x86/cfi: Add 'cfi=warn' boot option

Rebuilding with CONFIG_CFI_PERMISSIVE=y enabled is such a pain, esp. since
clang is so slow.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Kees Cook <kees@kernel.org>
Link: https://lore.kernel.org/r/20250224124159.924496481@infradead.org

arch/x86/kernel/alternative.c

@@ -1022,6 +1022,9 @@ static __init int cfi_parse_cmdline(char *str)
 			cfi_mode = CFI_FINEIBT;
 		} else if (!strcmp(str, "norand")) {
 			cfi_rand = false;
+		} else if (!strcmp(str, "warn")) {
+			pr_alert("CFI mismatch non-fatal!\n");
+			cfi_warn = true;
 		} else {
 			pr_err("Ignoring unknown cfi option (%s).", str);
 		}

include/linux/cfi.h

@@ -11,6 +11,8 @@
 #include <linux/module.h>
 #include <asm/cfi.h>
 
+extern bool cfi_warn;
+
 #ifndef cfi_get_offset
 static inline int cfi_get_offset(void)
 {

kernel/cfi.c

@@ -7,6 +7,8 @@
 
 #include <linux/cfi.h>
 
+bool cfi_warn __ro_after_init = IS_ENABLED(CONFIG_CFI_PERMISSIVE);
+
 enum bug_trap_type report_cfi_failure(struct pt_regs *regs, unsigned long addr,
 				      unsigned long *target, u32 type)
 {
@@ -17,7 +19,7 @@ enum bug_trap_type report_cfi_failure(struct pt_regs *regs, unsigned long addr,
 		pr_err("CFI failure at %pS (no target information)\n",
 		       (void *)addr);
 
-	if (IS_ENABLED(CONFIG_CFI_PERMISSIVE)) {
+	if (cfi_warn) {
 		__warn(NULL, 0, (void *)addr, 0, regs, NULL);
 		return BUG_TRAP_TYPE_WARN;
 	}