| Message ID | 20190410161612.18545-4-keescook@chromium.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Kconfig: Refactor memory initialization hardening | expand |
On Thu, Apr 11, 2019 at 1:16 AM Kees Cook <keescook@chromium.org> wrote: > > CONFIG_INIT_STACK_ALL turns on stack initialization based on > -ftrivial-auto-var-init in Clang builds and on > -fplugin-arg-structleak_plugin-byref-all in GCC builds. Is CONFIG_INIT_STACK_ALL wired up to GCC plugin in any way? I could not understand it from the code. > > -ftrivial-auto-var-init is a Clang flag that 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) 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. > > The proposed config builds the kernel with > -ftrivial-auto-var-init=pattern when selected. > > Developers have the possibility to opt-out of this feature on a > per-variable basis by using __attribute__((uninitialized)). > > 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 | 14 ++++++++++++++ > 2 files changed, 19 insertions(+) > > 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 9942d9869864..d744e20140b4 100644 > --- a/security/Kconfig.hardening > +++ b/security/Kconfig.hardening > @@ -19,9 +19,13 @@ 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 CC_HAS_AUTO_VAR_INIT || GCC_PLUGINS > + default INIT_STACK_ALL if CC_HAS_AUTO_VAR_INIT Why should this be enabled by default? Ins't it a performance regression since it inserts instructions in function prologue?
On Thu, Apr 11, 2019 at 1:06 AM Masahiro Yamada <yamada.masahiro@socionext.com> wrote: > > On Thu, Apr 11, 2019 at 1:16 AM Kees Cook <keescook@chromium.org> wrote: > > > > CONFIG_INIT_STACK_ALL turns on stack initialization based on > > -ftrivial-auto-var-init in Clang builds and on > > -fplugin-arg-structleak_plugin-byref-all in GCC builds. > > Is CONFIG_INIT_STACK_ALL wired up to GCC plugin in any way? > I could not understand it from the code. No, it's only available under Clang. Clang is all-or-nothing, and the GCC plugin has a degrees up to "all passed by reference" which isn't truly "all" (i.e. Clang will initialize variables that aren't passed by reference and trigger a compiler warning about being uninitialized.) > > choice > > prompt "Initialize kernel stack variables at function entry" > > depends on CC_HAS_AUTO_VAR_INIT || GCC_PLUGINS > > + default INIT_STACK_ALL if CC_HAS_AUTO_VAR_INIT > > Why should this be enabled by default? > Ins't it a performance regression > since it inserts instructions in function prologue? There are very few users of Clang right now (mainly Android), so I figured it'd be nice to start Clang builds from a "protected by default" here, especially given Linus's thoughts on making this always happen[1]. I don't want to do it for GCC yet, since that would likely come as a huge surprise to everyone else. :) But I'm happy to change this, of course. -Kees [1] https://lkml.kernel.org/r/CA+55aFykZL+cSBJjBBts7ebEFfyGPdMzTmLSxKnT_29=j942dA@mail.gmail.com -- Kees Cook
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 9942d9869864..d744e20140b4 100644 --- a/security/Kconfig.hardening +++ b/security/Kconfig.hardening @@ -19,9 +19,13 @@ 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 CC_HAS_AUTO_VAR_INIT || GCC_PLUGINS + default INIT_STACK_ALL if CC_HAS_AUTO_VAR_INIT default INIT_STACK_NONE help This option enables initialization of stack variables at @@ -77,6 +81,16 @@ 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
CONFIG_INIT_STACK_ALL turns on stack initialization based on -ftrivial-auto-var-init in Clang builds and on -fplugin-arg-structleak_plugin-byref-all in GCC builds. -ftrivial-auto-var-init is a Clang flag that 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) 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. The proposed config builds the kernel with -ftrivial-auto-var-init=pattern when selected. Developers have the possibility to opt-out of this feature on a per-variable basis by using __attribute__((uninitialized)). 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 | 14 ++++++++++++++ 2 files changed, 19 insertions(+)