Re: [PATCH v2 08/12] 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 <redacted>
---
  arch/powerpc/include/asm/book3s/32/kup.h | 27 +++++++++++++++++++++++-
  arch/powerpc/mm/book3s32/kuap.c          | 11 ++++++----
  2 files changed, 33 insertions(+), 5 deletions(-)

diff --git a/arch/powerpc/include/asm/book3s/32/kup.h b/arch/powerpc/include/asm/book3s/32/kup.h
index 2854d970dabe..68fbe28c6d7e 100644
--- a/arch/powerpc/include/asm/book3s/32/kup.h
+++ b/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);
  }
  

diff --git a/arch/powerpc/mm/book3s32/kuap.c b/arch/powerpc/mm/book3s32/kuap.c
index 5533ed92ab3d..a4ce6cdc28e5 100644
--- a/arch/powerpc/mm/book3s32/kuap.c
+++ b/arch/powerpc/mm/book3s32/kuap.c

@@ -3,15 +3,18 @@
  #include <asm/kup.h>
  #include <asm/smp.h>
  
+struct static_key_false 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");
  }