linux-kernel - Re: [PATCH] hardening: Default to INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT

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Message-ID: <202109141214.630BB3A@keescook>
Date:   Tue, 14 Sep 2021 12:14:47 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] hardening: Default to INIT_STACK_ALL_ZERO if
 CC_HAS_AUTO_VAR_INIT_ZERO

On Tue, Sep 14, 2021 at 11:53:38AM -0700, Nick Desaulniers wrote:
> 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?

v2:

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

-- 
Kees Cook