Re: [PATCH v3] ARM: Implement SLS mitigation

[Nathan Chancellor]

Hi Jian,

On Fri, Feb 19, 2021 at 12:18:40PM -0800, 'Jian Cai' via Clang Built Linux wrote:
> 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@xxxxxxxxxx>
> Suggested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> Suggested-by: Nathan Chancellor  <nathan@xxxxxxxxxx>
> Suggested-by: David Laight <David.Laight@xxxxxxxxxx>
> Suggested-by: Will Deacon <will@xxxxxxxxxx>
> Reviewed-by: Nathan Chancellor <nathan@xxxxxxxxxx>

My review still stands but in the future, if you significantly change
how a patch is structured or works, please drop my tag and let me re-add
it.

One comment below.

> Signed-off-by: Jian Cai <jiancai@xxxxxxxxxx>
> ---
> 
> 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)

This is a much more convoluted way of writing:

depends on $(cc-option,-mharden-sls=all)

"default n" is the default and "def_bool" is short for:

bool
default <expr>

which is defeated by the "default n".

> +	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
>