powerpc: Implement user_access_save() and user_access_restore()

Implement user_access_save() and user_access_restore()

On 8xx and radix:
  - On save, get the value of the associated special register then
    prevent user access.
  - On restore, set back the saved value to the associated special
    register.

On book3s/32:
  - On save, get the value stored in current->thread.kuap and prevent
    user access.
  - On restore, regenerate address range from the stored value and
    reopen read/write access for that range.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/54f2f74938006b33c55a416674807b42ef222068.1579866752.git.christophe.leroy@c-s.fr

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

@@ -153,6 +153,29 @@ static __always_inline void prevent_user_access(void __user *to, const void __us
 	kuap_update_sr(mfsrin(addr) | SR_KS, addr, end);	/* set Ks */
 }
 
+static inline unsigned long prevent_user_access_return(void)
+{
+	unsigned long flags = current->thread.kuap;
+	unsigned long addr = flags & 0xf0000000;
+	unsigned long end = flags << 28;
+	void __user *to = (__force void __user *)addr;
+
+	if (flags)
+		prevent_user_access(to, to, end - addr, KUAP_READ_WRITE);
+
+	return flags;
+}
+
+static inline void restore_user_access(unsigned long flags)
+{
+	unsigned long addr = flags & 0xf0000000;
+	unsigned long end = flags << 28;
+	void __user *to = (__force void __user *)addr;
+
+	if (flags)
+		allow_user_access(to, to, end - addr, KUAP_READ_WRITE);
+}
+
 static inline bool
 bad_kuap_fault(struct pt_regs *regs, unsigned long address, bool is_write)
 {

arch/powerpc/include/asm/book3s/64/kup-radix.h

@@ -63,6 +63,14 @@
  * because that would require an expensive read/modify write of the AMR.
  */
 
+static inline unsigned long get_kuap(void)
+{
+	if (!early_mmu_has_feature(MMU_FTR_RADIX_KUAP))
+		return 0;
+
+	return mfspr(SPRN_AMR);
+}
+
 static inline void set_kuap(unsigned long value)
 {
 	if (!early_mmu_has_feature(MMU_FTR_RADIX_KUAP))
@@ -98,6 +106,20 @@ static inline void prevent_user_access(void __user *to, const void __user *from,
 	set_kuap(AMR_KUAP_BLOCKED);
 }
 
+static inline unsigned long prevent_user_access_return(void)
+{
+	unsigned long flags = get_kuap();
+
+	set_kuap(AMR_KUAP_BLOCKED);
+
+	return flags;
+}
+
+static inline void restore_user_access(unsigned long flags)
+{
+	set_kuap(flags);
+}
+
 static inline bool
 bad_kuap_fault(struct pt_regs *regs, unsigned long address, bool is_write)
 {

arch/powerpc/include/asm/kup.h

@@ -55,6 +55,8 @@ static inline void allow_user_access(void __user *to, const void __user *from,
 				     unsigned long size, unsigned long dir) { }
 static inline void prevent_user_access(void __user *to, const void __user *from,
 				       unsigned long size, unsigned long dir) { }
+static inline unsigned long prevent_user_access_return(void) { return 0UL; }
+static inline void restore_user_access(unsigned long flags) { }
 static inline bool
 bad_kuap_fault(struct pt_regs *regs, unsigned long address, bool is_write)
 {

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

@@ -46,6 +46,20 @@ static inline void prevent_user_access(void __user *to, const void __user *from,
 	mtspr(SPRN_MD_AP, MD_APG_KUAP);
 }
 
+static inline unsigned long prevent_user_access_return(void)
+{
+	unsigned long flags = mfspr(SPRN_MD_AP);
+
+	mtspr(SPRN_MD_AP, MD_APG_KUAP);
+
+	return flags;
+}
+
+static inline void restore_user_access(unsigned long flags)
+{
+	mtspr(SPRN_MD_AP, flags);
+}
+
 static inline bool
 bad_kuap_fault(struct pt_regs *regs, unsigned long address, bool is_write)
 {

arch/powerpc/include/asm/uaccess.h

@@ -465,9 +465,8 @@ static __must_check inline bool user_access_begin(const void __user *ptr, size_t
 }
 #define user_access_begin	user_access_begin
 #define user_access_end		prevent_current_access_user
-
-static inline unsigned long user_access_save(void) { return 0UL; }
-static inline void user_access_restore(unsigned long flags) { }
+#define user_access_save	prevent_user_access_return
+#define user_access_restore	restore_user_access
 
 #define unsafe_op_wrap(op, err) do { if (unlikely(op)) goto err; } while (0)
 #define unsafe_get_user(x, p, e) unsafe_op_wrap(__get_user_allowed(x, p), e)