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

[Alexander Mikhalitsyn <alex@xxxxxxxxxxxx>]

On Tue, 2026-11-10 at 10:14 +0100, 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>

Reviewed-by: Alexander Mikhalitsyn <alex@xxxxxxxxxxxx>

> ---
>  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;
>