stack-protector: test compiler capability in Kconfig and drop AUTO mode

Move the test for -fstack-protector(-strong) option to Kconfig.

If the compiler does not support the option, the corresponding menu
is automatically hidden.  If STRONG is not supported, it will fall
back to REGULAR.  If REGULAR is not supported, it will be disabled.
This means, AUTO is implicitly handled by 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.

X86 has additional shell scripts in case the compiler supports those
options, 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.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Kees Cook <keescook@chromium.org>

5 files changed, 28 insertions, 120 deletions

diff --git a/Makefile b/Makefile
index 747edaf11559..ca9d98b4a71b 100644
--- a/Makefile
+++ b/Makefile
@@ -686,55 +686,11 @@ 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 += $(call cc-option, -fno-stack-protector)
-  endif
-endif
-endif
+stackp-flags-$(CONFIG_CC_HAS_STACKPROTECTOR_NONE) := -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,)
@@ -1118,7 +1074,7 @@ 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) $(autoksyms_h) include/generated/utsrelease.h \
                    include/config/auto.conf
@@ -1144,43 +1100,6 @@ 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 8a7f7e1f2ca7..2c7c3cf8adfc 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -536,13 +536,16 @@ 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_NONE
+        def_bool $(cc-option,-fno-stack-protector)
+
+config CC_STACKPROTECTOR
+        bool "Stack Protector buffer overflow detection"
         depends on HAVE_CC_STACKPROTECTOR
-        default CC_STACKPROTECTOR_AUTO
+        depends on $(cc-option,-fstack-protector)
+        default y
         help
           This option turns on the "stack-protector" GCC feature. This
           feature puts, at the beginning of functions, a canary value on
@@ -552,14 +555,6 @@ 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.
 
@@ -571,7 +566,10 @@ config CC_STACKPROTECTOR_REGULAR
           by about 0.3%.
 
 config CC_STACKPROTECTOR_STRONG
-        bool "Strong"
+        bool "Strong Stack Protector"
+        depends on CC_STACKPROTECTOR
+        depends on $(cc-option,-fstack-protector-strong)
+        default y
         help
           Functions will have the stack-protector canary logic added in any
           of the following conditions:
@@ -589,14 +587,6 @@ 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 HAVE_ARCH_WITHIN_STACK_FRAMES
         bool
         help
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index cb6e3a219294..50a1b8ec9ad9 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -129,7 +129,7 @@ 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_CC_STACKPROTECTOR           if CC_HAS_SANE_STACKPROTECTOR
         select HAVE_CMPXCHG_DOUBLE
         select HAVE_CMPXCHG_LOCAL
         select HAVE_CONTEXT_TRACKING            if X86_64
@@ -341,6 +341,15 @@ config PGTABLE_LEVELS
         default 2
 
 source "init/Kconfig"
+
+config CC_HAS_SANE_STACKPROTECTOR
+        bool
+        default $(success,$(srctree)/scripts/gcc-x86_64-has-stack-protector.sh $(CC)) if 64BIT
+        default $(success,$(srctree)/scripts/gcc-x86_32-has-stack-protector.sh $(CC))
+        help
+           We have to make sure stack protector is unconditionally disabled if
+           the compiler produces broken code.
+
 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 6b2aeefb9cd3..f5c119495254 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 4a48bdcd4d6b..3755af0cd9f7 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