Messages in this thread Patch in this message | | | Date | Sun, 14 Jun 2020 16:45:34 +0200 | Subject | [PATCH] [RFC] security: allow using Clang's zero initialization for stack variables | From | glider@google ... |
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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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