fs/proc/task_nommu.c: don't use priv->task->mm

I do not know if CONFIG_PREEMPT/SMP is possible without CONFIG_MMU
but the usage of task->mm in m_stop(). The task can exit/exec before
we take mmap_sem, in this case m_stop() can hit NULL or unlock the
wrong rw_semaphore.

Also, this code uses priv->task != NULL to decide whether we need
up_read/mmput. This is correct, but we will probably kill priv->task.
Change m_start/m_stop to rely on IS_ERR_OR_NULL() like task_mmu.c does.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: Greg Ungerer <gerg@uclinux.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 10 insertions, 8 deletions

diff --git a/fs/proc/task_nommu.c b/fs/proc/task_nommu.c
index 9019f1de3f72..429cb7a5419e 100644
--- a/fs/proc/task_nommu.c
+++ b/fs/proc/task_nommu.c
@@ -217,17 +217,17 @@ static void *m_start(struct seq_file *m, loff_t *pos)
                 return ERR_PTR(-ESRCH);
 
         mm = priv->mm;
-        if (!mm || !atomic_inc_not_zero(&mm->mm_users)) {
-                put_task_struct(priv->task);
-                priv->task = NULL;
+        if (!mm || !atomic_inc_not_zero(&mm->mm_users))
                 return NULL;
-        }
-        down_read(&mm->mmap_sem);
 
+        down_read(&mm->mmap_sem);
         /* start from the Nth VMA */
         for (p = rb_first(&mm->mm_rb); p; p = rb_next(p))
                 if (n-- == 0)
                         return p;
+
+        up_read(&mm->mmap_sem);
+        mmput(mm);
         return NULL;
 }
 
@@ -235,11 +235,13 @@ static void m_stop(struct seq_file *m, void *_vml)
 {
         struct proc_maps_private *priv = m->private;
 
+        if (!IS_ERR_OR_NULL(_vml)) {
+                up_read(&priv->mm->mmap_sem);
+                mmput(priv->mm);
+        }
         if (priv->task) {
-                struct mm_struct *mm = priv->task->mm;
-                up_read(&mm->mmap_sem);
-                mmput(mm);
                 put_task_struct(priv->task);
+                priv->task = NULL;
         }
 }