Re: [PATCH] hardening: Default to INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_ZERO
From: Nick Desaulniers <ndesaulniers@google.com>
Date: 2021-09-14 18:53:54
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`On Tue, Sep 14, 2021 at 10:21 AM Kees Cook [off-list ref] wrote:
On Tue, Sep 14, 2021 at 08:58:12AM -0700, Nathan Chancellor wrote:quoted
On 9/14/2021 3:28 AM, Will Deacon wrote:quoted
CC_HAS_AUTO_VAR_INIT_ZERO requires a supported set of compiler options distinct from those needed by CC_HAS_AUTO_VAR_INIT_PATTERN, Fix up the Kconfig dependency for INIT_STACK_ALL_ZERO to test for the former instead of the latter, as these are the options passed by the top-level Makefile. Cc: Kees Cook <redacted> Cc: Nathan Chancellor <nathan@kernel.org> Cc: Nick Desaulniers <ndesaulniers@google.com> Cc: Gustavo A. R. Silva <gustavoars@kernel.org> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Fixes: dcb7c0b9461c ("hardening: Clarify Kconfig text for auto-var-init") Signed-off-by: Will Deacon <will@kernel.org>Reviewed-by: Nathan Chancellor <nathan@kernel.org> One comment below.quoted
--- I just noticed this while reading the code and I suspect it doesn't really matter in practice. security/Kconfig.hardening | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening index 90cbaff86e13..341e2fdcba94 100644 --- a/security/Kconfig.hardening +++ b/security/Kconfig.hardening@@ -29,7 +29,7 @@ choice prompt "Initialize kernel stack variables at function entry" default GCC_PLUGIN_STRUCTLEAK_BYREF_ALL if COMPILE_TEST && GCC_PLUGINS default INIT_STACK_ALL_PATTERN if COMPILE_TEST && CC_HAS_AUTO_VAR_INIT_PATTERN - default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_PATTERN + default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_ZERO default INIT_STACK_NONE help This option enables initialization of stack variables atWhile I think this change is correct in and of itself, CONFIG_INIT_STACK_ALL_ZERO is broken with GCC 12.x, as CONFIG_CC_HAS_AUTO_VAR_INIT_ZERO won't be set even though GCC now supports -ftrivial-auto-var-init=zero because GCC does not implement the -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang flag for obvious reasons ;) the cc-option call probably needs to be adjusted.GCC silently ignores the -enable flag, so things actually work correctly as-is.
So then would that mean that CC_HAS_AUTO_VAR_INIT_ZERO_WITH_ENABLE evaluates to true then, in your patch below? Rather than create 2 new kconfigs with 1 new invocation of the compiler via cc-option, how about just adding an `ifdef CONFIG_CC_IS_CLANG` guard around adding the obnoxious flag to `KBUILD_CFLAGS` in the top level Makefile?
quoted hunk ↗ jump to hunk
But, yes, it makes the command line long and doesn't make sense. How about we do this instead:diff --git a/Makefile b/Makefile index 34a0afc3a8eb..34439deac939 100644 --- a/Makefile +++ b/Makefile@@ -831,12 +831,11 @@ endif # Initialize all stack variables with a zero value. ifdef CONFIG_INIT_STACK_ALL_ZERO -# Future support for zero initialization is still being debated, see -# https://bugs.llvm.org/show_bug.cgi?id=45497. These flags are subject to being -# renamed or dropped. KBUILD_CFLAGS += -ftrivial-auto-var-init=zero +ifdef CONFIG_CC_HAS_AUTO_VAR_INIT_ZERO_WITH_ENABLE KBUILD_CFLAGS += -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang endif +endif # While VLAs have been removed, GCC produces unreachable stack probes # for the randomize_kstack_offset feature. Disable it for all compilers.diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening index 90cbaff86e13..beea81df3081 100644 --- a/security/Kconfig.hardening +++ b/security/Kconfig.hardening@@ -22,14 +22,22 @@ menu "Memory initialization" config CC_HAS_AUTO_VAR_INIT_PATTERN def_bool $(cc-option,-ftrivial-auto-var-init=pattern) +config CC_HAS_AUTO_VAR_INIT_ZERO_WITHOUT_ENABLE + def_bool $(cc-option,-ftrivial-auto-var-init=zero) + +config CC_HAS_AUTO_VAR_INIT_ZERO_WITH_ENABLE + # https://bugs.llvm.org/show_bug.cgi?id=45497 + def_bool !CC_HAS_AUTO_VAR_INIT_ZERO_WITHOUT_ENABLE && \ + $(cc-option,-ftrivial-auto-var-init=zero -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang) + config CC_HAS_AUTO_VAR_INIT_ZERO - def_bool $(cc-option,-ftrivial-auto-var-init=zero -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang) + def_bool CC_HAS_AUTO_VAR_INIT_ZERO_WITHOUT_ENABLE || CC_HAS_AUTO_VAR_INIT_ZERO_WITH_ENABLE choice prompt "Initialize kernel stack variables at function entry" default GCC_PLUGIN_STRUCTLEAK_BYREF_ALL if COMPILE_TEST && GCC_PLUGINS default INIT_STACK_ALL_PATTERN if COMPILE_TEST && CC_HAS_AUTO_VAR_INIT_PATTERN - default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_PATTERN + default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_ZERO default INIT_STACK_NONE help This option enables initialization of stack variables at --Kees Cook
-- Thanks, ~Nick Desaulniers