x86/retpoline: Use -mfunction-return

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>

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index a74886a..6e16057 100644 (file)
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -15,11 +15,13 @@ endif
 ifdef CONFIG_CC_IS_GCC
 RETPOLINE_CFLAGS       := $(call cc-option,-mindirect-branch=thunk-extern -mindirect-branch-register)
 RETPOLINE_CFLAGS       += $(call cc-option,-mindirect-branch-cs-prefix)
+RETPOLINE_CFLAGS       += $(call cc-option,-mfunction-return=thunk-extern)
 RETPOLINE_VDSO_CFLAGS  := $(call cc-option,-mindirect-branch=thunk-inline -mindirect-branch-register)
 endif
 ifdef CONFIG_CC_IS_CLANG
 RETPOLINE_CFLAGS       := -mretpoline-external-thunk
 RETPOLINE_VDSO_CFLAGS  := -mretpoline
+RETPOLINE_CFLAGS       += $(call cc-option,-mfunction-return=thunk-extern)
 endif
 export RETPOLINE_CFLAGS
 export RETPOLINE_VDSO_CFLAGS

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 5728539..829c9f8 100644 (file)
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -123,6 +123,8 @@
 typedef u8 retpoline_thunk_t[RETPOLINE_THUNK_SIZE];
 extern retpoline_thunk_t __x86_indirect_thunk_array[];
 
+extern void __x86_return_thunk(void);
+
 #ifdef CONFIG_RETPOLINE
 
 #define GEN(reg) \

diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
index 2cdd624..4467c21 100644 (file)
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -67,3 +67,16 @@ SYM_CODE_END(__x86_indirect_thunk_array)
 #define GEN(reg) EXPORT_THUNK(reg)
 #include <asm/GEN-for-each-reg.h>
 #undef GEN
+
+/*
+ * 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)
+       UNWIND_HINT_EMPTY
+       ANNOTATE_NOENDBR
+       ret
+       int3
+SYM_CODE_END(__x86_return_thunk)
+
+__EXPORT_THUNK(__x86_return_thunk)