s390/mm: fix address space detection in exception handling

Commit 0aaba41b ("s390: remove all code using the access register
mode") removed access register mode from the kernel, and also from the
address space detection logic. However, user space could still switch
to access register mode (trans_exc_code == 1), and exceptions in that
mode would not be correctly assigned.

Fix this by adding a check for trans_exc_code == 1 to get_fault_type(),
and remove the wrong comment line before that function.

Fixes: 0aaba41b ("s390: remove all code using the access register mode")
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: <stable@vger.kernel.org> # v4.15+
Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>

arch/s390/mm/fault.c

@@ -83,7 +83,6 @@ static inline int notify_page_fault(struct pt_regs *regs)
 
 /*
  * Find out which address space caused the exception.
- * Access register mode is impossible, ignore space == 3.
  */
 static enum fault_type get_fault_type(struct pt_regs *regs)
 {
@@ -108,6 +107,10 @@ static enum fault_type get_fault_type(struct pt_regs *regs)
 		}
 		return VDSO_FAULT;
 	}
+	if (trans_exc_code == 1) {
+		/* access register mode, not used in the kernel */
+		return USER_FAULT;
+	}
 	/* home space exception -> access via kernel ASCE */
 	return KERNEL_FAULT;
 }