x86/cpu: Cleanup the untrain mess

Since there can only be one active return_thunk, there only needs be
one (matching) untrain_ret. It fundamentally doesn't make sense to
allow multiple untrain_ret at the same time.

Fold all the 3 different untrain methods into a single (temporary)
helper stub.

Fixes: fb3bd91 ("x86/srso: Add a Speculative RAS Overflow mitigation")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230814121149.042774962@infradead.org

Author: Peter Zijlstra

arch/x86/include/asm/nospec-branch.h

@@ -272,9 +272,9 @@
 .endm
 
 #ifdef CONFIG_CPU_UNRET_ENTRY
-#define CALL_ZEN_UNTRAIN_RET	"call retbleed_untrain_ret"
+#define CALL_UNTRAIN_RET	"call entry_untrain_ret"
 #else
-#define CALL_ZEN_UNTRAIN_RET	""
+#define CALL_UNTRAIN_RET	""
 #endif
 
 /*
@@ -293,31 +293,21 @@
 	defined(CONFIG_CALL_DEPTH_TRACKING) || defined(CONFIG_CPU_SRSO)
 	VALIDATE_UNRET_END
 	ALTERNATIVE_3 "",						\
-		      CALL_ZEN_UNTRAIN_RET, X86_FEATURE_UNRET,		\
+		      CALL_UNTRAIN_RET, X86_FEATURE_UNRET,		\
 		      "call entry_ibpb", X86_FEATURE_ENTRY_IBPB,	\
 		      __stringify(RESET_CALL_DEPTH), X86_FEATURE_CALL_DEPTH
 #endif
-
-#ifdef CONFIG_CPU_SRSO
-	ALTERNATIVE_2 "", "call srso_untrain_ret", X86_FEATURE_SRSO, \
-			  "call srso_alias_untrain_ret", X86_FEATURE_SRSO_ALIAS
-#endif
 .endm
 
 .macro UNTRAIN_RET_FROM_CALL
 #if defined(CONFIG_CPU_UNRET_ENTRY) || defined(CONFIG_CPU_IBPB_ENTRY) || \
 	defined(CONFIG_CALL_DEPTH_TRACKING)
 	VALIDATE_UNRET_END
 	ALTERNATIVE_3 "",						\
-		      CALL_ZEN_UNTRAIN_RET, X86_FEATURE_UNRET,		\
+		      CALL_UNTRAIN_RET, X86_FEATURE_UNRET,		\
 		      "call entry_ibpb", X86_FEATURE_ENTRY_IBPB,	\
 		      __stringify(RESET_CALL_DEPTH_FROM_CALL), X86_FEATURE_CALL_DEPTH
 #endif
-
-#ifdef CONFIG_CPU_SRSO
-	ALTERNATIVE_2 "", "call srso_untrain_ret", X86_FEATURE_SRSO, \
-			  "call srso_alias_untrain_ret", X86_FEATURE_SRSO_ALIAS
-#endif
 .endm
 
 
@@ -355,6 +345,7 @@ extern void retbleed_untrain_ret(void);
 extern void srso_untrain_ret(void);
 extern void srso_alias_untrain_ret(void);
 
+extern void entry_untrain_ret(void);
 extern void entry_ibpb(void);
 
 extern void (*x86_return_thunk)(void);

arch/x86/kernel/cpu/bugs.c

@@ -2460,6 +2460,7 @@ static void __init srso_select_mitigation(void)
 			 * like ftrace, static_call, etc.
 			 */
 			setup_force_cpu_cap(X86_FEATURE_RETHUNK);
+			setup_force_cpu_cap(X86_FEATURE_UNRET);
 
 			if (boot_cpu_data.x86 == 0x19) {
 				setup_force_cpu_cap(X86_FEATURE_SRSO_ALIAS);

arch/x86/lib/retpoline.S

@@ -289,6 +289,13 @@ SYM_CODE_START(srso_return_thunk)
 	ud2
 SYM_CODE_END(srso_return_thunk)
 
+SYM_FUNC_START(entry_untrain_ret)
+	ALTERNATIVE_2 "jmp retbleed_untrain_ret", \
+		      "jmp srso_untrain_ret", X86_FEATURE_SRSO, \
+		      "jmp srso_alias_untrain_ret", X86_FEATURE_SRSO_ALIAS
+SYM_FUNC_END(entry_untrain_ret)
+__EXPORT_THUNK(entry_untrain_ret)
+
 SYM_CODE_START(__x86_return_thunk)
 	UNWIND_HINT_FUNC
 	ANNOTATE_NOENDBR