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

1 files changed, 14 insertions, 0 deletions

diff --git a/arch/powerpc/include/asm/nohash/32/kup-8xx.h b/arch/powerpc/include/asm/nohash/32/kup-8xx.h
index 1d70c80366fd..85ed2390fb99 100644
--- a/arch/powerpc/include/asm/nohash/32/kup-8xx.h
+++ b/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)
 {