[PATCH v2 08/11] x86/cpu: Cleanup the untrain mess

From: Peter Zijlstra
Date: Mon Aug 14 2023 - 08:20:36 EST


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: fb3bd914b3ec ("x86/srso: Add a Speculative RAS Overflow mitigation")
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
arch/x86/include/asm/nospec-branch.h | 19 +++++--------------
arch/x86/kernel/cpu/bugs.c | 1 +
arch/x86/lib/retpoline.S | 7 +++++++
3 files changed, 13 insertions(+), 14 deletions(-)

--- a/arch/x86/include/asm/nospec-branch.h
+++ b/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

/*
@@ -293,15 +293,10 @@
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
@@ -309,15 +304,10 @@
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


@@ -351,6 +341,7 @@ 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);
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -2449,6 +2449,7 @@ static void __init srso_select_mitigatio
* 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);
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -277,6 +277,13 @@ 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