[PATCH 4/4] powerpc/kuap: Make disabling KUAP at boottime optional

[Christophe Leroy]

It is possible to disable KUAP at boottime with 'nosmap' parameter.

That is implemented with jump_label hence adds a 'nop' in front
of each open/close of userspace access.

>From a security point of view it makes sence to disallow disabling
KUAP. And on processors like the 8xx where 'nop' is not seamless,
it saves a few cycles.

So add a CONFIG item to make it optionnal.

Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
---
 arch/powerpc/include/asm/kup.h         |  2 +-
 arch/powerpc/mm/init-common.c          |  3 +++
 arch/powerpc/platforms/Kconfig.cputype | 10 ++++++++++
 3 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/kup.h b/arch/powerpc/include/asm/kup.h
index 74b7f4cee2ed..f3280169aeec 100644
--- a/arch/powerpc/include/asm/kup.h
+++ b/arch/powerpc/include/asm/kup.h
@@ -53,7 +53,7 @@ extern struct static_key_false disable_kuap_key;
 
 static __always_inline bool kuap_is_disabled(void)
 {
-	return static_branch_unlikely(&disable_kuap_key);
+	return IS_ENABLED(CONFIG_PPC_KUAP_BOOTTIME) && static_branch_unlikely(&disable_kuap_key);
 }
 #endif
 #else
diff --git a/arch/powerpc/mm/init-common.c b/arch/powerpc/mm/init-common.c
index 74e140b1efef..994ee58f0092 100644
--- a/arch/powerpc/mm/init-common.c
+++ b/arch/powerpc/mm/init-common.c
@@ -48,6 +48,9 @@ early_param("nosmep", parse_nosmep);
 
 static int __init parse_nosmap(char *p)
 {
+	if (!IS_ENABLED(CONFIG_PPC_KUAP_BOOTTIME))
+		return 0;
+
 	disable_kuap = true;
 	pr_warn("Disabling Kernel Userspace Access Protection\n");
 	return 0;
diff --git a/arch/powerpc/platforms/Kconfig.cputype b/arch/powerpc/platforms/Kconfig.cputype
index 45fd975ef521..f75c2d5cd182 100644
--- a/arch/powerpc/platforms/Kconfig.cputype
+++ b/arch/powerpc/platforms/Kconfig.cputype
@@ -502,6 +502,16 @@ config PPC_KUAP
 
 	  If you're unsure, say Y.
 
+config PPC_KUAP_BOOTTIME
+	bool "Allow disabling Kernel Userspace Access Protection at boottime"
+	depends on PPC_KUAP
+	default y
+	help
+	  Allow the user to disable Kernel Userspace Access Protection (KUAP)
+	  at boot time using 'nosmap' kernel parameter.
+
+	  If you're unsure, say Y.
+
 config PPC_KUAP_DEBUG
 	bool "Extra debugging for Kernel Userspace Access Protection"
 	depends on PPC_KUAP
-- 
2.40.1