mm_for_maps: take ->cred_guard_mutex to fix the race with exec

The problem is minor, but without ->cred_guard_mutex held we can race
with exec() and get the new ->mm but check old creds.

Now we do not need to re-check task->mm after ptrace_may_access(), it
can't be changed to the new mm under us.

Strictly speaking, this also fixes another very minor problem. Unless
security check fails or the task exits mm_for_maps() should never
return NULL, the caller should get either old or new ->mm.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Signed-off-by: James Morris <jmorris@namei.org>

fs/proc/base.c

@@ -234,19 +234,19 @@ static int check_mem_permission(struct task_struct *task)
 
 struct mm_struct *mm_for_maps(struct task_struct *task)
 {
-	struct mm_struct *mm = get_task_mm(task);
+	struct mm_struct *mm;
 
-	if (mm && mm != current->mm) {
-		/*
-		 * task->mm can be changed before security check,
-		 * in that case we must notice the change after.
-		 */
-		if (!ptrace_may_access(task, PTRACE_MODE_READ) ||
-		    mm != task->mm) {
-			mmput(mm);
-			mm = NULL;
-		}
+	if (mutex_lock_killable(&task->cred_guard_mutex))
+		return NULL;
+
+	mm = get_task_mm(task);
+	if (mm && mm != current->mm &&
+			!ptrace_may_access(task, PTRACE_MODE_READ)) {
+		mmput(mm);
+		mm = NULL;
 	}
+	mutex_unlock(&task->cred_guard_mutex);
+
 	return mm;
 }