[PATCH v3] ARM: Implement SLS mitigation

[Jian Cai]

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.

Links:
https://reviews.llvm.org/D93221
https://reviews.llvm.org/D81404
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 at google.com>
Suggested-by: Nick Desaulniers <ndesaulniers at google.com>
Suggested-by: Nathan Chancellor  <nathan at kernel.org>
Suggested-by: David Laight <David.Laight at aculab.com>
Suggested-by: Will Deacon <will at kernel.org>
Reviewed-by: Nathan Chancellor <nathan at kernel.org>
Signed-off-by: Jian Cai <jiancai at google.com>
---

Changes v2 -> v3:
  Modify linker scripts as Nick suggested to address boot failure
  (verified with qemu). Added more details in Kconfig.hardening
  description. Disable the config by default.

 arch/arm/Makefile                  |  4 ++++
 arch/arm/include/asm/vmlinux.lds.h |  4 ++++
 arch/arm/kernel/vmlinux.lds.S      |  1 +
 arch/arm64/Makefile                |  4 ++++
 arch/arm64/kernel/vmlinux.lds.S    |  5 +++++
 security/Kconfig.hardening         | 10 ++++++++++
 6 files changed, 28 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/arm/include/asm/vmlinux.lds.h b/arch/arm/include/asm/vmlinux.lds.h
index 4a91428c324d..c7f9717511ca 100644
--- a/arch/arm/include/asm/vmlinux.lds.h
+++ b/arch/arm/include/asm/vmlinux.lds.h
@@ -145,3 +145,7 @@
 		__edtcm_data = .;					\
 	}								\
 	. = __dtcm_start + SIZEOF(.data_dtcm);
+
+#define SLS_TEXT							\
+		ALIGN_FUNCTION();					\
+		*(.text.__llvm_slsblr_thunk_*)
diff --git a/arch/arm/kernel/vmlinux.lds.S b/arch/arm/kernel/vmlinux.lds.S
index f7f4620d59c3..e71f2bc97bae 100644
--- a/arch/arm/kernel/vmlinux.lds.S
+++ b/arch/arm/kernel/vmlinux.lds.S
@@ -63,6 +63,7 @@ SECTIONS
 	.text : {			/* Real text segment		*/
 		_stext = .;		/* Text and read-only data	*/
 		ARM_TEXT
+		SLS_TEXT
 	}
 
 #ifdef CONFIG_DEBUG_ALIGN_RODATA
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/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
index 4c0b0c89ad59..f8912e42ffcd 100644
--- a/arch/arm64/kernel/vmlinux.lds.S
+++ b/arch/arm64/kernel/vmlinux.lds.S
@@ -93,6 +93,10 @@ jiffies = jiffies_64;
 #define TRAMP_TEXT
 #endif
 
+#define SLS_TEXT					\
+	ALIGN_FUNCTION();				\
+	*(.text.__llvm_slsblr_thunk_*)
+
 /*
  * The size of the PE/COFF section that covers the kernel image, which
  * runs from _stext to _edata, must be a round multiple of the PE/COFF
@@ -144,6 +148,7 @@ SECTIONS
 			HIBERNATE_TEXT
 			TRAMP_TEXT
 			*(.fixup)
+			SLS_TEXT
 			*(.gnu.warning)
 		. = ALIGN(16);
 		*(.got)			/* Global offset table		*/
diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
index 269967c4fc1b..e70f227019e1 100644
--- a/security/Kconfig.hardening
+++ b/security/Kconfig.hardening
@@ -121,6 +121,16 @@ choice
 
 endchoice
 
+config HARDEN_SLS_ALL
+	bool "enable SLS vulnerability hardening"
+	default n
+	def_bool $(cc-option,-mharden-sls=all)
+	help
+	  Enables straight-line speculation vulnerability hardening on ARM and ARM64
+	  architectures. It inserts speculation barrier sequences (SB or DSB+ISB
+	  depending on the target architecture) after RET and BR, and replacing
+	  BLR with BL+BR sequence.
+
 config GCC_PLUGIN_STRUCTLEAK_VERBOSE
 	bool "Report forcefully initialized variables"
 	depends on GCC_PLUGIN_STRUCTLEAK
-- 
2.30.0.617.g56c4b15f3c-goog