mm_for_maps: shift down_read(mmap_sem) to the caller

mm_for_maps() takes ->mmap_sem after security checks, this looks
strange and obfuscates the locking rules. Move this lock to its
single caller, m_start().
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

@@ -235,9 +235,8 @@ 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);
-	if (!mm)
-		return NULL;
-	if (mm != current->mm) {
+
+	if (mm && mm != current->mm) {
 		/*
 		 * task->mm can be changed before security check,
 		 * in that case we must notice the change after.
@@ -245,10 +244,9 @@ struct mm_struct *mm_for_maps(struct task_struct *task)
 		if (!ptrace_may_access(task, PTRACE_MODE_READ) ||
 		    mm != task->mm) {
 			mmput(mm);
-			return NULL;
+			mm = NULL;
 		}
 	}
-	down_read(&mm->mmap_sem);
 	return mm;
 }
 

fs/proc/task_mmu.c

@@ -119,6 +119,7 @@ static void *m_start(struct seq_file *m, loff_t *pos)
 	mm = mm_for_maps(priv->task);
 	if (!mm)
 		return NULL;
+	down_read(&mm->mmap_sem);
 
 	tail_vma = get_gate_vma(priv->task);
 	priv->tail_vma = tail_vma;

fs/proc/task_nommu.c

@@ -189,6 +189,7 @@ static void *m_start(struct seq_file *m, loff_t *pos)
 		priv->task = NULL;
 		return NULL;
 	}
+	down_read(&mm->mmap_sem);
 
 	/* start from the Nth VMA */
 	for (p = rb_first(&mm->mm_rb); p; p = rb_next(p))