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Subject[PATCH] [RFC] security: allow using Clang's zero initialization for stack variables
From
In addition to -ftrivial-auto-var-init=pattern (used by
CONFIG_INIT_STACK_ALL now) Clang also supports zero initialization for
locals enabled by -ftrivial-auto-var-init=zero.
The future of this flag is still being debated, see
https://bugs.llvm.org/show_bug.cgi?id=45497
Right now it is guarded by another flag,
-enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang,
which means it may not be supported by future Clang releases.
Another possible resolution is that -ftrivial-auto-var-init=zero will
persist (as certain users have already started depending on it), but the
name of the guard flag will change.

In the meantime, zero initialization has proven itself as a good
production mitigation measure against uninitialized locals. Unlike
pattern initialization, which has a higher chance of triggering existing
bugs, zero initialization provides safe defaults for strings, pointers,
indexes, and sizes. On the other hand, pattern initialization remains
safer for return values.
Performance-wise, the difference between pattern and zero initialization
is usually negligible, although the generated code for zero
initialization is more compact.

The proposed config, CONFIG_USE_CLANG_ZERO_INITIALIZATION, makes
CONFIG_INIT_STACK_ALL use zero initialization if the corresponding flags
are supported by Clang.

Cc: Kees Cook <keescook@chromium.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Alexander Potapenko <glider@google.com>
---
Makefile | 15 ++++++++++++++-
security/Kconfig.hardening | 16 ++++++++++++++++
2 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index fd31992bf918..2860bad7e39a 100644
--- a/Makefile
+++ b/Makefile
@@ -802,9 +802,22 @@ KBUILD_CFLAGS += -fomit-frame-pointer
endif
endif

-# Initialize all stack variables with a pattern, if desired.
+# Initialize all stack variables, if desired.
ifdef CONFIG_INIT_STACK_ALL
+
+# Use pattern initialization by default.
+ifndef CONFIG_USE_CLANG_ZERO_INITIALIZATION
KBUILD_CFLAGS += -ftrivial-auto-var-init=pattern
+else
+
+ifdef CONFIG_CC_HAS_AUTO_VAR_ZERO_INIT
+# 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 -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
+endif
+
+endif
endif

DEBUG_CFLAGS := $(call cc-option, -fno-var-tracking-assignments)
diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
index af4c979b38ee..299d27c6d78c 100644
--- a/security/Kconfig.hardening
+++ b/security/Kconfig.hardening
@@ -22,6 +22,9 @@ menu "Memory initialization"
config CC_HAS_AUTO_VAR_INIT
def_bool $(cc-option,-ftrivial-auto-var-init=pattern)

+config CC_HAS_AUTO_VAR_ZERO_INIT
+ 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
@@ -100,6 +103,19 @@ choice

endchoice

+config USE_CLANG_ZERO_INITIALIZATION
+ bool "Use Clang's zero initialization for local variables"
+ depends on CC_HAS_AUTO_VAR_ZERO_INIT
+ depends on INIT_STACK_ALL
+ help
+ If set, uses zeros instead of 0xAA to initialize local variables in
+ INIT_STACK_ALL. Zeroing the stack provides safer defaults for strings,
+ pointers, indexes, and sizes. The downsides are less-safe defaults for
+ return values, and exposing fewer bugs where the underlying code
+ relies on zero initialization.
+ The corresponding flag isn't officially supported by Clang and may
+ sooner or later go away or get renamed.
+
config GCC_PLUGIN_STRUCTLEAK_VERBOSE
bool "Report forcefully initialized variables"
depends on GCC_PLUGIN_STRUCTLEAK
--
2.27.0.290.gba653c62da-goog
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