select CALL_PADDING
> > 
> > To that end, can we please make this a prompted choice?
> 
> How about something like so instead?
> 
> ---
> Subject: x86/cfi: Boot time selection of CFI scheme
> From: Peter Zijlstra <peterz@infradead.org>
> Date: Tue Oct 18 21:50:54 CEST 2022
> 
> Add the "cfi=" boot parameter to allow users to select a scheme at
> boot time.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  arch/x86/kernel/alternative.c |  103 +++++++++++++++++++++++++++++++++---------
>  1 file changed, 83 insertions(+), 20 deletions(-)
> 
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -702,6 +702,47 @@ void __init_or_module noinline apply_ibt
>  #endif /* CONFIG_X86_KERNEL_IBT */
>  
>  #ifdef CONFIG_FINEIBT
> +
> +enum cfi_mode {
> +	CFI_DEFAULT,
> +	CFI_OFF,
> +	CFI_KCFI,
> +	CFI_FINEIBT,
> +};

Is there a reason not to default to FineIBT if the hardware supports it?

If we're going to give the user choices then my previous rant about
documentation still applies:

  https://lkml.kernel.org/lkml/20220503220244.vyz5flk3gg3y6rbw@treble

-- 
Josh