linux-kernel - [PATCH 12/18] arm64: reserve x18 only with Shadow Call Stack

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Message-Id: <20191018161033.261971-13-samitolvanen@google.com>
Date:   Fri, 18 Oct 2019 09:10:27 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Dave Martin <Dave.Martin@....com>,
        Kees Cook <keescook@...omium.org>,
        Laura Abbott <labbott@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com,
        kernel-hardening@...ts.openwall.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Sami Tolvanen <samitolvanen@...gle.com>
Subject: [PATCH 12/18] arm64: reserve x18 only with Shadow Call Stack

Only reserve x18 with CONFIG_SHADOW_CALL_STACK. Note that all external
kernel modules must also have x18 reserved if the kernel uses SCS.

Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
---
 arch/arm64/Makefile | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index 1c7b276bc7c5..ef76101201b2 100644
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -55,7 +55,7 @@ endif
 
 KBUILD_CFLAGS	+= -mgeneral-regs-only $(lseinstr) $(brokengasinst)	\
 		   $(compat_vdso) $(cc_has_k_constraint)
-KBUILD_CFLAGS	+= -fno-asynchronous-unwind-tables -ffixed-x18
+KBUILD_CFLAGS	+= -fno-asynchronous-unwind-tables
 KBUILD_CFLAGS	+= $(call cc-disable-warning, psabi)
 KBUILD_AFLAGS	+= $(lseinstr) $(brokengasinst) $(compat_vdso)
 
@@ -72,6 +72,10 @@ stack_protector_prepare: prepare0
 					include/generated/asm-offsets.h))
 endif
 
+ifeq ($(CONFIG_SHADOW_CALL_STACK), y)
+KBUILD_CFLAGS	+= -ffixed-x18
+endif
+
 ifeq ($(CONFIG_CPU_BIG_ENDIAN), y)
 KBUILD_CPPFLAGS	+= -mbig-endian
 CHECKFLAGS	+= -D__AARCH64EB__
-- 
2.23.0.866.gb869b98d4c-goog