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 <samitolvanen@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>

diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index 85e4149cc5d5c142d20b2924d3e10f31cf2a240d..409a6c1be8cc9a73ea3b3202478c2adc0baaee50 100644 (file)
--- a/arch/arm64/Makefile
+++ b/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__