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Subject[PATCH v2 3/3] security: Implement Clang's stack initialization
Date
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@google.com>
Signed-off-by: Alexander Potapenko <glider@google.com>
Signed-off-by: Kees Cook <keescook@chromium.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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