blk-mq-debugfs: don't allow write on attributes with seq_operations set

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>

1 files changed, 5 insertions, 1 deletions

diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
index fa31ceaa8de6..21cbc1f071c6 100644
--- a/block/blk-mq-debugfs.c
+++ b/block/blk-mq-debugfs.c
@@ -697,7 +697,11 @@ static ssize_t blk_mq_debugfs_write(struct file *file, const char __user *buf,
         const struct blk_mq_debugfs_attr *attr = m->private;
         void *data = d_inode(file->f_path.dentry->d_parent)->i_private;
 
-        if (!attr->write)
+        /*
+         * Attributes that only implement .seq_ops are read-only and 'attr' is
+         * the same with 'data' in this case.
+         */
+        if (attr == data || !attr->write)
                 return -EPERM;
 
         return attr->write(data, buf, count, ppos);