Re: [RFC PATCH v9 05/27] x86/cet/shstk: Add Kconfig option for user-mode Shadow Stack protection

On Wed, Feb 26, 2020 at 10:05 AM Dave Hansen [off-list ref] wrote:

On 2/5/20 10:19 AM, Yu-cheng Yu wrote:

quoted

+# Check assembler Shadow Stack suppot

                                  ^ support

quoted

+ifdef CONFIG_X86_INTEL_SHADOW_STACK_USER
+  ifeq ($(call as-instr, saveprevssp, y),)
+      $(error CONFIG_X86_INTEL_SHADOW_STACK_USER not supported by the assembler)
+  endif
+endif

Is this *just* looking for instruction support in the assembler?

We usually just .byte them, like this for pkeys:

        asm volatile(".byte 0x0f,0x01,0xee\n\t"
                     : "=a" (pkru), "=d" (edx)
                     : "c" (ecx));

That way everybody with old toolchains can still build the kernel (and
run/test code with your config option on, btw...).

CET requires a complete new OS image from kernel, toolchain, run-time.
CET enabled kernel without the rest of updated OS won't give you CET
at all.

-- 
H.J.