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Date: Thu, 11 Apr 2019 11:01:17 -0700
From: Kees Cook <keescook@...omium.org>
To: Alexander Potapenko <glider@...gle.com>
Cc: Kees Cook <keescook@...omium.org>,
Masahiro Yamada <yamada.masahiro@...ionext.com>,
James Morris <jmorris@...ei.org>,
Alexander Popov <alex.popov@...ux.com>,
Nick Desaulniers <ndesaulniers@...gle.com>,
Kostya Serebryany <kcc@...gle.com>,
Dmitry Vyukov <dvyukov@...gle.com>,
Sandeep Patil <sspatil@...roid.com>,
Laura Abbott <labbott@...hat.com>,
Randy Dunlap <rdunlap@...radead.org>,
Michal Marek <michal.lkml@...kovi.net>,
Emese Revfy <re.emese@...il.com>,
"Serge E. Hallyn" <serge@...lyn.com>,
kernel-hardening@...ts.openwall.com,
linux-security-module@...r.kernel.org,
linux-kbuild@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: [PATCH v2 3/3] security: Implement Clang's stack initialization
CONFIG_INIT_STACK_ALL turns on stack initialization based on
-ftrivial-auto-var-init in Clang builds, which has greater coverage
than CONFIG_GCC_PLUGINS_STRUCTLEAK_BYREF_ALL.
-ftrivial-auto-var-init Clang option provides trivial initializers for
uninitialized local variables, variable fields and padding.
It has three possible values:
pattern - uninitialized locals are filled with a fixed pattern
(mostly 0xAA on 64-bit platforms, see https://reviews.llvm.org/D54604
for more details, but 0x000000AA for 32-bit pointers) likely to cause
crashes when uninitialized value is used;
zero (it's still debated whether this flag makes it to the official
Clang release) - uninitialized locals are filled with zeroes;
uninitialized (default) - uninitialized locals are left intact.
This patch uses only the "pattern" mode when CONFIG_INIT_STACK_ALL is
enabled.
Developers have the possibility to opt-out of this feature on a
per-variable basis by using __attribute__((uninitialized)), but such
use should be well justified in comments.
Co-developed-by: Alexander Potapenko <glider@...gle.com>
Signed-off-by: Alexander Potapenko <glider@...gle.com>
Signed-off-by: Kees Cook <keescook@...omium.org>
---
Makefile | 5 +++++
security/Kconfig.hardening | 15 ++++++++++++++-
2 files changed, 19 insertions(+), 1 deletion(-)
diff --git a/Makefile b/Makefile
index c0a34064c574..a7d9c6cd0267 100644
--- a/Makefile
+++ b/Makefile
@@ -745,6 +745,11 @@ KBUILD_CFLAGS += -fomit-frame-pointer
endif
endif
+# Initialize all stack variables with a pattern, if desired.
+ifdef CONFIG_INIT_STACK_ALL
+KBUILD_CFLAGS += -ftrivial-auto-var-init=pattern
+endif
+
DEBUG_CFLAGS := $(call cc-option, -fno-var-tracking-assignments)
ifdef CONFIG_DEBUG_INFO
diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
index 3dd7a28c3822..5dd61770d3f0 100644
--- a/security/Kconfig.hardening
+++ b/security/Kconfig.hardening
@@ -18,9 +18,12 @@ config GCC_PLUGIN_STRUCTLEAK
menu "Memory initialization"
+config CC_HAS_AUTO_VAR_INIT
+ def_bool $(cc-option,-ftrivial-auto-var-init=pattern)
+
choice
prompt "Initialize kernel stack variables at function entry"
- depends on GCC_PLUGINS
+ depends on CC_HAS_AUTO_VAR_INIT || GCC_PLUGINS
default INIT_STACK_NONE
help
This option enables initialization of stack variables at
@@ -76,6 +79,16 @@ choice
of uninitialized stack variable exploits and information
exposures.
+ config INIT_STACK_ALL
+ bool "0xAA-init everything on the stack (strongest)"
+ depends on CC_HAS_AUTO_VAR_INIT
+ help
+ Initializes everything on the stack with a 0xAA
+ pattern. This is intended to eliminate all classes
+ of uninitialized stack variable exploits and information
+ exposures, even variables that were warned to have been
+ left uninitialized.
+
endchoice
config GCC_PLUGIN_STRUCTLEAK_VERBOSE
--
2.17.1
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