powerpc/8xx: Allow disabling KUAP at boot time

PPC64 uses MMU features to enable/disable KUAP at boot time.
But feature fixups are applied way too early on PPC32.

But since commit c1672883 ("powerpc/32: Manage KUAP in C"),
all KUAP is in C so it is now possible to use static branches.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/3dca510ce555335261a47c4799167da698f569c0.1622782111.git.christophe.leroy@csgroup.eu

arch/powerpc/include/asm/nohash/32/kup-8xx.h

@@ -9,10 +9,22 @@
 
 #ifndef __ASSEMBLY__
 
+#include <linux/jump_label.h>
+
 #include <asm/reg.h>
 
+extern struct static_key_false disable_kuap_key;
+
+static __always_inline bool kuap_is_disabled(void)
+{
+	return static_branch_unlikely(&disable_kuap_key);
+}
+
 static inline void kuap_save_and_lock(struct pt_regs *regs)
 {
+	if (kuap_is_disabled())
+		return;
+
 	regs->kuap = mfspr(SPRN_MD_AP);
 	mtspr(SPRN_MD_AP, MD_APG_KUAP);
 }
@@ -23,12 +35,20 @@ static inline void kuap_user_restore(struct pt_regs *regs)
 
 static inline void kuap_kernel_restore(struct pt_regs *regs, unsigned long kuap)
 {
+	if (kuap_is_disabled())
+		return;
+
 	mtspr(SPRN_MD_AP, regs->kuap);
 }
 
 static inline unsigned long kuap_get_and_assert_locked(void)
 {
-	unsigned long kuap = mfspr(SPRN_MD_AP);
+	unsigned long kuap;
+
+	if (kuap_is_disabled())
+		return MD_APG_INIT;
+
+	kuap = mfspr(SPRN_MD_AP);
 
 	if (IS_ENABLED(CONFIG_PPC_KUAP_DEBUG))
 		WARN_ON_ONCE(kuap >> 16 != MD_APG_KUAP >> 16);
@@ -38,25 +58,36 @@ static inline unsigned long kuap_get_and_assert_locked(void)
 
 static inline void kuap_assert_locked(void)
 {
-	if (IS_ENABLED(CONFIG_PPC_KUAP_DEBUG))
+	if (IS_ENABLED(CONFIG_PPC_KUAP_DEBUG) && !kuap_is_disabled())
 		kuap_get_and_assert_locked();
 }
 
 static inline void allow_user_access(void __user *to, const void __user *from,
 				     unsigned long size, unsigned long dir)
 {
+	if (kuap_is_disabled())
+		return;
+
 	mtspr(SPRN_MD_AP, MD_APG_INIT);
 }
 
 static inline void prevent_user_access(void __user *to, const void __user *from,
 				       unsigned long size, unsigned long dir)
 {
+	if (kuap_is_disabled())
+		return;
+
 	mtspr(SPRN_MD_AP, MD_APG_KUAP);
 }
 
 static inline unsigned long prevent_user_access_return(void)
 {
-	unsigned long flags = mfspr(SPRN_MD_AP);
+	unsigned long flags;
+
+	if (kuap_is_disabled())
+		return MD_APG_INIT;
+
+	flags = mfspr(SPRN_MD_AP);
 
 	mtspr(SPRN_MD_AP, MD_APG_KUAP);
 
@@ -65,12 +96,18 @@ static inline unsigned long prevent_user_access_return(void)
 
 static inline void restore_user_access(unsigned long flags)
 {
+	if (kuap_is_disabled())
+		return;
+
 	mtspr(SPRN_MD_AP, flags);
 }
 
 static inline bool
 bad_kuap_fault(struct pt_regs *regs, unsigned long address, bool is_write)
 {
+	if (kuap_is_disabled())
+		return false;
+
 	return !((regs->kuap ^ MD_APG_KUAP) & 0xff000000);
 }
 

arch/powerpc/mm/nohash/8xx.c

@@ -256,12 +256,17 @@ void __init setup_kuep(bool disabled)
 #endif
 
 #ifdef CONFIG_PPC_KUAP
+struct static_key_false disable_kuap_key;
+EXPORT_SYMBOL(disable_kuap_key);
+
 void __init setup_kuap(bool disabled)
 {
-	pr_info("Activating Kernel Userspace Access Protection\n");
+	if (disabled) {
+		static_branch_enable(&disable_kuap_key);
+		return;
+	}
 
-	if (disabled)
-		pr_warn("KUAP cannot be disabled yet on 8xx when compiled in\n");
+	pr_info("Activating Kernel Userspace Access Protection\n");
 
 	mtspr(SPRN_MD_AP, MD_APG_KUAP);
 }