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Date:   Fri, 12 Feb 2021 11:52:53 -0800
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Subject: [PATCH v2] ARM: Implement Clang's SLS mitigation
From:   Jian Cai <jiancai@google.com>
Cc:     ndesaulniers@google.com, manojgupta@google.com, llozano@google.com,
        clang-built-linux@googlegroups.com, Jian Cai <jiancai@google.com>,
        Nathan Chancellor <nathan@kernel.org>,
        David Laight <David.Laight@aculab.com>,
        Russell King <linux@armlinux.org.uk>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        James Morris <jmorris@namei.org>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Krzysztof Kozlowski <krzk@kernel.org>,
        Ard Biesheuvel <ardb@kernel.org>,
        "=?UTF-8?q?Andreas=20F=C3=A4rber?=" <afaerber@suse.de>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-security-module@vger.kernel.org
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This patch adds CONFIG_HARDEN_SLS_ALL that can be used to turn on
-mharden-sls=all, which mitigates the straight-line speculation
vulnerability, speculative execution of the instruction following some
unconditional jumps. Notice -mharden-sls= has other options as below,
and this config turns on the strongest option.

all: enable all mitigations against Straight Line Speculation that are implemented.
none: disable all mitigations against Straight Line Speculation.
retbr: enable the mitigation against Straight Line Speculation for RET and BR instructions.
blr: enable the mitigation against Straight Line Speculation for BLR instructions.

Link: https://reviews.llvm.org/D93221
Link: https://reviews.llvm.org/D81404
Link: https://developer.arm.com/support/arm-security-updates/speculative-processor-vulnerability/downloads/straight-line-speculation
https://developer.arm.com/support/arm-security-updates/speculative-processor-vulnerability/frequently-asked-questions#SLS2

Suggested-by: Manoj Gupta <manojgupta@google.com>
Suggested-by: Nathan Chancellor  <nathan@kernel.org>
Suggested-by: David Laight <David.Laight@aculab.com>
Signed-off-by: Jian Cai <jiancai@google.com>
---

Changes v1 -> v2:
 Update the description and patch based on Nathan and David's comments. 

 arch/arm/Makefile          | 4 ++++
 arch/arm64/Makefile        | 4 ++++
 security/Kconfig.hardening | 7 +++++++
 3 files changed, 15 insertions(+)

diff --git a/arch/arm/Makefile b/arch/arm/Makefile
index 4aaec9599e8a..11d89ef32da9 100644
--- a/arch/arm/Makefile
+++ b/arch/arm/Makefile
@@ -48,6 +48,10 @@ CHECKFLAGS	+= -D__ARMEL__
 KBUILD_LDFLAGS	+= -EL
 endif
 
+ifeq ($(CONFIG_HARDEN_SLS_ALL), y)
+KBUILD_CFLAGS  += -mharden-sls=all
+endif
+
 #
 # The Scalar Replacement of Aggregates (SRA) optimization pass in GCC 4.9 and
 # later may result in code being generated that handles signed short and signed
diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index 90309208bb28..ca7299b356a9 100644
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -34,6 +34,10 @@ $(warning LSE atomics not supported by binutils)
   endif
 endif
 
+ifeq ($(CONFIG_HARDEN_SLS_ALL), y)
+KBUILD_CFLAGS  += -mharden-sls=all
+endif
+
 cc_has_k_constraint := $(call try-run,echo				\
 	'int main(void) {						\
 		asm volatile("and w0, w0, %w0" :: "K" (4294967295));	\
diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
index 269967c4fc1b..9266d8d1f78f 100644
--- a/security/Kconfig.hardening
+++ b/security/Kconfig.hardening
@@ -121,6 +121,13 @@ choice
 
 endchoice
 
+config HARDEN_SLS_ALL
+	bool "enable SLS vulnerability hardening"
+	def_bool $(cc-option,-mharden-sls=all)
+        help
+          Enables straight-line speculation vulnerability hardening
+	  at highest level.
+
 config GCC_PLUGIN_STRUCTLEAK_VERBOSE
 	bool "Report forcefully initialized variables"
 	depends on GCC_PLUGIN_STRUCTLEAK
-- 
2.30.0.478.g8a0d178c01-goog