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: [email protected]
Signed-off-by: Christian Brauner (Amutable) <[email protected]>
---
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
