arm64: Reserve register x18 from general allocation with SCS

Reserve the x18 register from general allocation when SCS is enabled,
because the compiler uses the register to store the current task's
shadow stack pointer. Note that all external kernel modules must also be
compiled with -ffixed-x18 if the kernel has SCS enabled.

Signed-off-by: Sami Tolvanen <[email protected]>
Reviewed-by: Nick Desaulniers <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Acked-by: Will Deacon <[email protected]>
Signed-off-by: Will Deacon <[email protected]>

Author: samitolvanen

arch/arm64/Makefile

@@ -81,6 +81,10 @@ endif
 
 KBUILD_CFLAGS += $(branch-prot-flags-y)
 
+ifeq ($(CONFIG_SHADOW_CALL_STACK), y)
+KBUILD_CFLAGS	+= -ffixed-x18
+endif
+
 ifeq ($(CONFIG_CPU_BIG_ENDIAN), y)
 KBUILD_CPPFLAGS	+= -mbig-endian
 CHECKFLAGS	+= -D__AARCH64EB__