From: Peter Zijlstra <peterz@infradead.org>
Date: Mon, 14 Aug 2023 13:44:30 +0200
Subject: x86/alternative: Make custom return thunk unconditional
Git-commit: 095b8303f3835c68ac4a8b6d754ca1c3b6230711
Patch-mainline: v6.5-rc7
References: git-fixes

There is infrastructure to rewrite return thunks to point to any
random thunk one desires, unwrap that from CALL_THUNKS, which up to
now was the sole user of that.

  [ bp: Make the thunks visible on 32-bit and add ifdeffery for the
    32-bit builds. ]

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230814121148.775293785@infradead.org

Acked-by: nborisov <nik.borisov@suse.com>
---
 arch/x86/include/asm/nospec-branch.h |    5 ++++-
 arch/x86/kernel/alternative.c        |    2 --
 arch/x86/kernel/cpu/bugs.c           |    2 ++
 3 files changed, 6 insertions(+), 3 deletions(-)

--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -177,8 +177,11 @@

 typedef u8 retpoline_thunk_t[RETPOLINE_THUNK_SIZE];
 extern retpoline_thunk_t __x86_indirect_thunk_array[];
-
+#ifdef CONFIG_RETPOLINE
 extern void __x86_return_thunk(void);
+#else
+static inline void __x86_return_thunk(void) {}
+#endif
 extern void zen_untrain_ret(void);
 extern void srso_untrain_ret(void);
 extern void srso_untrain_ret_alias(void);
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -572,8 +572,6 @@ void __init_or_module noinline apply_ret
 	}
 }

-void (*x86_return_thunk)(void) __ro_after_init = &__x86_return_thunk;
-
 /*
  * Rewrite the compiler generated return thunk tail-calls.
  *
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -61,6 +61,8 @@ EXPORT_SYMBOL_GPL(x86_pred_cmd);

 static DEFINE_MUTEX(spec_ctrl_mutex);

+void (*x86_return_thunk)(void) __ro_after_init = &__x86_return_thunk;
+
 /* Update SPEC_CTRL MSR and its cached copy unconditionally */
 static void update_spec_ctrl(u64 val)
 {