Re: [PATCH] x86/kcfi: Require FRED for FineIBT

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 14/02/2025 7:22 pm, Kees Cook wrote:
> diff --git a/arch/x86/include/asm/cfi.h b/arch/x86/include/asm/cfi.h
> index ef5e0a698253..dfa2ba4cceca 100644
> --- a/arch/x86/include/asm/cfi.h
> +++ b/arch/x86/include/asm/cfi.h
> @@ -93,7 +93,7 @@
>   *
>   */
>  enum cfi_mode {
> -	CFI_AUTO,	/* FineIBT if hardware has IBT, otherwise kCFI */
> +	CFI_AUTO,	/* FineIBT if hardware has IBT, FRED, and XOM */

You discuss XOM in the commit message, but there's no check ...

>  	CFI_OFF,	/* Taditional / IBT depending on .config */
>  	CFI_KCFI,	/* Optionally CALL_PADDING, IBT, RETPOLINE */
>  	CFI_FINEIBT,	/* see arch/x86/kernel/alternative.c */
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index 97422292b609..acc12a6efc18 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -1323,7 +1323,9 @@ static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
>  
>  	if (cfi_mode == CFI_AUTO) {
>  		cfi_mode = CFI_KCFI;
> -		if (HAS_KERNEL_IBT && cpu_feature_enabled(X86_FEATURE_IBT))
> +		/* FineIBT requires IBT and will only be safe with FRED */
> +		if (HAS_KERNEL_IBT && cpu_feature_enabled(X86_FEATURE_IBT) &&
> +		    cpu_feature_enabled(X86_FEATURE_FRED))

... here.

Is this meant to be "/* TODO: wire up XOM */" or is that accounted for
somewhere else?


Also, while I hate to come back and contradict myself from earlier...

Architecturally, FineIBT without FRED seems to be no improvement over
simple IBT.  (I'd love to find some way of hardening the entrypoints,
but I can't see a robust way of doing so.)

However, micro-architecturally, FineIBT is still far better than simple
IBT for speculation issue, seeing as Intel keep on staunchly refusing to
turn off the indirect predictors by default like AMD do.

A security conscious user ought to be using FineIBT for this, given a
choice, even if it's not perfect in other regards.

~Andrew