linux-kernel - [PATCH 5.18 10/61] x86/retpoline: Use -mfunction-return

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Message-Id: <20220712183237.359715627@linuxfoundation.org>
Date:   Tue, 12 Jul 2022 20:39:07 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Borislav Petkov <bp@...e.de>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Thadeu Lima de Souza Cascardo <cascardo@...onical.com>
Subject: [PATCH 5.18 10/61] x86/retpoline: Use -mfunction-return

From: Peter Zijlstra <peterz@...radead.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@...radead.org>
Signed-off-by: Borislav Petkov <bp@...e.de>
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
Reviewed-by: Josh Poimboeuf <jpoimboe@...nel.org>
Tested-by: Nick Desaulniers <ndesaulniers@...gle.com>
Signed-off-by: Borislav Petkov <bp@...e.de>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@...onical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 arch/x86/Makefile                    |    2 ++
 arch/x86/include/asm/nospec-branch.h |    2 ++
 arch/x86/lib/retpoline.S             |   13 +++++++++++++
 3 files changed, 17 insertions(+)

--- 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
--- 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) \
--- 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)