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

[sashiko-bot@kernel.org]

> 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@vger.kernel.org
> Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

Sashiko has reviewed this patch and found no issues.

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