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From:   Sasha Levin <Alexander.Levin@microsoft.com>
To:     "stable@vger.kernel.org" <stable@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
CC:     Eryu Guan <eguan@redhat.com>, Ming Lei <ming.lei@redhat.com>,
        Jens Axboe <axboe@kernel.dk>,
        Sasha Levin <Alexander.Levin@microsoft.com>
Subject: [PATCH AUTOSEL for 4.14 070/161] blk-mq-debugfs: don't allow write on
 attributes with seq_operations set
Thread-Topic: [PATCH AUTOSEL for 4.14 070/161] blk-mq-debugfs: don't allow
 write on attributes with seq_operations set
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Date:   Mon, 9 Apr 2018 00:20:43 +0000
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From: Eryu Guan <eguan@redhat.com>

[ Upstream commit 6b136a24b05c81a24e0b648a4bd938bcd0c4f69e ]

Attributes that only implement .seq_ops are read-only, any write to
them should be rejected. But currently kernel would crash when
writing to such debugfs entries, e.g.

chmod +w /sys/kernel/debug/block/<dev>/requeue_list
echo 0 > /sys/kernel/debug/block/<dev>/requeue_list
chmod -w /sys/kernel/debug/block/<dev>/requeue_list

Fix it by returning -EPERM in blk_mq_debugfs_write() when writing to
such attributes.

Cc: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
---
 block/blk-mq-debugfs.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
index de294d775acf..d95439154556 100644
--- a/block/blk-mq-debugfs.c
+++ b/block/blk-mq-debugfs.c
@@ -704,7 +704,11 @@ static ssize_t blk_mq_debugfs_write(struct file *file,=
 const char __user *buf,
 	const struct blk_mq_debugfs_attr *attr =3D m->private;
 	void *data =3D d_inode(file->f_path.dentry->d_parent)->i_private;
=20
-	if (!attr->write)
+	/*
+	 * Attributes that only implement .seq_ops are read-only and 'attr' is
+	 * the same with 'data' in this case.
+	 */
+	if (attr =3D=3D data || !attr->write)
 		return -EPERM;
=20
 	return attr->write(data, buf, count, ppos);
--=20
2.15.1