loop: Add sanity check for read/write_iter

Some file systems do not support read_iter/write_iter, such as selinuxfs
in this issue.
So before calling them, first confirm that the interface is supported and
then call it.

It is releavant in that vfs_iter_read/write have the check, and removal
of their used caused szybot to be able to hit this issue.

Fixes: f2fed44 ("loop: stop using vfs_iter__{read,write} for buffered I/O")
Reported-by: syzbot+6af973a3b8dfd2faefdc@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=6af973a3b8dfd2faefdc
Signed-off-by: Lizhi Xu <lizhi.xu@windriver.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20250428143626.3318717-1-lizhi.xu@windriver.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/loop.c

@@ -505,6 +505,17 @@ static void loop_assign_backing_file(struct loop_device *lo, struct file *file)
 	lo->lo_min_dio_size = loop_query_min_dio_size(lo);
 }
 
+static int loop_check_backing_file(struct file *file)
+{
+	if (!file->f_op->read_iter)
+		return -EINVAL;
+
+	if ((file->f_mode & FMODE_WRITE) && !file->f_op->write_iter)
+		return -EINVAL;
+
+	return 0;
+}
+
 /*
  * loop_change_fd switched the backing store of a loopback device to
  * a new file. This is useful for operating system installers to free up
@@ -526,6 +537,10 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,
 	if (!file)
 		return -EBADF;
 
+	error = loop_check_backing_file(file);
+	if (error)
+		return error;
+
 	/* suppress uevents while reconfiguring the device */
 	dev_set_uevent_suppress(disk_to_dev(lo->lo_disk), 1);
 
@@ -963,6 +978,14 @@ static int loop_configure(struct loop_device *lo, blk_mode_t mode,
 
 	if (!file)
 		return -EBADF;
+
+	if ((mode & BLK_OPEN_WRITE) && !file->f_op->write_iter)
+		return -EINVAL;
+
+	error = loop_check_backing_file(file);
+	if (error)
+		return error;
+
 	is_loop = is_loop_device(file);
 
 	/* This is safe, since we have a reference from open(). */