powerpc/32s: 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.

Now that all KUAP related actions are in C following the
conversion of KUAP initial setup and context switch in C,
static branches can be used to enable/disable KUAP.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[mpe: Export disable_kuap_key to fix build errors]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/cd79e8008455fba5395d099f9bb1305c039b931c.1622708530.git.christophe.leroy@csgroup.eu

arch/powerpc/include/asm/book3s/32/kup.h

@@ -9,11 +9,12 @@
 
 #include <linux/jump_label.h>
 
+extern struct static_key_false disable_kuap_key;
 extern struct static_key_false disable_kuep_key;
 
 static __always_inline bool kuap_is_disabled(void)
 {
-	return !IS_ENABLED(CONFIG_PPC_KUAP);
+	return !IS_ENABLED(CONFIG_PPC_KUAP) || static_branch_unlikely(&disable_kuap_key);
 }
 
 static __always_inline bool kuep_is_disabled(void)
@@ -62,6 +63,9 @@ static inline void kuap_save_and_lock(struct pt_regs *regs)
 	u32 addr = kuap & 0xf0000000;
 	u32 end = kuap << 28;
 
+	if (kuap_is_disabled())
+		return;
+
 	regs->kuap = kuap;
 	if (unlikely(!kuap))
 		return;
@@ -79,6 +83,9 @@ static inline void kuap_kernel_restore(struct pt_regs *regs, unsigned long kuap)
 	u32 addr = regs->kuap & 0xf0000000;
 	u32 end = regs->kuap << 28;
 
+	if (kuap_is_disabled())
+		return;
+
 	current->thread.kuap = regs->kuap;
 
 	if (unlikely(regs->kuap == kuap))
@@ -91,6 +98,9 @@ static inline unsigned long kuap_get_and_assert_locked(void)
 {
 	unsigned long kuap = current->thread.kuap;
 
+	if (kuap_is_disabled())
+		return 0;
+
 	WARN_ON_ONCE(IS_ENABLED(CONFIG_PPC_KUAP_DEBUG) && kuap != 0);
 
 	return kuap;
@@ -106,6 +116,9 @@ static __always_inline void allow_user_access(void __user *to, const void __user
 {
 	u32 addr, end;
 
+	if (kuap_is_disabled())
+		return;
+
 	BUILD_BUG_ON(!__builtin_constant_p(dir));
 	BUILD_BUG_ON(dir & ~KUAP_READ_WRITE);
 
@@ -128,6 +141,9 @@ static __always_inline void prevent_user_access(void __user *to, const void __us
 {
 	u32 addr, end;
 
+	if (kuap_is_disabled())
+		return;
+
 	BUILD_BUG_ON(!__builtin_constant_p(dir));
 
 	if (dir & KUAP_CURRENT_WRITE) {
@@ -159,6 +175,9 @@ static inline unsigned long prevent_user_access_return(void)
 	unsigned long end = flags << 28;
 	void __user *to = (__force void __user *)addr;
 
+	if (kuap_is_disabled())
+		return 0;
+
 	if (flags)
 		prevent_user_access(to, to, end - addr, KUAP_READ_WRITE);
 
@@ -171,6 +190,9 @@ static inline void restore_user_access(unsigned long flags)
 	unsigned long end = flags << 28;
 	void __user *to = (__force void __user *)addr;
 
+	if (kuap_is_disabled())
+		return;
+
 	if (flags)
 		allow_user_access(to, to, end - addr, KUAP_READ_WRITE);
 }
@@ -181,6 +203,9 @@ bad_kuap_fault(struct pt_regs *regs, unsigned long address, bool is_write)
 	unsigned long begin = regs->kuap & 0xf0000000;
 	unsigned long end = regs->kuap << 28;
 
+	if (kuap_is_disabled())
+		return false;
+
 	return is_write && (address < begin || address >= end);
 }
 

arch/powerpc/mm/book3s32/kuap.c

@@ -3,15 +3,19 @@
 #include <asm/kup.h>
 #include <asm/smp.h>
 
+struct static_key_false disable_kuap_key;
+EXPORT_SYMBOL(disable_kuap_key);
+
 void __init setup_kuap(bool disabled)
 {
-	kuap_update_sr(mfsr(0) | SR_KS, 0, TASK_SIZE);
+	if (!disabled)
+		kuap_update_sr(mfsr(0) | SR_KS, 0, TASK_SIZE);
 
 	if (smp_processor_id() != boot_cpuid)
 		return;
 
-	pr_info("Activating Kernel Userspace Access Protection\n");
-
 	if (disabled)
-		pr_warn("KUAP cannot be disabled yet on 6xx when compiled in\n");
+		static_branch_enable(&disable_kuap_key);
+	else
+		pr_info("Activating Kernel Userspace Access Protection\n");
 }