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

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Message-ID: <20250714104919.GR905792@noisy.programming.kicks-ass.net>
Date: Mon, 14 Jul 2025 12:49:19 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: x86@...nel.org
Cc: kys@...rosoft.com, haiyangz@...rosoft.com, wei.liu@...nel.org,
	decui@...rosoft.com, tglx@...utronix.de, mingo@...hat.com,
	bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com,
	seanjc@...gle.com, pbonzini@...hat.com, ardb@...nel.org,
	kees@...nel.org, Arnd Bergmann <arnd@...db.de>,
	gregkh@...uxfoundation.org, jpoimboe@...nel.org,
	linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, linux-efi@...r.kernel.org,
	samitolvanen@...gle.com, ojeda@...nel.org
Subject: Re: [PATCH v3 16/16] objtool: Validate kCFI calls

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 {