Nearly all modern compilers support a stack-protector option, and nearly all modern distributions enable the kernel stack-protector, so enabling this by default in kernel builds would make sense. However, Kconfig does not have knowledge of available compiler features, so it isn't safe to force on, as this would unconditionally break builds for the compilers or architectures that don't have support. Instead, this introduces a new option, CONFIG_CC_STACKPROTECTOR_AUTO, which attempts to discover the best possible stack-protector available, and will allow builds to proceed even if the compiler doesn't support any stack-protector. This option is made the default so that kernels built with modern compilers will be protected-by-default against stack buffer overflows, avoiding things like the recent BlueBorne attack. Selection of a specific stack-protector option remains available, including disabling it. Cc: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx> Cc: Michal Marek <mmarek@xxxxxxxx> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Cc: Ingo Molnar <mingo@xxxxxxxxxx> Cc: Laura Abbott <labbott@xxxxxxxxxx> Cc: Nicholas Piggin <npiggin@xxxxxxxxx> Cc: linux-kbuild@xxxxxxxxxxxxxxx Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> --- Makefile | 11 +++++++++++ arch/Kconfig | 8 +++++++- 2 files changed, 18 insertions(+), 1 deletion(-) diff --git a/Makefile b/Makefile index e122a9cf0399..9bd334b35003 100644 --- a/Makefile +++ b/Makefile @@ -676,6 +676,10 @@ endif # This selects the stack protector compiler flag. Testing it is delayed # until after .config has been reprocessed, in the prepare-compiler-check # target. +ifdef CONFIG_CC_STACKPROTECTOR_AUTO + stackp-flag := $(call cc-option,-fstack-protector-strong,$(call cc-option,-fstack-protector)) + stackp-name := AUTO +else ifdef CONFIG_CC_STACKPROTECTOR_REGULAR stackp-flag := -fstack-protector stackp-name := REGULAR @@ -688,6 +692,7 @@ else stackp-flag := $(call cc-option, -fno-stack-protector) endif endif +endif ifdef stackp-name # If the stack protector has been selected, inform the rest of the build. KBUILD_CFLAGS += -DCONFIG_CC_STACKPROTECTOR @@ -1084,6 +1089,12 @@ PHONY += prepare-compiler-check prepare-compiler-check: FORCE # Make sure compiler supports requested stack protector flag. ifdef stackp-name + # Warn about CONFIG_CC_STACKPROTECTOR_AUTO having found no option. + ifeq ($(stackp-flag),) + @echo CONFIG_CC_STACKPROTECTOR_$(stackp-name): \ + Compiler does not support any known stack-protector >&2 + endif + # Fail if specifically requested stack protector is missing. ifeq ($(call cc-option, $(stackp-flag)),) @echo Cannot use CONFIG_CC_STACKPROTECTOR_$(stackp-name): \ $(stackp-flag) not supported by compiler >&2 && exit 1 diff --git a/arch/Kconfig b/arch/Kconfig index 7007c1bfa79c..cd2676ff5d5d 100644 --- a/arch/Kconfig +++ b/arch/Kconfig @@ -526,7 +526,7 @@ config HAVE_CC_STACKPROTECTOR choice prompt "Stack Protector buffer overflow detection" depends on HAVE_CC_STACKPROTECTOR - default CC_STACKPROTECTOR_NONE + default CC_STACKPROTECTOR_AUTO help This option turns on the "stack-protector" GCC feature. This feature puts, at the beginning of functions, a canary value on @@ -573,6 +573,12 @@ config CC_STACKPROTECTOR_STRONG about 20% of all kernel functions, which increases the kernel code size by about 2%. +config CC_STACKPROTECTOR_AUTO + bool "Automatic" + help + If the compiler supports it, the best available stack-protector + option will be chosen. + endchoice config THIN_ARCHIVES -- 2.7.4 -- To unsubscribe from this list: send the line "unsubscribe linux-kbuild" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
