[PATCH] proc: task_mmu bug fix.

[Eric W. Biederman]

This should fix the big bug that has been crashing kernels when
fuser is called.  At least it is the bug I observed here.  It seems
you need the right access pattern on /proc/<pid>/maps to trigger this.

seq_operations ->stop is only called once per start making it safe to
call put_task_struct there.  However m_next was calling m_stop which
totally messed me up.  

Technically the task_struct needs to be held for the duration, so
split m_stop into two functions such that only vma_stop is called
multiple times per start. 

Signed-off-by: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>


---

 fs/proc/task_mmu.c |   18 ++++++++++++------
 1 files changed, 12 insertions(+), 6 deletions(-)

4217fed6dbbf2b5615d8a498b39aad5ee28d3e5f
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 4772543..f299538 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -363,17 +363,13 @@ out:
 	return priv->tail_vma;
 }
 
-static void m_stop(struct seq_file *m, void *v)
+static void vma_stop(struct proc_maps_private *priv, struct vm_area_struct *vma)
 {
-	struct proc_maps_private *priv = m->private;
-	struct vm_area_struct *vma = v;
 	if (vma && vma != priv->tail_vma) {
 		struct mm_struct *mm = vma->vm_mm;
 		up_read(&mm->mmap_sem);
 		mmput(mm);
 	}
-	if (priv->task)
-		put_task_struct(priv->task);
 }
 
 static void *m_next(struct seq_file *m, void *v, loff_t *pos)
@@ -385,10 +381,20 @@ static void *m_next(struct seq_file *m, 
 	(*pos)++;
 	if (vma && (vma != tail_vma) && vma->vm_next)
 		return vma->vm_next;
-	m_stop(m, v);
+	vma_stop(priv, vma);
 	return (vma != tail_vma)? tail_vma: NULL;
 }
 
+static void m_stop(struct seq_file *m, void *v)
+{
+	struct proc_maps_private *priv = m->private;
+	struct vm_area_struct *vma = v;
+
+	vma_stop(priv, vma);
+	if (priv->task)
+		put_task_struct(priv->task);
+}
+
 static struct seq_operations proc_pid_maps_op = {
 	.start	= m_start,
 	.next	= m_next,
-- 
1.2.2.g709a-dirty

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