[tip: x86/urgent] x86/retpolines: Enable the default thunk warning only on relevant configs

["tip-bot2 for Borislav Petkov (AMD)" <tip-bot2@linutronix.de>]

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     6376306adde5b252ee7c73572e35d13fb13f6f18
Gitweb:        https://git.kernel.org/tip/6376306adde5b252ee7c73572e35d13fb13f6f18
Author:        Borislav Petkov (AMD) <bp@alien8.de>
AuthorDate:    Mon, 15 Apr 2024 18:15:43 +02:00
Committer:     Borislav Petkov (AMD) <bp@alien8.de>
CommitterDate: Wed, 17 Apr 2024 18:02:05 +02:00

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: 4461438a8405 ("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
---
 arch/x86/lib/retpoline.S | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
index e674ccf..391059b 100644
--- a/arch/x86/lib/retpoline.S
+++ b/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)