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* [PATCH v2] hardening: Avoid harmless Clang option under CONFIG_INIT_STACK_ALL_ZERO
@ 2021-09-14 20:02 Kees Cook
  2021-09-15  7:50 ` Will Deacon
  0 siblings, 1 reply; 2+ messages in thread
From: Kees Cook @ 2021-09-14 20:02 UTC (permalink / raw)
  To: Kees Cook
  Cc: Greg Kroah-Hartman, Masahiro Yamada, llvm, Will Deacon,
	Nick Desaulniers, Nathan Chancellor, Gustavo A. R. Silva,
	Michal Marek, James Morris, Serge E. Hallyn, linux-kernel,
	linux-kbuild, linux-security-module, linux-hardening

Currently under Clang, CC_HAS_AUTO_VAR_INIT_ZERO requires an extra
-enable flag compared to CC_HAS_AUTO_VAR_INIT_PATTERN. GCC does not,
and will happily ignore the Clang-specific flag. However, its presence
on the command-line is both cumbersome and confusing. Due to GCC's
tolerant behavior, though, we can continue to use a single Kconfig
cc-option test for the feature on both compilers, but then drop the
Clang-specific option in the Makefile.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: llvm@lists.linux.dev
Fixes: dcb7c0b9461c ("hardening: Clarify Kconfig text for auto-var-init")
Suggested-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/lkml/20210914102837.6172-1-will@kernel.org/
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 Makefile                   | 6 +++---
 security/Kconfig.hardening | 5 ++++-
 2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/Makefile b/Makefile
index 34a0afc3a8eb..72d165ffabdb 100644
--- a/Makefile
+++ b/Makefile
@@ -831,12 +831,12 @@ 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_IS_CLANG
+# https://bugs.llvm.org/show_bug.cgi?id=45497
 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..ded17b8abce2 100644
--- a/security/Kconfig.hardening
+++ b/security/Kconfig.hardening
@@ -23,13 +23,16 @@ config CC_HAS_AUTO_VAR_INIT_PATTERN
 	def_bool $(cc-option,-ftrivial-auto-var-init=pattern)
 
 config CC_HAS_AUTO_VAR_INIT_ZERO
+	# GCC ignores the -enable flag, so we can test for the feature with
+	# a single invocation using the flag, but drop it as appropriate in
+	# the Makefile, depending on the presence of Clang.
 	def_bool $(cc-option,-ftrivial-auto-var-init=zero -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang)
 
 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
-- 
2.30.2


^ permalink raw reply	[flat|nested] 2+ messages in thread

* Re: [PATCH v2] hardening: Avoid harmless Clang option under CONFIG_INIT_STACK_ALL_ZERO
  2021-09-14 20:02 [PATCH v2] hardening: Avoid harmless Clang option under CONFIG_INIT_STACK_ALL_ZERO Kees Cook
@ 2021-09-15  7:50 ` Will Deacon
  0 siblings, 0 replies; 2+ messages in thread
From: Will Deacon @ 2021-09-15  7:50 UTC (permalink / raw)
  To: Kees Cook
  Cc: Greg Kroah-Hartman, Masahiro Yamada, llvm, Nick Desaulniers,
	Nathan Chancellor, Gustavo A. R. Silva, Michal Marek,
	James Morris, Serge E. Hallyn, linux-kernel, linux-kbuild,
	linux-security-module, linux-hardening

On Tue, Sep 14, 2021 at 01:02:03PM -0700, Kees Cook wrote:
> Currently under Clang, CC_HAS_AUTO_VAR_INIT_ZERO requires an extra
> -enable flag compared to CC_HAS_AUTO_VAR_INIT_PATTERN. GCC does not,
> and will happily ignore the Clang-specific flag. However, its presence
> on the command-line is both cumbersome and confusing. Due to GCC's
> tolerant behavior, though, we can continue to use a single Kconfig
> cc-option test for the feature on both compilers, but then drop the
> Clang-specific option in the Makefile.
> 
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Masahiro Yamada <masahiroy@kernel.org>
> Cc: llvm@lists.linux.dev
> Fixes: dcb7c0b9461c ("hardening: Clarify Kconfig text for auto-var-init")
> Suggested-by: Will Deacon <will@kernel.org>
> Link: https://lore.kernel.org/lkml/20210914102837.6172-1-will@kernel.org/
> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
> Reviewed-by: Nathan Chancellor <nathan@kernel.org>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  Makefile                   | 6 +++---
>  security/Kconfig.hardening | 5 ++++-
>  2 files changed, 7 insertions(+), 4 deletions(-)

Acked-by: Will Deacon <will@kernel.org>

Cheers for sorting this out!

Will

^ permalink raw reply	[flat|nested] 2+ messages in thread

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