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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
        Zhenzhong Duan <zhenzhong.duan@oracle.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        David Woodhouse <dwmw@amazon.co.uk>,
        Borislav Petkov <bp@suse.de>,
        Daniel Borkmann <daniel@iogearbox.net>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Andy Lutomirski <luto@kernel.org>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Michal Marek <michal.lkml@markovi.net>,
        srinivas.eeda@oracle.com
Subject: [PATCH 4.14 078/146] x86/retpoline: Make CONFIG_RETPOLINE depend on compiler support
Date:   Tue,  4 Dec 2018 11:49:24 +0100
Message-Id: <20181204103729.971223628@linuxfoundation.org>
In-Reply-To: <20181204103726.750894136@linuxfoundation.org>
References: <20181204103726.750894136@linuxfoundation.org>
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4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Zhenzhong Duan zhenzhong.duan@oracle.com

commit 4cd24de3a0980bf3100c9dcb08ef65ca7c31af48 upstream

Since retpoline capable compilers are widely available, make
CONFIG_RETPOLINE hard depend on the compiler capability.

Break the build when CONFIG_RETPOLINE is enabled and the compiler does not
support it. Emit an error message in that case:

 "arch/x86/Makefile:226: *** You are building kernel with non-retpoline
  compiler, please update your compiler..  Stop."

[dwmw: Fail the build with non-retpoline compiler]

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: Borislav Petkov <bp@suse.de>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Michal Marek <michal.lkml@markovi.net>
Cc: <srinivas.eeda@oracle.com>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/cca0cb20-f9e2-4094-840b-fb0f8810cd34@default
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 arch/x86/Kconfig                     |    4 ----
 arch/x86/Makefile                    |    5 +++--
 arch/x86/include/asm/nospec-branch.h |   10 ++++++----
 arch/x86/kernel/cpu/bugs.c           |    2 +-
 scripts/Makefile.build               |    2 --
 5 files changed, 10 insertions(+), 13 deletions(-)

--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -440,10 +440,6 @@ config RETPOLINE
 	  branches. Requires a compiler with -mindirect-branch=thunk-extern
 	  support for full protection. The kernel may run slower.
 
-	  Without compiler support, at least indirect branches in assembler
-	  code are eliminated. Since this includes the syscall entry path,
-	  it is not entirely pointless.
-
 config INTEL_RDT
 	bool "Intel Resource Director Technology support"
 	default n
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -241,9 +241,10 @@ KBUILD_CFLAGS += -fno-asynchronous-unwin
 
 # Avoid indirect branches in kernel to deal with Spectre
 ifdef CONFIG_RETPOLINE
-ifneq ($(RETPOLINE_CFLAGS),)
-  KBUILD_CFLAGS += $(RETPOLINE_CFLAGS) -DRETPOLINE
+ifeq ($(RETPOLINE_CFLAGS),)
+  $(error You are building kernel with non-retpoline compiler, please update your compiler.)
 endif
+  KBUILD_CFLAGS += $(RETPOLINE_CFLAGS)
 endif
 
 archscripts: scripts_basic
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -162,11 +162,12 @@
 	_ASM_PTR " 999b\n\t"					\
 	".popsection\n\t"
 
-#if defined(CONFIG_X86_64) && defined(RETPOLINE)
+#ifdef CONFIG_RETPOLINE
+#ifdef CONFIG_X86_64
 
 /*
- * Since the inline asm uses the %V modifier which is only in newer GCC,
- * the 64-bit one is dependent on RETPOLINE not CONFIG_RETPOLINE.
+ * Inline asm uses the %V modifier which is only in newer GCC
+ * which is ensured when CONFIG_RETPOLINE is defined.
  */
 # define CALL_NOSPEC						\
 	ANNOTATE_NOSPEC_ALTERNATIVE				\
@@ -181,7 +182,7 @@
 	X86_FEATURE_RETPOLINE_AMD)
 # define THUNK_TARGET(addr) [thunk_target] "r" (addr)
 
-#elif defined(CONFIG_X86_32) && defined(CONFIG_RETPOLINE)
+#else /* CONFIG_X86_32 */
 /*
  * For i386 we use the original ret-equivalent retpoline, because
  * otherwise we'll run out of registers. We don't care about CET
@@ -211,6 +212,7 @@
 	X86_FEATURE_RETPOLINE_AMD)
 
 # define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
+#endif
 #else /* No retpoline for C / inline asm */
 # define CALL_NOSPEC "call *%[thunk_target]\n"
 # define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -251,7 +251,7 @@ static void __init spec2_print_if_secure
 
 static inline bool retp_compiler(void)
 {
-	return __is_defined(RETPOLINE);
+	return __is_defined(CONFIG_RETPOLINE);
 }
 
 static inline bool match_option(const char *arg, int arglen, const char *opt)
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -272,10 +272,8 @@ else
 objtool_args += $(call cc-ifversion, -lt, 0405, --no-unreachable)
 endif
 ifdef CONFIG_RETPOLINE
-ifneq ($(RETPOLINE_CFLAGS),)
   objtool_args += --retpoline
 endif
-endif
 
 
 ifdef CONFIG_MODVERSIONS