| Message ID | 20200614144534.237035-1-glider@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [RFC] security: allow using Clang's zero initialization for stack variables | expand |
On Sun, Jun 14, 2020 at 04:45:34PM +0200, glider@google.com 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 <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 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. :)
On Sun, Jun 14, 2020 at 7:04 PM Kees Cook <keescook@chromium.org> wrote: > > On Sun, Jun 14, 2020 at 04:45:34PM +0200, glider@google.com 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 <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 > > 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 <keescook@chromium.org> wrote: > > > > On Sun, Jun 14, 2020 at 04:45:34PM +0200, glider@google.com 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 <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 > > > > 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...
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
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(-)