Re: [PATCH] [PATCH v2] AARCH64: Add gcc Shadow Call Stack support
From: Kees Cook <hidden>
Date: 2022-02-25 20:58:19
Also in:
linux-hardening, lkml, llvm
On Thu, Feb 24, 2022 at 07:24:10PM -0800, Dan Li wrote:
Shadow call stacks will be available in GCC >= 12, this patch makes the corresponding kernel configuration available when compiling the kernel with the gcc. Note that the implementation in GCC is slightly different from Clang. With SCS enabled, functions will only pop x30 once in the epilogue, like: str x30, [x18], #8 stp x29, x30, [sp, #-16]! ...... - ldp x29, x30, [sp], #16 //clang + ldr x29, [sp], #16 //GCC ldr x30, [x18, #-8]! Link: https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=ce09ab17ddd21f73ff2caf6eec3b0ee9b0e1a11e Signed-off-by: Dan Li <redacted>
Thanks for the tweaks!
---
FYI:
This function can be used to test if the shadow call stack works:
//noinline void __noscs scs_test(void)
noinline void scs_test(void)
{
unsigned long * lr = (unsigned long *)__builtin_frame_address(0) + 1;
asm volatile("str xzr, [%0]\n\t": : "r"(lr) : "x30");
} Not a big deal, but just FYI, there's a lot of whitespace trailing the "}" above...
ffff800008012770 <scs_test>: ffff800008012770: d503245f bti c ffff800008012774: d503233f paciasp ffff800008012778: f800865e str x30, [x18], #8 ffff80000801277c: a9bf7bfd stp x29, x30, [sp, #-16]! ffff800008012780: 910003fd mov x29, sp ffff800008012784: 910023a0 add x0, x29, #0x8 ffff800008012788: f900001f str xzr, [x0] ffff80000801278c: f85f8e5e ldr x30, [x18, #-8]! ffff800008012790: f84107fd ldr x29, [sp], #16 ffff800008012794: d50323bf autiasp ffff800008012798: d65f03c0 ret If SCS protection is enabled, this function will return normally. If the function has __noscs attribute (scs disabled), it will crash due to 0 address access.
It would be cool to turn this into an LKDTM test... (see things like the CFI_FORWARD_PROTO test). I imagine this should be CFI_BACKWARD_SHADOW or something... Also, I assume you're using real hardware to test this? It'd be nice to see if qemu can be convinced to run with the needed features. Whenever I've tried this it becomes impossibly slow. :)
quoted hunk ↗ jump to hunk
arch/Kconfig | 19 ++++++++++--------- arch/arm64/Kconfig | 2 +- include/linux/compiler-gcc.h | 4 ++++ 3 files changed, 15 insertions(+), 10 deletions(-)diff --git a/arch/Kconfig b/arch/Kconfig index 678a80713b21..c92683362ac2 100644 --- a/arch/Kconfig +++ b/arch/Kconfig@@ -599,21 +599,22 @@ config STACKPROTECTOR_STRONG config ARCH_SUPPORTS_SHADOW_CALL_STACK bool help - An architecture should select this if it supports Clang's Shadow - Call Stack and implements runtime support for shadow stack + An architecture should select this if it supports the compiler's + Shadow Call Stack and implements runtime support for shadow stack switching. config SHADOW_CALL_STACK - bool "Clang Shadow Call Stack" - depends on CC_IS_CLANG && ARCH_SUPPORTS_SHADOW_CALL_STACK + bool "Shadow Call Stack" + depends on ARCH_SUPPORTS_SHADOW_CALL_STACK depends on DYNAMIC_FTRACE_WITH_REGS || !FUNCTION_GRAPH_TRACER help - This option enables Clang's Shadow Call Stack, which uses a - shadow stack to protect function return addresses from being - overwritten by an attacker. More information can be found in - Clang's documentation: + This option enables the compiler's Shadow Call Stack, which + uses a shadow stack to protect function return addresses from + being overwritten by an attacker. More information can be found + in the compiler's documentation: - https://clang.llvm.org/docs/ShadowCallStack.html + - Clang (https://clang.llvm.org/docs/ShadowCallStack.html) + - GCC (https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html#Instrumentation-Options) Note that security guarantees in the kernel differ from the ones documented for user space. The kernel must store addressesdiff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig index 09b885cc4db5..b7145337efae 100644 --- a/arch/arm64/Kconfig +++ b/arch/arm64/Kconfig@@ -1255,7 +1255,7 @@ config HW_PERF_EVENTS config ARCH_HAS_FILTER_PGPROT def_bool y -# Supported by clang >= 7.0 +# Supported by clang >= 7.0 or GCC >= 12.0.0 config CC_HAVE_SHADOW_CALL_STACK def_bool $(cc-option, -fsanitize=shadow-call-stack -ffixed-x18)diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h index ccbbd31b3aae..deff5b308470 100644 --- a/include/linux/compiler-gcc.h +++ b/include/linux/compiler-gcc.h@@ -97,6 +97,10 @@ #define KASAN_ABI_VERSION 4 #endif +#ifdef CONFIG_SHADOW_CALL_STACK +#define __noscs __attribute__((__no_sanitize__("shadow-call-stack"))) +#endif
I initially wondered if we need a separate __no_sanitize(STUFF) patch to
make the compiler-clang.h macros easier, but I see there are places
where we do multiple ("address", "hwaddress") and have specialized
macros, so I think this is fine. And since GCC doesn't support
"__has_feature", I think this is the correct location for this.
+ #if __has_attribute(__no_sanitize_address__) #define __no_sanitize_address __attribute__((no_sanitize_address)) #else -- 2.17.1
Reviewed-by: Kees Cook <redacted> -- Kees Cook _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel