Re: [PATCH RFC 01/41] loop: refuse backing files that aren't open for reading

[Jan Kara <jack@xxxxxxx>]

On Tue 10-11-26 10:14:17, Christian Brauner wrote:
> loop_configure() takes any file as long as it implements ->read_iter().
> Loop itself never checked whether the file was actually opened for
> reading. Buffered I/O went through vfs_iter_read() which refuses files
> without FMODE_READ. Direct I/O has called ->read_iter() of the backing
> file directly ever since loop grew support for it.
> 
> Since commit f2fed441c69b ("loop: stop using vfs_iter_{read,write} for
> buffered I/O") buffered I/O calls ->read_iter() directly as well. That
> commit was about O_DIRECT handling and the FMODE_READ check went away
> as a side effect.
> 
> So whoever can set up a loop device can read a file it was only allowed
> to open for writing. Open the file O_WRONLY, configure a loop device
> with it and read the contents through the loop device.
> 
> Refuse backing files that aren't open for reading. Loop always reads
> from its backing file so a write-only backing file never made sense in
> the first place.
> 
> Fixes: bc07c10a3603 ("block: loop: support DIO & AIO")
> Fixes: f2fed441c69b ("loop: stop using vfs_iter_{read,write} for buffered 
> I/O")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Christian Brauner (Amutable) <brauner@xxxxxxxxxx>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

                                                                Honza

> ---
>  drivers/block/loop.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index a3325ceab484..c96e35c3ef42 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -528,6 +528,9 @@ static void loop_assign_backing_file(struct loop_device 
> *lo, struct file *file)
>  
>  static int loop_check_backing_file(struct file *file)
>  {
> +     if (!(file->f_mode & FMODE_READ))
> +             return -EBADF;
> +
>       if (!file->f_op->read_iter)
>               return -EINVAL;
>  
> 
> -- 
> 2.53.0
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR