From: Peter Zijlstra <peterz@infradead.org>
Date: Tue, 14 Jun 2022 23:15:36 +0200
Subject: x86/retpoline: Use -mfunction-return
Git-commit: 0b53c374b9eff2255a386f1f1cfb9a928e52a5ae
Patch-mainline: v5.19-rc7
References: bsc#1199657 CVE-2022-29900 CVE-2022-29901

Utilize -mfunction-return=thunk-extern when available to have the
compiler replace RET instructions with direct JMPs to the symbol
__x86_return_thunk. This does not affect assembler (.S) sources, only C
sources.

-mfunction-return=thunk-extern has been available since gcc 7.3 and
clang 15.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
---
 Makefile                             |    2 ++
 arch/x86/include/asm/nospec-branch.h |    2 ++
 arch/x86/lib/retpoline.S             |   11 +++++++++++
 3 files changed, 15 insertions(+)

--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -157,6 +157,8 @@
 #ifdef CONFIG_RETPOLINE
 #ifdef CONFIG_X86_64
 
+extern void __x86_return_thunk(void);
+
 /*
  * Inline asm uses the %V modifier which is only in newer GCC
  * which is ensured when CONFIG_RETPOLINE is defined.
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -46,3 +46,14 @@ GENERATE_THUNK(r13)
 GENERATE_THUNK(r14)
 GENERATE_THUNK(r15)
 #endif
+
+/*
+ * This function name is magical and is used by -mfunction-return=thunk-extern
+ * for the compiler to generate JMPs to it.
+ */
+SYM_CODE_START(__x86_return_thunk)
+	ret
+	int3
+SYM_CODE_END(__x86_return_thunk)
+
+__EXPORT_THUNK(__x86_return_thunk)
--- a/Makefile
+++ b/Makefile
@@ -639,8 +639,10 @@ endif
 
 RETPOLINE_CFLAGS_GCC := -mindirect-branch=thunk-extern -mindirect-branch-register
 RETPOLINE_CFLAGS_GCC += $(call cc-option,-mindirect-branch-cs-prefix)
+RETPOLINE_CFLAGS_GCC += $(call cc-option,-mfunction-return=thunk-extern)
 RETPOLINE_VDSO_CFLAGS_GCC := -mindirect-branch=thunk-inline -mindirect-branch-register
 RETPOLINE_CFLAGS_CLANG := -mretpoline-external-thunk
+RETPOLINE_CFLAGS_CLANG += $(call cc-option,-mfunction-return=thunk-extern)
 RETPOLINE_VDSO_CFLAGS_CLANG := -mretpoline
 RETPOLINE_CFLAGS := $(call cc-option,$(RETPOLINE_CFLAGS_GCC),$(call cc-option,$(RETPOLINE_CFLAGS_CLANG)))
 RETPOLINE_VDSO_CFLAGS := $(call cc-option,$(RETPOLINE_VDSO_CFLAGS_GCC),$(call cc-option,$(RETPOLINE_VDSO_CFLAGS_CLANG)))