Before, the stack protector flag was sanity checked before .config had
been reprocessed. This meant the build couldn't be aborted early, and
only a warning could be emitted followed later by the compiler blowing
up with an unknown flag. This has caused a lot of confusion over time,
so this splits the flag selection from sanity checking and performs the
sanity checking after the make has been restarted from a reprocessed
.config, so builds can be aborted as early as possible now.
Additionally moves the x86-specific sanity check to the same location,
since it suffered from the same warn-then-wait-for-compiler-failure
problem.
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
Makefile | 69 +++++++++++++++++++++++++++++++++----------------------
arch/x86/Makefile | 8 -------
2 files changed, 42 insertions(+), 35 deletions(-)
diff --git a/Makefile b/Makefile
index 6a170fc856b2..ab124a0e5e0d 100644
--- a/Makefile
+++ b/Makefile
@@ -656,41 +656,28 @@ ifneq ($(CONFIG_FRAME_WARN),0)
KBUILD_CFLAGS += $(call cc-option,-Wframe-larger-than=${CONFIG_FRAME_WARN})
endif
-# Handle stack protector mode.
-#
-# Since kbuild can potentially perform two passes (first with the old
-# .config values and then with updated .config values), we cannot error out
-# if a desired compiler option is unsupported. If we were to error, kbuild
-# could never get to the second pass and actually notice that we changed
-# the option to something that was supported.
-#
-# Additionally, we don't want to fallback and/or silently change which compiler
-# flags will be used, since that leads to producing kernels with different
-# security feature characteristics depending on the compiler used. ("But I
-# selected CC_STACKPROTECTOR_STRONG! Why did it build with _REGULAR?!")
-#
-# The middle ground is to warn here so that the failed option is obvious, but
-# to let the build fail with bad compiler flags so that we can't produce a
-# kernel when there is a CONFIG and compiler mismatch.
-#
+# 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_REGULAR
stackp-flag := -fstack-protector
- ifeq ($(call cc-option, $(stackp-flag)),)
- $(warning Cannot use CONFIG_CC_STACKPROTECTOR_REGULAR: \
- -fstack-protector not supported by compiler)
- endif
+ stackp-name := REGULAR
else
ifdef CONFIG_CC_STACKPROTECTOR_STRONG
stackp-flag := -fstack-protector-strong
- ifeq ($(call cc-option, $(stackp-flag)),)
- $(warning Cannot use CONFIG_CC_STACKPROTECTOR_STRONG: \
- -fstack-protector-strong not supported by compiler)
- endif
+ stackp-name := STRONG
else
# Force off for distro compilers that enable stack protector by default.
stackp-flag := $(call cc-option, -fno-stack-protector)
endif
endif
+# Find arch-specific stack protector compiler sanity-checking script.
+ifdef CONFIG_CC_STACKPROTECTOR
+ stackp-path := $(srctree)/scripts/gcc-$(ARCH)_$(BITS)-has-stack-protector.sh
+ ifneq ($(wildcard $(stackp-path)),)
+ stackp-check := $(stackp-path)
+ endif
+endif
KBUILD_CFLAGS += $(stackp-flag)
ifeq ($(cc-name),clang)
@@ -1018,8 +1005,10 @@ ifneq ($(KBUILD_SRC),)
fi;
endif
-# prepare2 creates a makefile if using a separate output directory
-prepare2: prepare3 outputmakefile asm-generic
+# prepare2 creates a makefile if using a separate output directory.
+# From this point forward, .config has been reprocessed, so any rules
+# that need to depend on updated CONFIG_* values can be checked here.
+prepare2: prepare3 prepare-compiler-check outputmakefile asm-generic
prepare1: prepare2 $(version_h) include/generated/utsrelease.h \
include/config/auto.conf
@@ -1050,6 +1039,32 @@ endif
PHONY += prepare-objtool
prepare-objtool: $(objtool_target)
+# Check for CONFIG flags that require compiler support. Abort the build
+# after .config has been processed, but before the kernel build starts.
+#
+# For security-sensitive CONFIG options, we don't want to fallback and/or
+# silently change which compiler flags will be used, since that leads to
+# producing kernels with different security feature characteristics
+# depending on the compiler used. (For example, "But I selected
+# CC_STACKPROTECTOR_STRONG! Why did it build with _REGULAR?!")
+PHONY += prepare-compiler-check
+prepare-compiler-check: FORCE
+# Make sure compiler supports requested stack protector flag.
+ifdef stackp-name
+ ifeq ($(call cc-option, $(stackp-flag)),)
+ @echo Cannot use CONFIG_CC_STACKPROTECTOR_$(stackp-name): \
+ $(stackp-flag) not supported by compiler >&2 && exit 1
+ endif
+endif
+# Make sure compiler does not have buggy stack-protector support.
+ifdef stackp-check
+ ifneq ($(shell $(CONFIG_SHELL) $(stackp-check) $(CC) $(KBUILD_CPPFLAGS) $(biarch)),y)
+ @echo Cannot use CONFIG_CC_STACKPROTECTOR_$(stackp-name): \
+ $(stackp-flag) available but compiler is broken >&2 && exit 1
+ endif
+endif
+ @:
+
# Generate some files
# ---------------------------------------------------------------------------
diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 6fce7f096b88..830ed391e7ef 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -126,14 +126,6 @@ else
KBUILD_CFLAGS += $(call cc-option,-maccumulate-outgoing-args)
endif
-# Make sure compiler does not have buggy stack-protector support.
-ifdef CONFIG_CC_STACKPROTECTOR
- cc_has_sp := $(srctree)/scripts/gcc-x86_$(BITS)-has-stack-protector.sh
- ifneq ($(shell $(CONFIG_SHELL) $(cc_has_sp) $(CC) $(KBUILD_CPPFLAGS) $(biarch)),y)
- $(warning stack-protector enabled but compiler support broken)
- endif
-endif
-
ifdef CONFIG_X86_X32
x32_ld_ok := $(call try-run,\
/bin/echo -e '1: .quad 1b' | \
--
2.7.4
--
Kees Cook
Chrome OS & Brillo Security
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