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 <[email protected]>
Cc: Nick Desaulniers <[email protected]>
Signed-off-by: Alexander Potapenko <[email protected]>
---
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
On Sun, Jun 14, 2020 at 04:45:34PM +0200, [email protected] wrote:
> 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 <[email protected]>
> Cc: Nick Desaulniers <[email protected]>
> Signed-off-by: Alexander Potapenko <[email protected]>
> ---
> 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
I'd prefer this be split instead of built as a nested if (i.e. entirely
control section via the Kconfig -- see below).
>
> 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)
> +
I'd like to be more specific here. Let's rename CC_HAS_AUTO_VAR_INIT to
CC_HAS_AUTO_VAR_INIT_PATTERN, and change the other to
CC_HAS_AUTO_VAR_INIT_ZERO (they then both match the word order of the
option, and the thing that changes is the last word).
> 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.
> +
Similarly, I'd like to rename INIT_STACK_ALL to INIT_STACK_ALL_PATTERN
and then add INIT_STACK_ALL_ZERO.
> config GCC_PLUGIN_STRUCTLEAK_VERBOSE
> bool "Report forcefully initialized variables"
> depends on GCC_PLUGIN_STRUCTLEAK
> --
> 2.27.0.290.gba653c62da-goog
>
But yes, let's get this change implemented. :)
--
Kees Cook
On Sun, Jun 14, 2020 at 7:04 PM Kees Cook <[email protected]> wrote:
>
> On Sun, Jun 14, 2020 at 04:45:34PM +0200, [email protected] wrote:
> > 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 <[email protected]>
> > Cc: Nick Desaulniers <[email protected]>
> > Signed-off-by: Alexander Potapenko <[email protected]>
> > ---
> > 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
>
> I'd prefer this be split instead of built as a nested if (i.e. entirely
> control section via the Kconfig -- see below).
>
> >
> > 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)
> > +
>
> I'd like to be more specific here. Let's rename CC_HAS_AUTO_VAR_INIT to
> CC_HAS_AUTO_VAR_INIT_PATTERN, and change the other to
> CC_HAS_AUTO_VAR_INIT_ZERO (they then both match the word order of the
> option, and the thing that changes is the last word).
>
> > 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.
> > +
>
> Similarly, I'd like to rename INIT_STACK_ALL to INIT_STACK_ALL_PATTERN
> and then add INIT_STACK_ALL_ZERO.
What are the policies regarding keeping the existing config flags?
Don't we need to preserve INIT_STACK_ALL?
On Sun, Jun 14, 2020 at 07:16:33PM +0200, Alexander Potapenko wrote:
> On Sun, Jun 14, 2020 at 7:04 PM Kees Cook <[email protected]> wrote:
> >
> > On Sun, Jun 14, 2020 at 04:45:34PM +0200, [email protected] wrote:
> > > 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 <[email protected]>
> > > Cc: Nick Desaulniers <[email protected]>
> > > Signed-off-by: Alexander Potapenko <[email protected]>
> > > ---
> > > 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
> >
> > I'd prefer this be split instead of built as a nested if (i.e. entirely
> > control section via the Kconfig -- see below).
> >
> > >
> > > 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)
> > > +
> >
> > I'd like to be more specific here. Let's rename CC_HAS_AUTO_VAR_INIT to
> > CC_HAS_AUTO_VAR_INIT_PATTERN, and change the other to
> > CC_HAS_AUTO_VAR_INIT_ZERO (they then both match the word order of the
> > option, and the thing that changes is the last word).
> >
> > > 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.
> > > +
> >
> > Similarly, I'd like to rename INIT_STACK_ALL to INIT_STACK_ALL_PATTERN
> > and then add INIT_STACK_ALL_ZERO.
>
> What are the policies regarding keeping the existing config flags?
> Don't we need to preserve INIT_STACK_ALL?
There isn't a strong policy -- it's mostly a question of "how painful
will it be for end-users to change this?" Given the users of this are
the Clang folks, and it's a very new option, I would prefer we just
change it and carry on.
That said, I'd also like at some point do:
choice
prompt "Initialize kernel stack variables at function entry"
default GCC_PLUGIN_STRUCTLEAK_BYREF_ALL if COMPILE_TEST && GCC_PLUGINS
- default INIT_STACK_ALL_ZERO if COMPILE_TEST && CC_HAS_AUTO_VAR_INIT_ZERO
+ default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT
default INIT_STACK_NONE
help
This option enables initialization of stack variables at
But not at this time...
--
Kees Cook