[PATCH AUTOSEL 6.10 089/121] x86/alternatives: Make FineIBT mode Kconfig selectable

[Sasha Levin]

From: Kees Cook <keescook@xxxxxxxxxxxx>

[ Upstream commit d6f635bcaca8d38dfa47ee20658705f9eff156b5 ]

Since FineIBT performs checking at the destination, it is weaker against
attacks that can construct arbitrary executable memory contents. As such,
some system builders want to run with FineIBT disabled by default. Allow
the "cfi=kcfi" boot param mode to be selectable through Kconfig via the
newly introduced CONFIG_CFI_AUTO_DEFAULT.

Reviewed-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
Reviewed-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Tested-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20240501000218.work.998-kees@xxxxxxxxxx
Signed-off-by: Kees Cook <kees@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 arch/x86/Kconfig              | 9 +++++++++
 arch/x86/include/asm/cfi.h    | 2 +-
 arch/x86/kernel/alternative.c | 8 ++++----
 3 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 1d7122a1883e8..56e301921d2a1 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2427,6 +2427,15 @@ config STRICT_SIGALTSTACK_SIZE
 
 	  Say 'N' unless you want to really enforce this check.
 
+config CFI_AUTO_DEFAULT
+	bool "Attempt to use FineIBT by default at boot time"
+	depends on FINEIBT
+	default y
+	help
+	  Attempt to use FineIBT by default at boot time. If enabled,
+	  this is the same as booting with "cfi=auto". If disabled,
+	  this is the same as booting with "cfi=kcfi".
+
 source "kernel/livepatch/Kconfig"
 
 endmenu
diff --git a/arch/x86/include/asm/cfi.h b/arch/x86/include/asm/cfi.h
index 7cd7525579051..31d19c815f992 100644
--- a/arch/x86/include/asm/cfi.h
+++ b/arch/x86/include/asm/cfi.h
@@ -93,7 +93,7 @@
  *
  */
 enum cfi_mode {
-	CFI_DEFAULT,	/* FineIBT if hardware has IBT, otherwise kCFI */
+	CFI_AUTO,	/* FineIBT if hardware has IBT, otherwise kCFI */
 	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 89de612432728..7fcba437abaee 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -885,8 +885,8 @@ void __init_or_module apply_seal_endbr(s32 *start, s32 *end) { }
 
 #endif /* CONFIG_X86_KERNEL_IBT */
 
-#ifdef CONFIG_FINEIBT
-#define __CFI_DEFAULT	CFI_DEFAULT
+#ifdef CONFIG_CFI_AUTO_DEFAULT
+#define __CFI_DEFAULT	CFI_AUTO
 #elif defined(CONFIG_CFI_CLANG)
 #define __CFI_DEFAULT	CFI_KCFI
 #else
@@ -994,7 +994,7 @@ static __init int cfi_parse_cmdline(char *str)
 		}
 
 		if (!strcmp(str, "auto")) {
-			cfi_mode = CFI_DEFAULT;
+			cfi_mode = CFI_AUTO;
 		} else if (!strcmp(str, "off")) {
 			cfi_mode = CFI_OFF;
 			cfi_rand = false;
@@ -1254,7 +1254,7 @@ static void __apply_fineibt(s32 *start_retpoline, s32 *end_retpoline,
 		      "FineIBT preamble wrong size: %ld", fineibt_preamble_size))
 		return;
 
-	if (cfi_mode == CFI_DEFAULT) {
+	if (cfi_mode == CFI_AUTO) {
 		cfi_mode = CFI_KCFI;
 		if (HAS_KERNEL_IBT && cpu_feature_enabled(X86_FEATURE_IBT))
 			cfi_mode = CFI_FINEIBT;
-- 
2.43.0