fs/proc/task_mmu.c: don't use task->mm in m_start() and show_*map()

get_gate_vma(priv->task->mm) looks ugly and wrong, task->mm can be NULL or
it can changed by exec right after mm_access().

And in theory this race is not harmless, the task can exec and then later
exit and free the new mm_struct.  In this case get_task_mm(oldmm) can't
help, get_gate_vma(task->mm) can read the freed/unmapped memory.

I think that priv->task should simply die and hold_task_mempolicy() logic
can be simplified.  tail_vma logic asks for cleanups too.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/task_mmu.c

@@ -170,7 +170,7 @@ static void *m_start(struct seq_file *m, loff_t *pos)
 		return mm;
 	down_read(&mm->mmap_sem);
 
-	tail_vma = get_gate_vma(priv->task->mm);
+	tail_vma = get_gate_vma(mm);
 	priv->tail_vma = tail_vma;
 	hold_task_mempolicy(priv);
 	/* Start with last addr hint */
@@ -351,12 +351,11 @@ static int show_map(struct seq_file *m, void *v, int is_pid)
 {
 	struct vm_area_struct *vma = v;
 	struct proc_maps_private *priv = m->private;
-	struct task_struct *task = priv->task;
 
 	show_map_vma(m, vma, is_pid);
 
 	if (m->count < m->size)  /* vma is copied successfully */
-		m->version = (vma != get_gate_vma(task->mm))
+		m->version = (vma != priv->tail_vma)
 			? vma->vm_start : 0;
 	return 0;
 }
@@ -584,7 +583,6 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma)
 static int show_smap(struct seq_file *m, void *v, int is_pid)
 {
 	struct proc_maps_private *priv = m->private;
-	struct task_struct *task = priv->task;
 	struct vm_area_struct *vma = v;
 	struct mem_size_stats mss;
 	struct mm_walk smaps_walk = {
@@ -639,7 +637,7 @@ static int show_smap(struct seq_file *m, void *v, int is_pid)
 	show_smap_vma_flags(m, vma);
 
 	if (m->count < m->size)  /* vma is copied successfully */
-		m->version = (vma != get_gate_vma(task->mm))
+		m->version = (vma != priv->tail_vma)
 			? vma->vm_start : 0;
 	return 0;
 }