Re: [PATCH 3/5] x86/cfi: Add option for cfi=debug bootparam

[Nathan Chancellor <nathan@kernel.org>]

On Mon, Aug 25, 2025 at 07:25:50AM -0700, Kees Cook wrote:
> From: Kees Cook <kees@outflux.net>
> 
> Add "debug" option for "cfi=" bootparam to get details on early CFI
> initialization steps. Standardize CFI pr_info() lines to use "CFI:"
> prefix. Standardize "CFI: Using ..." to always report which CFI mode is
> being used, regardless of CONFIG_FINEIBT. Document all the "cfi=" options.
> 
> Signed-off-by: Kees Cook <kees@outflux.net>

I am not sure if the x86 maintainers are "patch count adverse" but it
feels like this would be a little easier to review as four separate
patches. Every sentence in the commit message is basically its own
change.

1. The initial documentation for cfi= and its current values.
2. Standardization of pr_info() calls to use "CFI:"
3. Adding "CFI: Using" to __apply_fineibt()
4. Adding cfi=debug

Patch four would become much simpler to understand, especially with
Peter's suggested change.

> ---
>  .../admin-guide/kernel-parameters.txt         | 18 +++++++++
>  arch/x86/kernel/alternative.c                 | 39 +++++++++++++++----
>  2 files changed, 50 insertions(+), 7 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 747a55abf494..7b4bddb5a030 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -608,6 +608,24 @@
>  	ccw_timeout_log	[S390]
>  			See Documentation/arch/s390/common_io.rst for details.
>  
> +	cfi=		[X86-64] Set Control Flow Integrity checking features
> +			when CONFIG_FINEIBT is enabled.
> +			Format: feature[,feature...]
> +			Default: auto
> +
> +			auto:	  Use FineIBT if IBT available, otherwise kCFI.
> +				  Under FineIBT, enable "paranoid" mode when
> +				  FRED is not available.
> +			off:	  Turn off CFI checking.
> +			kcfi:	  Use kCFI (disable FineIBT).
> +			fineibt:  Use FineIBT (even if IBT not available).
> +			norand:   Do not re-randomize CFI hashes.
> +			paranoid: Add caller hash checking under FineIBT.
> +			bhi:	  Enable register poisoning to stop speculation
> +				  across FineIBT. (Disabled by default.)
> +			warn:	  Do not enforce CFI checking: warn only.
> +			debug:	  Report CFI initialization details.
> +
>  	cgroup_disable=	[KNL] Disable a particular controller or optional feature
>  			Format: {name of the controller(s) or feature(s) to disable}
>  			The effects of cgroup_disable=foo are:
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index 7bde68247b5f..5d80ae77c042 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -1225,6 +1225,7 @@ int cfi_get_func_arity(void *func)
>  
>  static bool cfi_rand __ro_after_init = true;
>  static u32  cfi_seed __ro_after_init;
> +static bool cfi_debug __ro_after_init;
>  
>  /*
>   * Re-hash the CFI hash with a boot-time seed while making sure the result is
> @@ -1259,6 +1260,8 @@ static __init int cfi_parse_cmdline(char *str)
>  		} else if (!strcmp(str, "off")) {
>  			cfi_mode = CFI_OFF;
>  			cfi_rand = false;
> +		} else if (!strcmp(str, "debug")) {
> +			cfi_debug = true;
>  		} else if (!strcmp(str, "kcfi")) {
>  			cfi_mode = CFI_KCFI;
>  		} else if (!strcmp(str, "fineibt")) {
> @@ -1266,26 +1269,26 @@ static __init int cfi_parse_cmdline(char *str)
>  		} else if (!strcmp(str, "norand")) {
>  			cfi_rand = false;
>  		} else if (!strcmp(str, "warn")) {
> -			pr_alert("CFI mismatch non-fatal!\n");
> +			pr_alert("CFI: mismatch non-fatal!\n");
>  			cfi_warn = true;
>  		} else if (!strcmp(str, "paranoid")) {
>  			if (cfi_mode == CFI_FINEIBT) {
>  				cfi_paranoid = true;
>  			} else {
> -				pr_err("Ignoring paranoid; depends on fineibt.\n");
> +				pr_err("CFI: ignoring paranoid; depends on fineibt.\n");
>  			}
>  		} else if (!strcmp(str, "bhi")) {
>  #ifdef CONFIG_FINEIBT_BHI
>  			if (cfi_mode == CFI_FINEIBT) {
>  				cfi_bhi = true;
>  			} else {
> -				pr_err("Ignoring bhi; depends on fineibt.\n");
> +				pr_err("CFI: ignoring bhi; depends on fineibt.\n");
>  			}
>  #else
> -			pr_err("Ignoring bhi; depends on FINEIBT_BHI=y.\n");
> +			pr_err("CFI: ignoring bhi; depends on FINEIBT_BHI=y.\n");
>  #endif
>  		} else {
> -			pr_err("Ignoring unknown cfi option (%s).", str);
> +			pr_err("CFI: Ignoring unknown option (%s).", str);

You lowercase "Ignoring" earlier but not here, intentional? There are a
couple of other messages that have a capital first letter but not
others.

>  		}
>  
>  		str = next;
> @@ -1734,6 +1737,8 @@ static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
>  	 * rewrite them. This disables all CFI. If this succeeds but any of the
>  	 * later stages fails, we're without CFI.
>  	 */
> +	if (builtin && cfi_debug)
> +		pr_info("CFI: disabling all indirect call checking\n");
>  	ret = cfi_disable_callers(start_retpoline, end_retpoline);
>  	if (ret)
>  		goto err;
> @@ -1744,43 +1749,61 @@ static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
>  			cfi_bpf_hash = cfi_rehash(cfi_bpf_hash);
>  			cfi_bpf_subprog_hash = cfi_rehash(cfi_bpf_subprog_hash);
>  		}
> +		if (builtin && cfi_debug)
> +			pr_info("CFI: cfi_seed: 0x%08x\n", cfi_seed);
>  
> +		if (builtin && cfi_debug)
> +			pr_info("CFI: rehashing all preambles\n");
>  		ret = cfi_rand_preamble(start_cfi, end_cfi);
>  		if (ret)
>  			goto err;
>  
> +		if (builtin && cfi_debug)
> +			pr_info("CFI: rehashing all indirect calls\n");
>  		ret = cfi_rand_callers(start_retpoline, end_retpoline);
>  		if (ret)
>  			goto err;
> +	} else {
> +		if (builtin && cfi_debug)
> +			pr_info("CFI: rehashing disabled\n");
>  	}
>  
>  	switch (cfi_mode) {
>  	case CFI_OFF:
>  		if (builtin)
> -			pr_info("Disabling CFI\n");
> +			pr_info("CFI: disabled\n");
>  		return;
>  
>  	case CFI_KCFI:
> +		if (builtin && cfi_debug)
> +			pr_info("CFI: enabling all indirect call checking\n");
>  		ret = cfi_enable_callers(start_retpoline, end_retpoline);
>  		if (ret)
>  			goto err;
>  
>  		if (builtin)
> -			pr_info("Using kCFI\n");
> +			pr_info("CFI: Using %s kCFI\n",
> +				cfi_rand ? "rehashed" : "retpoline");
>  		return;
>  
>  	case CFI_FINEIBT:
> +		if (builtin && cfi_debug)
> +			pr_info("CFI: adding FineIBT to all preambles\n");
>  		/* place the FineIBT preamble at func()-16 */
>  		ret = cfi_rewrite_preamble(start_cfi, end_cfi);
>  		if (ret)
>  			goto err;
>  
>  		/* rewrite the callers to target func()-16 */
> +		if (builtin && cfi_debug)
> +			pr_info("CFI: rewriting indirect call sites to use FineIBT\n");
>  		ret = cfi_rewrite_callers(start_retpoline, end_retpoline);
>  		if (ret)
>  			goto err;
>  
>  		/* now that nobody targets func()+0, remove ENDBR there */
> +		if (builtin && cfi_debug)
> +			pr_info("CFI: removing old endbr insns\n");
>  		cfi_rewrite_endbr(start_cfi, end_cfi);
>  
>  		if (builtin) {
> @@ -2005,6 +2028,8 @@ bool decode_fineibt_insn(struct pt_regs *regs, unsigned long *target, u32 *type)
>  static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
>  			    s32 *start_cfi, s32 *end_cfi, bool builtin)
>  {
> +	if (builtin)
> +		pr_info("CFI: Using standard kCFI\n");
>  }
>  
>  #ifdef CONFIG_X86_KERNEL_IBT
> -- 
> 2.34.1
>