x86/ibt: Add exact_endbr() helper

For when we want to exactly match ENDBR, and not everything that we
can scribble it with.

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/20250224124200.059556588@infradead.org

arch/x86/kernel/alternative.c

@@ -863,6 +863,21 @@ Efault:
 	return false;
 }
 
+#ifdef CONFIG_FINEIBT
+
+static __noendbr bool exact_endbr(u32 *val)
+{
+	u32 endbr;
+
+	__get_kernel_nofault(&endbr, val, u32, Efault);
+	return endbr == gen_endbr();
+
+Efault:
+	return false;
+}
+
+#endif
+
 static void poison_cfi(void *addr);
 
 static void __init_or_module poison_endbr(void *addr)
@@ -1426,10 +1441,9 @@ static void poison_cfi(void *addr)
 bool decode_fineibt_insn(struct pt_regs *regs, unsigned long *target, u32 *type)
 {
 	unsigned long addr = regs->ip - fineibt_preamble_ud2;
-	u32 endbr, hash;
+	u32 hash;
 
-	__get_kernel_nofault(&endbr, addr, u32, Efault);
-	if (endbr != gen_endbr())
+	if (!exact_endbr((void *)addr))
 		return false;
 
 	*target = addr + fineibt_preamble_size;