[PATCH] proc: nommu: /proc/<pid>/maps: release mmap read lock

[Ben Wolsieffer <ben.wolsieffer@xxxxxxxxxxx>]

From: Ben Wolsieffer <Ben.Wolsieffer@xxxxxxxxxxx>

The no-MMU implementation of /proc/<pid>/map doesn't normally release
the mmap read lock, because it uses !IS_ERR_OR_NULL(_vml) to determine
whether to release the lock. Since _vml is NULL when the end of the
mappings is reached, the lock is not released.

This code was incorrectly adapted from the MMU implementation, which
at the time released the lock in m_next() before returning the last entry.

The MMU implementation has diverged further from the no-MMU version
since then, so this patch brings their locking and error handling into
sync, fixing the bug and hopefully avoiding similar issues in the
future.

Fixes: 47fecca15c09 ("fs/proc/task_nommu.c: don't use priv->task->mm")
Signed-off-by: Ben Wolsieffer <ben.wolsieffer@xxxxxxxxxxx>
---
 fs/proc/task_nommu.c | 27 +++++++++++++++------------
 1 file changed, 15 insertions(+), 12 deletions(-)

diff --git a/fs/proc/task_nommu.c b/fs/proc/task_nommu.c
index 2c8b62265981..061bd3f82756 100644
--- a/fs/proc/task_nommu.c
+++ b/fs/proc/task_nommu.c
@@ -205,11 +205,16 @@ static void *m_start(struct seq_file *m, loff_t *pos)
 		return ERR_PTR(-ESRCH);
 
 	mm = priv->mm;
-	if (!mm || !mmget_not_zero(mm))
+	if (!mm || !mmget_not_zero(mm)) {
+		put_task_struct(priv->task);
+		priv->task = NULL;
 		return NULL;
+	}
 
 	if (mmap_read_lock_killable(mm)) {
 		mmput(mm);
+		put_task_struct(priv->task);
+		priv->task = NULL;
 		return ERR_PTR(-EINTR);
 	}
 
@@ -218,23 +223,21 @@ static void *m_start(struct seq_file *m, loff_t *pos)
 	if (vma)
 		return vma;
 
-	mmap_read_unlock(mm);
-	mmput(mm);
 	return NULL;
 }
 
-static void m_stop(struct seq_file *m, void *_vml)
+static void m_stop(struct seq_file *m, void *v)
 {
 	struct proc_maps_private *priv = m->private;
+	struct mm_struct *mm = priv->mm;
 
-	if (!IS_ERR_OR_NULL(_vml)) {
-		mmap_read_unlock(priv->mm);
-		mmput(priv->mm);
-	}
-	if (priv->task) {
-		put_task_struct(priv->task);
-		priv->task = NULL;
-	}
+	if (!priv->task)
+		return;
+
+	mmap_read_unlock(mm);
+	mmput(mm);
+	put_task_struct(priv->task);
+	priv->task = NULL;
 }
 
 static void *m_next(struct seq_file *m, void *_p, loff_t *pos)
-- 
2.42.0