Subject: [PATCH v13 05/12] arm64: reserve x18 from general allocation with SCS

From: Sami Tolvanen <[email-masked]>
Date: Mon, 27 Apr 2020 09:00:11 -0700

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-masked]>
Reviewed-by: Nick Desaulniers <[email-masked]>
Reviewed-by: Kees Cook <[email-masked]>
Acked-by: Will Deacon <[email-masked]>
---
 arch/arm64/Makefile | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index 85e4149cc5d5..409a6c1be8cc 100644
--- 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__
-- 
2.26.2.303.gf8c07b1a785-goog

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