| Message ID | 20131125221400.GA11041@www.outflux.net (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On 11/25/2013 02:14 PM, Kees Cook wrote: > Build the kernel with -fstack-protector-strong when it is available > (gcc 4.9 and later). This increases the coverage of the stack protector > without the heavy performance hit of -fstack-protector-all. What is the difference between the various options? -hpa
On Mon, Nov 25, 2013 at 3:16 PM, H. Peter Anvin <hpa@zytor.com> wrote: > On 11/25/2013 02:14 PM, Kees Cook wrote: >> Build the kernel with -fstack-protector-strong when it is available >> (gcc 4.9 and later). This increases the coverage of the stack protector >> without the heavy performance hit of -fstack-protector-all. > > What is the difference between the various options? -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 Chrome OS has been using -fstack-protector-strong for its kernel builds for the last 8 months with no problems. -Kees
On Mon, 25 Nov 2013, Kees Cook wrote: > On Mon, Nov 25, 2013 at 3:16 PM, H. Peter Anvin <hpa@zytor.com> wrote: > > On 11/25/2013 02:14 PM, Kees Cook wrote: > >> Build the kernel with -fstack-protector-strong when it is available > >> (gcc 4.9 and later). This increases the coverage of the stack protector > >> without the heavy performance hit of -fstack-protector-all. > > > > What is the difference between the various options? > > -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 > > Chrome OS has been using -fstack-protector-strong for its kernel > builds for the last 8 months with no problems. Could you get this information inside the commit log for your patch please? This is very valuable info to have right next to the change in the repository without having to dig into the gcc manual or finding the relevant email thread. Nicolas
* Nicolas Pitre <nicolas.pitre@linaro.org> wrote: > On Mon, 25 Nov 2013, Kees Cook wrote: > > > On Mon, Nov 25, 2013 at 3:16 PM, H. Peter Anvin <hpa@zytor.com> wrote: > > > On 11/25/2013 02:14 PM, Kees Cook wrote: > > >> Build the kernel with -fstack-protector-strong when it is available > > >> (gcc 4.9 and later). This increases the coverage of the stack protector > > >> without the heavy performance hit of -fstack-protector-all. > > > > > > What is the difference between the various options? > > > > -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 > > > > Chrome OS has been using -fstack-protector-strong for its kernel > > builds for the last 8 months with no problems. > > Could you get this information inside the commit log for your patch > please? This is very valuable info to have right next to the change > in the repository without having to dig into the gcc manual or > finding the relevant email thread. Another piece of information we need for the changelog is a vmlinux kernel size comparison, with/without the patch, for a defconfig build (or a Ubuntu distro config build). Thanks, Ingo
diff --git a/arch/arm/Makefile b/arch/arm/Makefile index c99b1086d83d..c6d3ea1c063e 100644 --- a/arch/arm/Makefile +++ b/arch/arm/Makefile @@ -41,7 +41,8 @@ KBUILD_CFLAGS +=-fno-omit-frame-pointer -mapcs -mno-sched-prolog endif ifeq ($(CONFIG_CC_STACKPROTECTOR),y) -KBUILD_CFLAGS +=-fstack-protector +KBUILD_CFLAGS += $(call cc-option,-fstack-protector-strong,-fstack-protector) + endif ifeq ($(CONFIG_CPU_BIG_ENDIAN),y) 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; diff --git a/arch/x86/Makefile b/arch/x86/Makefile index 41250fb33985..4ebb054cc323 100644 --- a/arch/x86/Makefile +++ b/arch/x86/Makefile @@ -86,7 +86,7 @@ endif 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 + stackp-y := $(call cc-option,-fstack-protector-strong,-fstack-protector) KBUILD_CFLAGS += $(stackp-y) else $(warning stack protector enabled but no compiler support)
Build the kernel with -fstack-protector-strong when it is available (gcc 4.9 and later). This increases the coverage of the stack protector without the heavy performance hit of -fstack-protector-all. On a Chrome OS kernel build, this grows the uncompressed kernel image by less than 0.16% on x86: -rwxr-xr-x 1 keescook portage 118219343 Apr 17 12:26 vmlinux.old -rwxr-xr-x 1 keescook portage 118407919 Apr 19 15:00 vmlinux 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. Signed-off-by: Kees Cook <keescook@chromium.org> --- arch/arm/Makefile | 3 ++- arch/arm/boot/compressed/misc.c | 14 ++++++++++++++ arch/x86/Makefile | 2 +- 3 files changed, 17 insertions(+), 2 deletions(-)