mm_for_maps: simplify, use ptrace_may_access()

It would be nice to kill __ptrace_may_access(). It requires task_lock(),
but this lock is only needed to read mm->flags in the middle.

Convert mm_for_maps() to use ptrace_may_access(), this also simplifies
the code a little bit.

Also, we do not need to take ->mmap_sem in advance. In fact I think
mm_for_maps() should not play with ->mmap_sem at all, the caller should
take this lock.

With or without this patch, without ->cred_guard_mutex held we can race
with exec() and get the new ->mm but check old creds.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Serge Hallyn <serue@us.ibm.com>
Signed-off-by: James Morris <jmorris@namei.org>

fs/proc/base.c

@@ -237,20 +237,19 @@ struct mm_struct *mm_for_maps(struct task_struct *task)
 	struct mm_struct *mm = get_task_mm(task);
 	if (!mm)
 		return NULL;
+	if (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);
+			return NULL;
+		}
+	}
 	down_read(&mm->mmap_sem);
-	task_lock(task);
-	if (task->mm != mm)
-		goto out;
-	if (task->mm != current->mm &&
-	    __ptrace_may_access(task, PTRACE_MODE_READ) < 0)
-		goto out;
-	task_unlock(task);
 	return mm;
-out:
-	task_unlock(task);
-	up_read(&mm->mmap_sem);
-	mmput(mm);
-	return NULL;
 }
 
 static int proc_pid_cmdline(struct task_struct *task, char * buffer)