Re: [PATCH v2] ARM: Implement Clang's SLS mitigation
From: Nick Desaulniers <ndesaulniers@google.com>
Date: 2021-02-17 18:21:23
Also in:
linux-arm-kernel, lkml
On Fri, Feb 12, 2021 at 11:53 AM 'Jian Cai' via Clang Built Linux [off-list ref] wrote: The oneline of the commit is "ARM: Implement Clang's SLS mitigation," but that's not precise. GCC implements the same flag with the same arguments. There is nothing compiler specific about this patch. (Though perhaps different section names are used, see below).
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 <redacted> Suggested-by: Nathan Chancellor <nathan@kernel.org> Suggested-by: David Laight <redacted> Signed-off-by: Jian Cai <redacted>
I observe lots of linker warnings with this applied on linux-next: ld.lld: warning: init/built-in.a(main.o):(.text.__llvm_slsblr_thunk_x0) is being placed in '.text.__llvm_slsblr_thunk_x0' You need to modify arch/arm64/kernel/vmlinux.lds.S and arch/arm/kernel/vmlinux.lds.S (and possibly arch/arm/boot/compressed/vmlinux.lds.S as well) to add these sections back into .text so that the linkers don't place these orphaned sections in wild places. The resulting aarch64 kernel image doesn't even boot (under emulation). For 32b ARM: ld.lld: warning: init/built-in.a(main.o):(.text.__llvm_slsblr_thunk_arm_r0) is being placed in '.text.__llvm_slsblr_thunk_arm_r0' ... ld.lld: warning: init/built-in.a(main.o):(.text.__llvm_slsblr_thunk_thumb_r0) is being placed in '.text.__llvm_slsblr_thunk_thumb_r0' ... <trimmed, but there's close to 60 of these> And the image doesn't boot (under emulation).
quoted hunk ↗ jump to hunk
--- 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 signeddiff --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)
This fails to set CONFIG_HARDEN_SLS_ALL for me with: $ ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- make LLVM=1 LLVM_IAS=1 -j72 defconfig $ grep SLS_ALL .config # CONFIG_HARDEN_SLS_ALL is not set but it's flipped on there for arm64 defconfig: $ ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make LLVM=1 LLVM_IAS=1 -j72 defconfig $ grep SLS_ALL .config CONFIG_HARDEN_SLS_ALL=y What's going on there? Is the cc-option Kconfig macro broken for Clang when cross compiling 32b ARM? I can still enable CONFIG_HARDEN_SLS_ALL via menuconfig, but I wonder if the default value is funny because the cc-option check is failing?
+ 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
---- Thanks, ~Nick Desaulniers