[PATCH v4] provide -fstack-protector-strong build option

[PATCH v4] provide -fstack-protector-strong build option

[Kees Cook]

This reorganizes the build options for CONFIG_CC_STACKPROTECTOR so that
the new CONFIG_CC_STACKPROTECTOR_STRONG can be used when building with
a compiler that supports it.

Now with objdump analysis.

-Kees

[PATCH v4 1/2] create HAVE_CC_STACKPROTECTOR for centralized use

[Kees Cook]

Instead of duplicating the CC_STACKPROTECTOR Kconfig and Makefile logic
in each architecture, switch to using HAVE_CC_STACKPROTECTOR and keep
everything in one place. This retains the x86-specific bug verification
scripts.

Signed-off-by: Kees Cook <keescook@chromium.org>
---
 Makefile           |   14 +++++++++++---
 arch/Kconfig       |   22 ++++++++++++++++++++++
 arch/arm/Kconfig   |   13 +------------
 arch/arm/Makefile  |    4 ----
 arch/mips/Kconfig  |   14 +-------------
 arch/mips/Makefile |    4 ----
 arch/sh/Kconfig    |   15 +--------------
 arch/sh/Makefile   |    4 ----
 arch/x86/Kconfig   |   17 +----------------
 arch/x86/Makefile  |    8 +++-----
 10 files changed, 40 insertions(+), 75 deletions(-)

diff --git a/Makefile b/Makefile
index 858a147fd836..27ed3ff7cf4c 100644
--- a/Makefile
+++ b/Makefile
@@ -595,10 +595,18 @@ ifneq ($(CONFIG_FRAME_WARN),0)
 KBUILD_CFLAGS += $(call cc-option,-Wframe-larger-than=${CONFIG_FRAME_WARN})
 endif
 
-# Force gcc to behave correct even for buggy distributions
-ifndef CONFIG_CC_STACKPROTECTOR
-KBUILD_CFLAGS += $(call cc-option, -fno-stack-protector)
+# Handle stack protector mode.
+ifdef CONFIG_CC_STACKPROTECTOR
+  stackp-flag := $(call cc-option, -fstack-protector)
+  ifeq ($(stackp-flag),)
+   $(error Cannot use CONFIG_CC_STACKPROTECTOR: \
+	   -fstack-protector not supported by compiler))
+  endif
+else
+  # Force off for distro compilers that enable stack protector by default.
+  stackp-flag := $(call cc-option, -fno-stack-protector)
 endif
+KBUILD_CFLAGS += $(stackp-flag)
 
 # This warning generated too much noise in a regular build.
 # Use make W=1 to enable this warning (see scripts/Makefile.build)
diff --git a/arch/Kconfig b/arch/Kconfig
index f1cf895c040f..24e026d83072 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -336,6 +336,28 @@ config SECCOMP_FILTER
 
 	  See Documentation/prctl/seccomp_filter.txt for details.
 
+config HAVE_CC_STACKPROTECTOR
+	bool
+	help
+	  An arch should select this symbol if:
+	  - its compiler supports the -fstack-protector option
+	  - it has implemented a stack canary (e.g. __stack_chk_guard)
+
+config CC_STACKPROTECTOR
+	bool "Enable -fstack-protector buffer overflow detection"
+	depends on HAVE_CC_STACKPROTECTOR
+	help
+	  This option turns on the -fstack-protector GCC feature. This
+	  feature puts, at the beginning of functions, a canary value on
+	  the stack just before the return address, and validates
+	  the value just before actually returning.  Stack based buffer
+	  overflows (that need to overwrite this return address) now also
+	  overwrite the canary, which gets detected and the attack is then
+	  neutralized via a kernel panic.
+
+	  This feature requires gcc version 4.2 or above, or a distribution
+	  gcc with the feature backported.
+
 config HAVE_CONTEXT_TRACKING
 	bool
 	help
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index c1f1a7eee953..9c909fc29272 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -30,6 +30,7 @@ config ARM
 	select HAVE_BPF_JIT
 	select HAVE_CONTEXT_TRACKING
 	select HAVE_C_RECORDMCOUNT
+	select HAVE_CC_STACKPROTECTOR
 	select HAVE_DEBUG_KMEMLEAK
 	select HAVE_DMA_API_DEBUG
 	select HAVE_DMA_ATTRS
@@ -1856,18 +1857,6 @@ config SECCOMP
 	  and the task is only allowed to execute a few safe syscalls
 	  defined by each seccomp mode.
 
-config CC_STACKPROTECTOR
-	bool "Enable -fstack-protector buffer overflow detection (EXPERIMENTAL)"
-	help
-	  This option turns on the -fstack-protector GCC feature. This
-	  feature puts, at the beginning of functions, a canary value on
-	  the stack just before the return address, and validates
-	  the value just before actually returning.  Stack based buffer
-	  overflows (that need to overwrite this return address) now also
-	  overwrite the canary, which gets detected and the attack is then
-	  neutralized via a kernel panic.
-	  This feature requires gcc version 4.2 or above.
-
 config SWIOTLB
 	def_bool y
 
diff --git a/arch/arm/Makefile b/arch/arm/Makefile
index c99b1086d83d..55b4255ad6ed 100644
--- a/arch/arm/Makefile
+++ b/arch/arm/Makefile
@@ -40,10 +40,6 @@ ifeq ($(CONFIG_FRAME_POINTER),y)
 KBUILD_CFLAGS	+=-fno-omit-frame-pointer -mapcs -mno-sched-prolog
 endif
 
-ifeq ($(CONFIG_CC_STACKPROTECTOR),y)
-KBUILD_CFLAGS	+=-fstack-protector
-endif
-
 ifeq ($(CONFIG_CPU_BIG_ENDIAN),y)
 KBUILD_CPPFLAGS	+= -mbig-endian
 AS		+= -EB
diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index 650de3976e7a..c93d92beb3d6 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -47,6 +47,7 @@ config MIPS
 	select MODULES_USE_ELF_RELA if MODULES && 64BIT
 	select CLONE_BACKWARDS
 	select HAVE_DEBUG_STACKOVERFLOW
+	select HAVE_CC_STACKPROTECTOR
 
 menu "Machine selection"
 
@@ -2322,19 +2323,6 @@ config SECCOMP
 
 	  If unsure, say Y. Only embedded should say N here.
 
-config CC_STACKPROTECTOR
-	bool "Enable -fstack-protector buffer overflow detection (EXPERIMENTAL)"
-	help
-	  This option turns on the -fstack-protector GCC feature. This
-	  feature puts, at the beginning of functions, a canary value on
-	  the stack just before the return address, and validates
-	  the value just before actually returning.  Stack based buffer
-	  overflows (that need to overwrite this return address) now also
-	  overwrite the canary, which gets detected and the attack is then
-	  neutralized via a kernel panic.
-
-	  This feature requires gcc version 4.2 or above.
-
 config USE_OF
 	bool
 	select OF
diff --git a/arch/mips/Makefile b/arch/mips/Makefile
index de300b993607..efe50787cd89 100644
--- a/arch/mips/Makefile
+++ b/arch/mips/Makefile
@@ -232,10 +232,6 @@ bootvars-y	= VMLINUX_LOAD_ADDRESS=$(load-y) \
 
 LDFLAGS			+= -m $(ld-emul)
 
-ifdef CONFIG_CC_STACKPROTECTOR
-  KBUILD_CFLAGS += -fstack-protector
-endif
-
 ifdef CONFIG_MIPS
 CHECKFLAGS += $(shell $(CC) $(KBUILD_CFLAGS) -dM -E -x c /dev/null | \
 	egrep -vw '__GNUC_(|MINOR_|PATCHLEVEL_)_' | \
diff --git a/arch/sh/Kconfig b/arch/sh/Kconfig
index 9b0979f4df7a..ce298317a73e 100644
--- a/arch/sh/Kconfig
+++ b/arch/sh/Kconfig
@@ -66,6 +66,7 @@ config SUPERH32
 	select PERF_EVENTS
 	select ARCH_HIBERNATION_POSSIBLE if MMU
 	select SPARSE_IRQ
+	select HAVE_CC_STACKPROTECTOR
 
 config SUPERH64
 	def_bool ARCH = "sh64"
@@ -695,20 +696,6 @@ config SECCOMP
 
 	  If unsure, say N.
 
-config CC_STACKPROTECTOR
-	bool "Enable -fstack-protector buffer overflow detection (EXPERIMENTAL)"
-	depends on SUPERH32
-	help
-	  This option turns on the -fstack-protector GCC feature. This
-	  feature puts, at the beginning of functions, a canary value on
-	  the stack just before the return address, and validates
-	  the value just before actually returning.  Stack based buffer
-	  overflows (that need to overwrite this return address) now also
-	  overwrite the canary, which gets detected and the attack is then
-	  neutralized via a kernel panic.
-
-	  This feature requires gcc version 4.2 or above.
-
 config SMP
 	bool "Symmetric multi-processing support"
 	depends on SYS_SUPPORTS_SMP
diff --git a/arch/sh/Makefile b/arch/sh/Makefile
index aed701c7b11b..d4d16e4be07c 100644
--- a/arch/sh/Makefile
+++ b/arch/sh/Makefile
@@ -199,10 +199,6 @@ ifeq ($(CONFIG_DWARF_UNWINDER),y)
   KBUILD_CFLAGS += -fasynchronous-unwind-tables
 endif
 
-ifeq ($(CONFIG_CC_STACKPROTECTOR),y)
-  KBUILD_CFLAGS += -fstack-protector
-endif
-
 libs-$(CONFIG_SUPERH32)		:= arch/sh/lib/	$(libs-y)
 libs-$(CONFIG_SUPERH64)		:= arch/sh/lib64/ $(libs-y)
 
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index e903c71f7e69..4a814e6c526b 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -124,6 +124,7 @@ config X86
 	select RTC_LIB
 	select HAVE_DEBUG_STACKOVERFLOW
 	select HAVE_IRQ_EXIT_ON_IRQ_STACK if X86_64
+	select HAVE_CC_STACKPROTECTOR
 
 config INSTRUCTION_DECODER
 	def_bool y
@@ -1616,22 +1617,6 @@ config SECCOMP
 
 	  If unsure, say Y. Only embedded should say N here.
 
-config CC_STACKPROTECTOR
-	bool "Enable -fstack-protector buffer overflow detection"
-	---help---
-	  This option turns on the -fstack-protector GCC feature. This
-	  feature puts, at the beginning of functions, a canary value on
-	  the stack just before the return address, and validates
-	  the value just before actually returning.  Stack based buffer
-	  overflows (that need to overwrite this return address) now also
-	  overwrite the canary, which gets detected and the attack is then
-	  neutralized via a kernel panic.
-
-	  This feature requires gcc version 4.2 or above, or a distribution
-	  gcc with the feature backported. Older versions are automatically
-	  detected and for those versions, this configuration option is
-	  ignored. (and a warning is printed during bootup)
-
 source kernel/Kconfig.hz
 
 config KEXEC
diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 57d021507120..66d126ab9942 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -89,13 +89,11 @@ else
         KBUILD_CFLAGS += -maccumulate-outgoing-args
 endif
 
+# Make sure compiler does not have buggy stackprotector support.
 ifdef CONFIG_CC_STACKPROTECTOR
 	cc_has_sp := $(srctree)/scripts/gcc-x86_$(BITS)-has-stack-protector.sh
-        ifeq ($(shell $(CONFIG_SHELL) $(cc_has_sp) $(CC) $(KBUILD_CPPFLAGS) $(biarch)),y)
-                stackp-y := -fstack-protector
-                KBUILD_CFLAGS += $(stackp-y)
-        else
-                $(warning stack protector enabled but no compiler support)
+        ifneq ($(shell $(CONFIG_SHELL) $(cc_has_sp) $(CC) $(KBUILD_CPPFLAGS) $(biarch)),y)
+                $(error stack protector enabled but compiler support is broken)
         endif
 endif
 
-- 
1.7.9.5

[PATCH v4 2/2] provide -fstack-protector-strong build option

[Kees Cook]

This changes the stack protector config option into a choice of "None",
"Regular", and "Strong". For "Strong", the kernel is built with
-fstack-protector-strong (gcc 4.9 and later). This options increases
the coverage of the stack protector without the heavy performance hit
of -fstack-protector-all.

For reference, the stack protector options available in gcc are:

-fstack-protector-all:
Adds the stack-canary saving prefix and stack-canary checking suffix to
_all_ function entry and exit. Results in substantial use of stack space
for saving the canary for deep stack users (e.g. historically xfs), and
measurable (though shockingly still low) performance hit due to all the
saving/checking. Really not suitable for sane systems, and was entirely
removed as an option from the kernel many years ago.

-fstack-protector:
Adds the canary save/check to functions that define an 8
(--param=ssp-buffer-size=N, N=8 by default) or more byte local char
array. Traditionally, stack overflows happened with string-based
manipulations, so this was a way to find those functions. Very few
total functions actually get the canary; no measurable performance or
size overhead.

-fstack-protector-strong
Adds the canary for a wider set of functions, since it's not just those
with strings that have ultimately been vulnerable to stack-busting. With
this superset, more functions end up with a canary, but it still
remains small compared to all functions with no measurable change in
performance. Based on the original design document, a function gets the
canary when it contains any of:
- local variable's address used as part of the RHS of an assignment or
  function argument
- local variable is an array (or union containing an array), regardless
  of array type or length
- uses register local variables
https://docs.google.com/a/google.com/document/d/1xXBH6rRZue4f296vGt9YQcuLVQHeE516stHwt8M9xyU

Comparison of "size" and "objdump" output when built with gcc-4.9 in
three configurations:
- defconfig
	11430641 text size
	36110 function bodies
- defconfig + CONFIG_CC_STACKPROTECTOR
	11468490 text size (+0.33%)
	1015 of 36110 functions stack-protected (2.81%)
- defconfig + CONFIG_CC_STACKPROTECTOR_STRONG via this patch
	11692790 text size (+2.24%)
	7401 of 36110 functions stack-protected (20.5%)

With -strong, ARM's compressed boot code now triggers stack protection,
so a static guard was added. Since this is only used during decompression
and was never used before, the exposure here is very small. Once it
switches to the full kernel, the stack guard is back to normal.

Chrome OS has been using -fstack-protector-strong for its kernel builds
for the last 8 months with no problems.

Signed-off-by: Kees Cook <keescook@chromium.org>
---
v4:
 - add objdump analysis to comparison
v3:
 - split off type of stack protection as a distinct config
v2:
 - added description of all stack protector options
 - added size comparisons for Ubuntu and defconfig
---
 Makefile                        |    8 ++++++-
 arch/Kconfig                    |   44 +++++++++++++++++++++++++++++++++++++--
 arch/arm/boot/compressed/misc.c |   14 +++++++++++++
 3 files changed, 63 insertions(+), 3 deletions(-)

diff --git a/Makefile b/Makefile
index 27ed3ff7cf4c..9701b690e1ec 100644
--- a/Makefile
+++ b/Makefile
@@ -596,12 +596,18 @@ KBUILD_CFLAGS += $(call cc-option,-Wframe-larger-than=${CONFIG_FRAME_WARN})
 endif
 
 # Handle stack protector mode.
-ifdef CONFIG_CC_STACKPROTECTOR
+ifdef CONFIG_CC_STACKPROTECTOR_REGULAR
   stackp-flag := $(call cc-option, -fstack-protector)
   ifeq ($(stackp-flag),)
    $(error Cannot use CONFIG_CC_STACKPROTECTOR: \
 	   -fstack-protector not supported by compiler))
   endif
+else ifdef CONFIG_CC_STACKPROTECTOR_STRONG
+  stackp-flag := $(call cc-option, -fstack-protector-strong)
+  ifeq ($(stackp-flag),)
+   $(error Cannot use CONFIG_CC_STACKPROTECTOR_STRONG: \
+	   -fstack-protector-strong not supported by compiler)
+  endif
 else
   # Force off for distro compilers that enable stack protector by default.
   stackp-flag := $(call cc-option, -fno-stack-protector)
diff --git a/arch/Kconfig b/arch/Kconfig
index 24e026d83072..8dde0a5b76fd 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -344,10 +344,17 @@ config HAVE_CC_STACKPROTECTOR
 	  - it has implemented a stack canary (e.g. __stack_chk_guard)
 
 config CC_STACKPROTECTOR
-	bool "Enable -fstack-protector buffer overflow detection"
+	def_bool n
+	help
+	  Set when a stack-protector mode is enabled, so that the build
+	  can enable kernel-side support for the GCC feature.
+
+choice
+	prompt "Stack Protector buffer overflow detection"
 	depends on HAVE_CC_STACKPROTECTOR
+	default CC_STACKPROTECTOR_NONE
 	help
-	  This option turns on the -fstack-protector GCC feature. This
+	  This option turns on the "stack-protector" GCC feature. This
 	  feature puts, at the beginning of functions, a canary value on
 	  the stack just before the return address, and validates
 	  the value just before actually returning.  Stack based buffer
@@ -355,9 +362,42 @@ config CC_STACKPROTECTOR
 	  overwrite the canary, which gets detected and the attack is then
 	  neutralized via a kernel panic.
 
+config CC_STACKPROTECTOR_NONE
+	bool "None"
+	help
+	  Disable "stack-protector" GCC feature.
+
+config CC_STACKPROTECTOR_REGULAR
+	bool "Regular"
+	select CC_STACKPROTECTOR
+	help
+	  Functions will have the stack-protector canary logic added if they
+	  have an 8-byte or larger character array on the stack.
+
 	  This feature requires gcc version 4.2 or above, or a distribution
 	  gcc with the feature backported.
 
+	  On an x86 "defconfig" build, this increases the kernel text by 0.3%.
+
+config CC_STACKPROTECTOR_STRONG
+	bool "Strong"
+	select CC_STACKPROTECTOR
+	help
+	  Functions will have the stack-protector canary logic added in any
+	  of the following conditions:
+	  - local variable's address used as part of the RHS of an
+	    assignment or function argument
+	  - local variable is an array (or union containing an array),
+	    regardless of array type or length
+	  - uses register local variables
+
+	  This feature requires gcc version 4.9 or above, or a distribution
+	  gcc with the feature backported.
+
+	  On an x86 "defconfig" build, this increases the kernel text by 2%.
+
+endchoice
+
 config HAVE_CONTEXT_TRACKING
 	bool
 	help
diff --git a/arch/arm/boot/compressed/misc.c b/arch/arm/boot/compressed/misc.c
index 31bd43b82095..d4f891f56996 100644
--- a/arch/arm/boot/compressed/misc.c
+++ b/arch/arm/boot/compressed/misc.c
@@ -127,6 +127,18 @@ asmlinkage void __div0(void)
 	error("Attempting division by 0!");
 }
 
+unsigned long __stack_chk_guard;
+
+void __stack_chk_guard_setup(void)
+{
+	__stack_chk_guard = 0x000a0dff;
+}
+
+void __stack_chk_fail(void)
+{
+	error("stack-protector: Kernel stack is corrupted\n");
+}
+
 extern int do_decompress(u8 *input, int len, u8 *output, void (*error)(char *x));
 
 
@@ -137,6 +149,8 @@ decompress_kernel(unsigned long output_start, unsigned long free_mem_ptr_p,
 {
 	int ret;
 
+	__stack_chk_guard_setup();
+
 	output_data		= (unsigned char *)output_start;
 	free_mem_ptr		= free_mem_ptr_p;
 	free_mem_end_ptr	= free_mem_ptr_end_p;
-- 
1.7.9.5

[PATCH v4 2/2] provide -fstack-protector-strong build option

[Ingo Molnar]

* Kees Cook <keescook@chromium.org> wrote:

> This changes the stack protector config option into a choice of "None",
> "Regular", and "Strong". For "Strong", the kernel is built with
> -fstack-protector-strong (gcc 4.9 and later). This options increases
> the coverage of the stack protector without the heavy performance hit
> of -fstack-protector-all.
> 
> For reference, the stack protector options available in gcc are:
> 
> -fstack-protector-all:
> Adds the stack-canary saving prefix and stack-canary checking suffix to
> _all_ function entry and exit. Results in substantial use of stack space
> for saving the canary for deep stack users (e.g. historically xfs), and
> measurable (though shockingly still low) performance hit due to all the
> saving/checking. Really not suitable for sane systems, and was entirely
> removed as an option from the kernel many years ago.
> 
> -fstack-protector:
> Adds the canary save/check to functions that define an 8
> (--param=ssp-buffer-size=N, N=8 by default) or more byte local char
> array. Traditionally, stack overflows happened with string-based
> manipulations, so this was a way to find those functions. Very few
> total functions actually get the canary; no measurable performance or
> size overhead.
> 
> -fstack-protector-strong
> Adds the canary for a wider set of functions, since it's not just those
> with strings that have ultimately been vulnerable to stack-busting. With
> this superset, more functions end up with a canary, but it still
> remains small compared to all functions with no measurable change in
> performance. Based on the original design document, a function gets the
> canary when it contains any of:
> - local variable's address used as part of the RHS of an assignment or
>   function argument
> - local variable is an array (or union containing an array), regardless
>   of array type or length
> - uses register local variables
> https://docs.google.com/a/google.com/document/d/1xXBH6rRZue4f296vGt9YQcuLVQHeE516stHwt8M9xyU
> 
> Comparison of "size" and "objdump" output when built with gcc-4.9 in
> three configurations:
> - defconfig
> 	11430641 text size
> 	36110 function bodies
> - defconfig + CONFIG_CC_STACKPROTECTOR
> 	11468490 text size (+0.33%)
> 	1015 of 36110 functions stack-protected (2.81%)
> - defconfig + CONFIG_CC_STACKPROTECTOR_STRONG via this patch
> 	11692790 text size (+2.24%)
> 	7401 of 36110 functions stack-protected (20.5%)

Ok, these patches now look pretty good to me.

One final detail is that I think the information about the percentage 
of functions affected should be propagated into the help text:

> +config CC_STACKPROTECTOR_REGULAR
> +	bool "Regular"
> +	select CC_STACKPROTECTOR
> +	help
> +	  Functions will have the stack-protector canary logic added if they
> +	  have an 8-byte or larger character array on the stack.
> +
>  	  This feature requires gcc version 4.2 or above, or a distribution
>  	  gcc with the feature backported.
>  
> +	  On an x86 "defconfig" build, this increases the kernel text by 0.3%.
> +
> +config CC_STACKPROTECTOR_STRONG
> +	bool "Strong"
> +	select CC_STACKPROTECTOR
> +	help
> +	  Functions will have the stack-protector canary logic added in any
> +	  of the following conditions:
> +	  - local variable's address used as part of the RHS of an
> +	    assignment or function argument
> +	  - local variable is an array (or union containing an array),
> +	    regardless of array type or length
> +	  - uses register local variables
> +
> +	  This feature requires gcc version 4.9 or above, or a distribution
> +	  gcc with the feature backported.
> +
> +	  On an x86 "defconfig" build, this increases the kernel text by 2%.

It should say something like:

	  On an x86 "defconfig" build, this feature adds canary checks 
	  to about 3% of all kernel functions, which increases kernel
	  code size by about 0.3%.

and for the _STRONG option:

	  On an x86 "defconfig" build, this feature adds canary checks 
	  to ~20% of all kernel functions, which increases kernel code 
	  size by ~2%.

this way distibutions and users can make an informed decision about 
the level of checks they want to employ.

Thanks,

	Ingo