Re: [PATCH v3 16/16] objtool: Validate kCFI calls

[Peter Zijlstra]

On Mon, Jul 14, 2025 at 12:20:27PM +0200, Peter Zijlstra wrote:

> --- a/arch/x86/platform/efi/efi_stub_64.S
> +++ b/arch/x86/platform/efi/efi_stub_64.S
> @@ -11,6 +11,10 @@
>  #include <asm/nospec-branch.h>
>  
>  SYM_FUNC_START(__efi_call)
> +	/*
> +	 * The EFI code doesn't have any CFI, annotate away the CFI violation.
> +	 */
> +	ANNOTATE_NOCFI_SYM
>  	pushq %rbp
>  	movq %rsp, %rbp
>  	and $~0xf, %rsp

FWIW, we should probably do something like this as well.

---

--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -562,6 +562,13 @@ __noendbr u64 ibt_save(bool disable)
 {
 	u64 msr = 0;
 
+	/*
+	 * Firmware code will not provide the same level of
+	 * control-flow-integriry. Taint the kernel to let the user know.
+	 */
+	if (disable || (IS_ENABLED(CONFIG_CFI_CLANG) && cfi_mode != CFI_OFF))
+		add_taint(TAINT_CFI, LOCKDEP_STILL_OK);
+
 	if (cpu_feature_enabled(X86_FEATURE_IBT)) {
 		rdmsrq(MSR_IA32_S_CET, msr);
 		if (disable)
--- a/include/linux/panic.h
+++ b/include/linux/panic.h
@@ -73,7 +73,8 @@ static inline void set_arch_panic_timeou
 #define TAINT_RANDSTRUCT		17
 #define TAINT_TEST			18
 #define TAINT_FWCTL			19
-#define TAINT_FLAGS_COUNT		20
+#define TAINT_CFI			20
+#define TAINT_FLAGS_COUNT		21
 #define TAINT_FLAGS_MAX			((1UL << TAINT_FLAGS_COUNT) - 1)
 
 struct taint_flag {