[PATCH 10/23] stack-protector: test compiler capability in Kconfig and drop AUTO mode
From: Masahiro Yamada
Date: Fri Feb 16 2018 - 13:42:35 EST
Add CC_HAS_STACKPROTECTOR(_STRONG) to test if the compiler supports
-fstack-protector(-strong) option.
X86 has additional shell scripts in case the compiler supports the
option, but generates broken code. I added CC_HAS_SANE_STACKPROTECTOR
to test this. I had to add -m32 to gcc-x86_32-has-stack-protector.sh
to make it work correctly.
If the compiler does not support the option, the menu is automatically
hidden. If _STRONG is not supported, it will fall back to _REGULAR.
This means, _AUTO is implicitly supported in the dependency solver of
Kconfig, hence removed.
I also turned the 'choice' into only two boolean symbols. The use of
'choice' is not a good idea here, because all of all{yes,mod,no}config
would choose the first visible value, while we want allnoconfig to
disable as many features as possible.
I did not add CC_HAS_STACKPROTECTOR_NONE in the hope that GCC versions
we support will recognize -fno-stack-protector.
If this turns out to be a problem, it will be possible to do this:
stackp-flags-$(CONFIG_CC_HAS_STACKPROTECTOR_NONE) := -fno-stack-protector
stackp-flags-$(CONFIG_CC_STACKPROTECTOR) := -fstack-protector
stackp-flags-$(CONFIG_CC_STACKPROTECTOR_STRONG) := -fstack-protector-strong
Signed-off-by: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
---
Makefile | 93 ++-----------------------------
arch/Kconfig | 37 ++++++------
arch/x86/Kconfig | 8 ++-
scripts/gcc-x86_32-has-stack-protector.sh | 7 +--
scripts/gcc-x86_64-has-stack-protector.sh | 5 --
5 files changed, 30 insertions(+), 120 deletions(-)
diff --git a/Makefile b/Makefile
index 9a8c689..e9fc7c9 100644
--- a/Makefile
+++ b/Makefile
@@ -675,55 +675,11 @@ ifneq ($(CONFIG_FRAME_WARN),0)
KBUILD_CFLAGS += $(call cc-option,-Wframe-larger-than=${CONFIG_FRAME_WARN})
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
-else
-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.
- 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 $(stackp-path))
- # If the wildcard test matches a test script, run it to check functionality.
- ifdef stackp-check
- ifneq ($(shell $(CONFIG_SHELL) $(stackp-check) $(CC) $(KBUILD_CPPFLAGS) $(biarch)),y)
- stackp-broken := y
- endif
- endif
- 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 += $(call cc-option, -fno-stack-protector)
- endif
-endif
-endif
+stackp-flags-y := -fno-stack-protector
+stackp-flags-$(CONFIG_CC_STACKPROTECTOR) := -fstack-protector
+stackp-flags-$(CONFIG_CC_STACKPROTECTOR_STRONG) := -fstack-protector-strong
+
+KBUILD_CFLAGS += $(stackp-flags-y)
ifeq ($(cc-name),clang)
KBUILD_CPPFLAGS += $(call cc-option,-Qunused-arguments,)
@@ -1079,7 +1035,7 @@ endif
# 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
+prepare2: prepare3 outputmakefile asm-generic
prepare1: prepare2 $(version_h) include/generated/utsrelease.h \
include/config/auto.conf
@@ -1105,43 +1061,6 @@ uapi-asm-generic:
PHONY += prepare-objtool
prepare-objtool: $(objtool_target)
-# Check for CONFIG flags that require compiler support. Abort the build
-# after .config has been processed, but before the kernel build starts.
-#
-# For security-sensitive CONFIG options, we don't want to fallback and/or
-# silently change which compiler flags will be used, since that leads to
-# producing kernels with different security feature characteristics
-# depending on the compiler used. (For example, "But I selected
-# CC_STACKPROTECTOR_STRONG! Why did it build with _REGULAR?!")
-PHONY += prepare-compiler-check
-prepare-compiler-check: FORCE
-# Make sure compiler supports requested stack protector flag.
-ifdef stackp-name
- # Warn about CONFIG_CC_STACKPROTECTOR_AUTO having found no option.
- ifeq ($(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): \
- $(stackp-flag) not supported by compiler >&2 && exit 1
- endif
- endif
-endif
-# Make sure compiler does not have buggy stack-protector support. If a
-# specific stack-protector was requested, fail the build, otherwise warn.
-ifdef stackp-broken
- ifeq ($(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
- @:
-
# Generate some files
# ---------------------------------------------------------------------------
diff --git a/arch/Kconfig b/arch/Kconfig
index 76c0b54..9b7a628 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -535,13 +535,21 @@ config HAVE_CC_STACKPROTECTOR
bool
help
An arch should select this symbol if:
- - its compiler supports the -fstack-protector option
- it has implemented a stack canary (e.g. __stack_chk_guard)
-choice
- prompt "Stack Protector buffer overflow detection"
+config CC_HAS_STACKPROTECTOR
+ bool
+ default $(cc-option -fstack-protector)
+
+config CC_HAS_STACKPROTECTOR_STRONG
+ bool
+ default $(cc-option -fstack-protector-strong)
+
+config CC_STACKPROTECTOR
+ bool "Stack Protector buffer overflow detection"
depends on HAVE_CC_STACKPROTECTOR
- default CC_STACKPROTECTOR_AUTO
+ depends on CC_HAS_STACKPROTECTOR
+ default y
help
This option turns on the "stack-protector" GCC feature. This
feature puts, at the beginning of functions, a canary value on
@@ -551,14 +559,6 @@ choice
overwrite the canary, which gets detected and the attack is then
neutralized via a kernel panic.
-config CC_STACKPROTECTOR_NONE
- bool "None"
- help
- Disable "stack-protector" GCC feature.
-
-config CC_STACKPROTECTOR_REGULAR
- bool "Regular"
- help
Functions will have the stack-protector canary logic added if they
have an 8-byte or larger character array on the stack.
@@ -570,7 +570,10 @@ config CC_STACKPROTECTOR_REGULAR
by about 0.3%.
config CC_STACKPROTECTOR_STRONG
- bool "Strong"
+ bool "Strong Stack Protector"
+ depends on CC_HAS_STACKPROTECTOR_STRONG
+ depends on CC_STACKPROTECTOR
+ default y
help
Functions will have the stack-protector canary logic added in any
of the following conditions:
@@ -588,14 +591,6 @@ 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
def_bool y
help
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 54d943a..498694d 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -124,7 +124,6 @@ config X86
select HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD if X86_64
select HAVE_ARCH_VMAP_STACK if X86_64
select HAVE_ARCH_WITHIN_STACK_FRAMES
- select HAVE_CC_STACKPROTECTOR
select HAVE_CMPXCHG_DOUBLE
select HAVE_CMPXCHG_LOCAL
select HAVE_CONTEXT_TRACKING if X86_64
@@ -340,6 +339,13 @@ config PGTABLE_LEVELS
default 2
source "init/Kconfig"
+
+config CC_HAS_SANE_STACKPROTECTOR
+ bool
+ default $(shell $srctree/scripts/gcc-x86_64-has-stack-protector.sh $CC) if 64BIT
+ default $(shell $srctree/scripts/gcc-x86_32-has-stack-protector.sh $CC)
+ select HAVE_CC_STACKPROTECTOR
+
source "kernel/Kconfig.freezer"
menu "Processor type and features"
diff --git a/scripts/gcc-x86_32-has-stack-protector.sh b/scripts/gcc-x86_32-has-stack-protector.sh
index 6b2aeef..f5c1194 100755
--- a/scripts/gcc-x86_32-has-stack-protector.sh
+++ b/scripts/gcc-x86_32-has-stack-protector.sh
@@ -1,9 +1,4 @@
#!/bin/sh
# SPDX-License-Identifier: GPL-2.0
-echo "int foo(void) { char X[200]; return 3; }" | $* -S -x c -c -O0 -fstack-protector - -o - 2> /dev/null | grep -q "%gs"
-if [ "$?" -eq "0" ] ; then
- echo y
-else
- echo n
-fi
+echo "int foo(void) { char X[200]; return 3; }" | $* -S -x c -c -m32 -O0 -fstack-protector - -o - 2> /dev/null | grep -q "%gs"
diff --git a/scripts/gcc-x86_64-has-stack-protector.sh b/scripts/gcc-x86_64-has-stack-protector.sh
index 4a48bdc..3755af0 100755
--- a/scripts/gcc-x86_64-has-stack-protector.sh
+++ b/scripts/gcc-x86_64-has-stack-protector.sh
@@ -2,8 +2,3 @@
# SPDX-License-Identifier: GPL-2.0
echo "int foo(void) { char X[200]; return 3; }" | $* -S -x c -c -O0 -mcmodel=kernel -fno-PIE -fstack-protector - -o - 2> /dev/null | grep -q "%gs"
-if [ "$?" -eq "0" ] ; then
- echo y
-else
- echo n
-fi
--
2.7.4