[PATCH v2 1/2] arm64: Add ARM64_PTR_AUTH_KERNEL config option

[Daniel Kiss]

This new option makes possible to build the kernel with pointer
authentication support for the user space while the kernel is not built
with the pointer authentication. There is a similar config structure for BTI.

The default configuration will be the same after this patch.

Signed-off-by: Daniel Kiss <daniel.kiss at arm.com>
---
 arch/arm64/Kconfig        | 26 +++++++++++++++++---------
 arch/arm64/Makefile       |  2 +-
 drivers/misc/lkdtm/bugs.c |  6 +++---
 3 files changed, 21 insertions(+), 13 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 75aefc9990ea..b8af3297425a 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -1501,7 +1501,6 @@ config ARM64_PTR_AUTH
 	# which is only understood by binutils starting with version 2.33.1.
 	depends on LD_IS_LLD || LD_VERSION >= 233010000 || (CC_IS_GCC && GCC_VERSION < 90100)
 	depends on !CC_IS_CLANG || AS_HAS_CFI_NEGATE_RA_STATE
-	depends on (!FUNCTION_GRAPH_TRACER || DYNAMIC_FTRACE_WITH_REGS)
 	help
 	  Pointer authentication (part of the ARMv8.3 Extensions) provides
 	  instructions for signing and authenticating pointers against secret
@@ -1513,13 +1512,6 @@ config ARM64_PTR_AUTH
 	  for each process at exec() time, with these keys being
 	  context-switched along with the process.
 
-	  If the compiler supports the -mbranch-protection or
-	  -msign-return-address flag (e.g. GCC 7 or later), then this option
-	  will also cause the kernel itself to be compiled with return address
-	  protection. In this case, and if the target hardware is known to
-	  support pointer authentication, then CONFIG_STACKPROTECTOR can be
-	  disabled with minimal loss of protection.
-
 	  The feature is detected at runtime. If the feature is not present in
 	  hardware it will not be advertised to userspace/KVM guest nor will it
 	  be enabled.
@@ -1530,6 +1522,22 @@ config ARM64_PTR_AUTH
 	  but with the feature disabled. On such a system, this option should
 	  not be selected.
 
+config ARM64_PTR_AUTH_KERNEL
+	bool "Enable support for pointer authentication for kernel"
+	default y
+	depends on ARM64_PTR_AUTH
+	depends on (!FUNCTION_GRAPH_TRACER || DYNAMIC_FTRACE_WITH_REGS)
+	help
+	  Build the kernel with return address protection by
+	  pointer authentication.
+
+	  If the compiler supports the -mbranch-protection or
+	  -msign-return-address flag (e.g. GCC 7 or later), then this option
+	  will cause the kernel itself to be compiled with return address
+	  protection. In this case, and if the target hardware is known to
+	  support pointer authentication, then CONFIG_STACKPROTECTOR can be
+	  disabled with minimal loss of protection.
+
 	  This feature works with FUNCTION_GRAPH_TRACER option only if
 	  DYNAMIC_FTRACE_WITH_REGS is enabled.
 
@@ -1618,7 +1626,7 @@ config ARM64_BTI_KERNEL
 	bool "Use Branch Target Identification for kernel"
 	default y
 	depends on ARM64_BTI
-	depends on ARM64_PTR_AUTH
+	depends on ARM64_PTR_AUTH_KERNEL
 	depends on CC_HAS_BRANCH_PROT_PAC_RET_BTI
 	# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94697
 	depends on !CC_IS_GCC || GCC_VERSION >= 100100
diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index 6a87d592bd00..6e5d9de8c2b3 100644
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -70,7 +70,7 @@ endif
 # off, this will be overridden if we are using branch protection.
 branch-prot-flags-y += $(call cc-option,-mbranch-protection=none)
 
-ifeq ($(CONFIG_ARM64_PTR_AUTH),y)
+ifeq ($(CONFIG_ARM64_PTR_AUTH_KERNEL),y)
 branch-prot-flags-$(CONFIG_CC_HAS_SIGN_RETURN_ADDRESS) := -msign-return-address=all
 # We enable additional protection for leaf functions as there is some
 # narrow potential for ROP protection benefits and no substantial
diff --git a/drivers/misc/lkdtm/bugs.c b/drivers/misc/lkdtm/bugs.c
index a0675d4154d2..439fa33ae413 100644
--- a/drivers/misc/lkdtm/bugs.c
+++ b/drivers/misc/lkdtm/bugs.c
@@ -446,7 +446,7 @@ void lkdtm_DOUBLE_FAULT(void)
 #ifdef CONFIG_ARM64
 static noinline void change_pac_parameters(void)
 {
-	if (IS_ENABLED(CONFIG_ARM64_PTR_AUTH)) {
+	if (IS_ENABLED(CONFIG_ARM64_PTR_AUTH_KERNEL)) {
 		/* Reset the keys of current task */
 		ptrauth_thread_init_kernel(current);
 		ptrauth_thread_switch_kernel(current);
@@ -460,8 +460,8 @@ noinline void lkdtm_CORRUPT_PAC(void)
 #define CORRUPT_PAC_ITERATE	10
 	int i;
 
-	if (!IS_ENABLED(CONFIG_ARM64_PTR_AUTH))
-		pr_err("FAIL: kernel not built with CONFIG_ARM64_PTR_AUTH\n");
+	if (!IS_ENABLED(CONFIG_ARM64_PTR_AUTH_KERNEL))
+		pr_err("FAIL: kernel not built with CONFIG_ARM64_PTR_AUTH_KERNEL\n");
 
 	if (!system_supports_address_auth()) {
 		pr_err("FAIL: CPU lacks pointer authentication feature\n");
-- 
2.29.2