x86/retpolines: Enable the default thunk warning only on relevant configs

JIRA: https://issues.redhat.com/browse/RHEL-71467

commit 6376306
Author: Borislav Petkov (AMD) <bp@alien8.de>
Date:   Mon, 15 Apr 2024 18:15:43 +0200

    x86/retpolines: Enable the default thunk warning only on relevant configs

    The using-default-thunk warning check makes sense only with
    configurations which actually enable the special return thunks.

    Otherwise, it fires on unrelated 32-bit configs on which the special
    return thunks won't even work (they're 64-bit only) and, what is more,
    those configs even go off into the weeds when booting in the
    alternatives patching code, leading to a dead machine.

    Fixes: 4461438 ("x86/retpoline: Ensure default return thunk isn't used at runtime")
    Reported-by: Klara Modin <klarasmodin@gmail.com>
    Reported-by: Erhard Furtner <erhard_f@mailbox.org>
    Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
    Tested-by: Klara Modin <klarasmodin@gmail.com>
    Link: https://lore.kernel.org/r/78e0d19c-b77a-4169-a80f-2eef91f4a1d6@gmail.com
    Link: https://lore.kernel.org/r/20240413024956.488d474e@yea

Signed-off-by: Waiman Long <longman@redhat.com>

Author: Waiman-Long

arch/x86/lib/retpoline.S

@@ -382,8 +382,15 @@ SYM_FUNC_END(call_depth_return_thunk)
 SYM_CODE_START(__x86_return_thunk)
 	UNWIND_HINT_FUNC
 	ANNOTATE_NOENDBR
+#if defined(CONFIG_MITIGATION_UNRET_ENTRY) || \
+    defined(CONFIG_MITIGATION_SRSO) || \
+    defined(CONFIG_MITIGATION_CALL_DEPTH_TRACKING)
 	ALTERNATIVE __stringify(ANNOTATE_UNRET_SAFE; ret), \
 		   "jmp warn_thunk_thunk", X86_FEATURE_ALWAYS
+#else
+	ANNOTATE_UNRET_SAFE
+	ret
+#endif
 	int3
 SYM_CODE_END(__x86_return_thunk)
 EXPORT_SYMBOL(__x86_return_thunk)