[PATCH 5.15 28/78] x86/retpoline: Use -mfunction-return

Greg Kroah-Hartman
2022-07-12T19:01:16Z

From: Peter Zijlstra <peterz@infradead.org>

commit 0b53c374b9eff2255a386f1f1cfb9a928e52a5ae upstream.

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>
[cascardo: RETPOLINE_CFLAGS is at Makefile]
[cascardo: remove ANNOTATE_NOENDBR from __x86_return_thunk]
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
Makefile | 2 ++
arch/x86/include/asm/nospec-branch.h | 2 ++
arch/x86/lib/retpoline.S | 12 ++++++++++++
3 files changed, 16 insertions(+)

--- a/Makefile
+++ b/Makefile
@@ -687,11 +687,13 @@ endif

ifdef CONFIG_CC_IS_GCC
RETPOLINE_CFLAGS := $(call cc-option,-mindirect-branch=thunk-extern -mindirect-branch-register)
+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
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -120,6 +120,8 @@
_ASM_PTR " 999b\n\t" \
".popsection\n\t"

+extern void __x86_return_thunk(void);
+
#ifdef CONFIG_RETPOLINE

typedef u8 retpoline_thunk_t[RETPOLINE_THUNK_SIZE];
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -66,3 +66,15 @@ 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
+ ret
+ int3
+SYM_CODE_END(__x86_return_thunk)
+
+__EXPORT_THUNK(__x86_return_thunk)