linux-kernel - Re: [PATCH 11/11] x86/fineibt: Add FineIBT+BHI mitigation

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Message-ID: <202502140957.FE93863F@keescook>
Date: Fri, 14 Feb 2025 10:02:12 -0800
From: Kees Cook <kees@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, alyssa.milburn@...el.com,
	scott.d.constable@...el.com, joao@...rdrivepizza.com,
	andrew.cooper3@...rix.com, jpoimboe@...nel.org,
	jose.marchesi@...cle.com, hjl.tools@...il.com,
	ndesaulniers@...gle.com, samitolvanen@...gle.com, nathan@...nel.org,
	ojeda@...nel.org, alexei.starovoitov@...il.com, mhiramat@...nel.org
Subject: Re: [PATCH 11/11] x86/fineibt: Add FineIBT+BHI mitigation

On Fri, Feb 07, 2025 at 01:15:40PM +0100, Peter Zijlstra wrote:
> Due to FineIBT weakness, add an additional mitigation for BHI.
> [...]
> @@ -1020,6 +1036,8 @@ static __init int cfi_parse_cmdline(char
>  			cfi_mode = CFI_KCFI;
>  		} else if (!strcmp(str, "fineibt")) {
>  			cfi_mode = CFI_FINEIBT;
> +		} else if (IS_ENABLED(CONFIG_FINEIBT_BHI) && !strcmp(str, "fineibt+bhi")) {
> +			cfi_mode = CFI_FINEIBT_BHI;
>  		} else if (!strcmp(str, "norand")) {
>  			cfi_rand = false;
>  		} else {

While looking at FineIBT vs entry, I noticed that FineIBT+BHI must be
explicitly selected at boot. Did you want it to be enabled automatically
when the compiler supports it and FineIBT is enabled? Does there need to
be a check for BHI added?

-- 
Kees Cook