From 44c6dc940b190cf22b044a784f3e00a7e7f08b2f Mon Sep 17 00:00:00 2001 From: Kees Cook Date: Tue, 6 Feb 2018 15:37:45 -0800 Subject: [PATCH] Makefile: introduce CONFIG_CC_STACKPROTECTOR_AUTO 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. Additionally, tiny.config is adjusted to use CC_STACKPROTECTOR_NONE, since that's the option with the least code size (and it used to be the default, so we have to explicitly choose it there now). Link: http://lkml.kernel.org/r/1510076320-69931-4-git-send-email-keescook@chromium.org Signed-off-by: Kees Cook Tested-by: Laura Abbott Cc: Masahiro Yamada Cc: Arnd Bergmann Cc: Josh Triplett Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- Makefile | 37 ++++++++++++++++++++++++++++++++++--- arch/Kconfig | 8 +++++++- kernel/configs/tiny.config | 4 ++++ 3 files changed, 45 insertions(+), 4 deletions(-) diff --git a/Makefile b/Makefile index f0f934097f64..d192dd826cce 100644 --- a/Makefile +++ b/Makefile @@ -680,6 +680,10 @@ 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 @@ -688,12 +692,18 @@ ifdef CONFIG_CC_STACKPROTECTOR_STRONG stackp-flag := -fstack-protector-strong stackp-name := STRONG else + # If either there is no stack protector for this architecture or + # CONFIG_CC_STACKPROTECTOR_NONE is selected, we're done, and $(stackp-name) + # is empty, skipping all remaining stack protector tests. + # # Force off for distro compilers that enable stack protector by default. - stackp-flag := $(call cc-option, -fno-stack-protector) + KBUILD_CFLAGS += $(call cc-option, -fno-stack-protector) +endif endif endif # Find arch-specific stack protector compiler sanity-checking script. ifdef stackp-name +ifneq ($(stackp-flag),) stackp-path := $(srctree)/scripts/gcc-$(SRCARCH)_$(BITS)-has-stack-protector.sh stackp-check := $(wildcard $(stackp-path)) # If the wildcard test matches a test script, run it to check functionality. @@ -705,9 +715,17 @@ ifdef stackp-name ifndef stackp-broken # If the stack protector is functional, enable code that depends on it. KBUILD_CPPFLAGS += -DCONFIG_CC_STACKPROTECTOR + # Either we've already detected the flag (for AUTO) or we'll fail the + # build in the prepare-compiler-check rule (for specific flag). + KBUILD_CFLAGS += $(stackp-flag) + else + # We have to make sure stack protector is unconditionally disabled if + # the compiler is broken (in case we're going to continue the build in + # AUTO mode). + KBUILD_CFLAGS += $(call cc-option, -fno-stack-protector) endif endif -KBUILD_CFLAGS += $(stackp-flag) +endif ifeq ($(cc-name),clang) KBUILD_CPPFLAGS += $(call cc-option,-Qunused-arguments,) @@ -1102,15 +1120,28 @@ PHONY += prepare-compiler-check prepare-compiler-check: FORCE # Make sure compiler supports requested stack protector flag. ifdef stackp-name + # Warn about CONFIG_CC_STACKPROTECTOR_AUTO having found no option. + ifeq ($(stackp-flag),) + @echo CONFIG_CC_STACKPROTECTOR_$(stackp-name): \ + Compiler does not support any known stack-protector >&2 + else + # Fail if specifically requested stack protector is missing. ifeq ($(call cc-option, $(stackp-flag)),) @echo Cannot use CONFIG_CC_STACKPROTECTOR_$(stackp-name): \ $(stackp-flag) not supported by compiler >&2 && exit 1 endif + endif endif -# Make sure compiler does not have buggy stack-protector support. +# Make sure compiler does not have buggy stack-protector support. If a +# specific stack-protector was requested, fail the build, otherwise warn. ifdef stackp-broken + ifeq ($(stackp-name),AUTO) + @echo CONFIG_CC_STACKPROTECTOR_$(stackp-name): \ + $(stackp-flag) available but compiler is broken: disabling >&2 + else @echo Cannot use CONFIG_CC_STACKPROTECTOR_$(stackp-name): \ $(stackp-flag) available but compiler is broken >&2 && exit 1 + endif endif @: diff --git a/arch/Kconfig b/arch/Kconfig index 9330a9d79dc3..76c0b54443b1 100644 --- a/arch/Kconfig +++ b/arch/Kconfig @@ -541,7 +541,7 @@ 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 @@ -588,6 +588,12 @@ 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 diff --git a/kernel/configs/tiny.config b/kernel/configs/tiny.config index 7fa0c4ae6394..9bfdffc100da 100644 --- a/kernel/configs/tiny.config +++ b/kernel/configs/tiny.config @@ -10,3 +10,7 @@ CONFIG_OPTIMIZE_INLINING=y # CONFIG_SLAB is not set # CONFIG_SLUB is not set CONFIG_SLOB=y +CONFIG_CC_STACKPROTECTOR_NONE=y +# CONFIG_CC_STACKPROTECTOR_REGULAR is not set +# CONFIG_CC_STACKPROTECTOR_STRONG is not set +# CONFIG_CC_STACKPROTECTOR_AUTO is not set