Commit 082c4c81 authored by Peter Zijlstra's avatar Peter Zijlstra
Browse files

x86/cfi: Boot time selection of CFI scheme 



Add the "cfi=" boot parameter to allow people to select a CFI scheme
at boot time. Mostly useful for development / debugging.

Requested-by: default avatarKees Cook <keescook@chromium.org>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: default avatarKees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20221027092842.699804264@infradead.org
parent 931ab636

arch/x86/kernel/alternative.c

+81 −18
Original line number Diff line number Diff line
@@ -702,6 +702,47 @@ void __init_or_module noinline apply_ibt_endbr(s32 *start, s32 *end) { } 
#endif /* CONFIG_X86_KERNEL_IBT */



#ifdef CONFIG_FINEIBT



enum cfi_mode {

	CFI_DEFAULT,

	CFI_OFF,

	CFI_KCFI,

	CFI_FINEIBT,

};



static enum cfi_mode cfi_mode __ro_after_init = CFI_DEFAULT;



static __init int cfi_parse_cmdline(char *str)

{

	if (!str)

		return -EINVAL;



	while (str) {

		char *next = strchr(str, ',');

		if (next) {

			*next = 0;

			next++;

		}



		if (!strcmp(str, "auto")) {

			cfi_mode = CFI_DEFAULT;

		} else if (!strcmp(str, "off")) {

			cfi_mode = CFI_OFF;

		} else if (!strcmp(str, "kcfi")) {

			cfi_mode = CFI_KCFI;

		} else if (!strcmp(str, "fineibt")) {

			cfi_mode = CFI_FINEIBT;

		} else {

			pr_err("Ignoring unknown cfi option (%s).", str);

		}



		str = next;

	}



	return 0;

}

early_param("cfi", cfi_parse_cmdline);



/*

 * kCFI						FineIBT

 *
@@ -868,9 +909,28 @@ static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline, 
		      "FineIBT preamble wrong size: %ld", fineibt_preamble_size))

		return;



	if (!HAS_KERNEL_IBT || !cpu_feature_enabled(X86_FEATURE_IBT))

	if (cfi_mode == CFI_DEFAULT) {

		cfi_mode = CFI_KCFI;

		if (HAS_KERNEL_IBT && cpu_feature_enabled(X86_FEATURE_IBT))

			cfi_mode = CFI_FINEIBT;

	}



	switch (cfi_mode) {

	case CFI_OFF:

		ret = cfi_disable_callers(start_retpoline, end_retpoline);

		if (ret)

			goto err;



		if (builtin)

			pr_info("Disabling CFI\n");

		return;



	case CFI_KCFI:

		if (builtin)

			pr_info("Using kCFI\n");

		return;



	case CFI_FINEIBT:

		/*

		 * Rewrite the callers to not use the __cfi_ stubs, such that we might

		 * rewrite them. This disables all CFI. If this succeeds but any of the
@@ -890,9 +950,12 @@ static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline, 


		if (builtin)

			pr_info("Using FineIBT CFI\n");



		return;



	default:

		break;

	}



err:

	pr_err("Something went horribly wrong trying to rewrite the CFI implementation.\n");

}