iio: event: Fix event FIFO reset race

`iio_event_getfd()` creates the event file descriptor with
`anon_inode_getfd()`, which allocates a new fd, creates the anonymous
file and installs it in the process fd table before returning to the
caller.

The IIO code resets the event FIFO after `anon_inode_getfd()` has returned,
but before `IIO_GET_EVENT_FD_IOCTL` has copied the fd number to userspace.
But since fd tables are shared between threads, another thread can guess
the newly allocated fd number and issue a `read()` on it as soon as the fd
has been installed.

This means the `kfifo_to_user()` in `iio_event_chrdev_read()` can run in
parallel with the `kfifo_reset_out()` in `iio_event_getfd()`.

The kfifo documentation says that `kfifo_reset_out()` is only safe when it
is called from the reader thread and there is only one concurrent reader.
Otherwise it is dangerous and must be handled in the same way as
`kfifo_reset()`.

If that happens, `kfifo_to_user()` can advance the FIFO `out` index based
on state from before the reset, after the reset has already moved the `out`
index to the current `in` index. That can leave the FIFO with an `out`
index past the `in` index. A later `read()` can then see an underflowed
FIFO length and copy more data than the event FIFO buffer contains. This
can result in an out-of-bounds read and leak adjacent kernel memory to
userspace.

Move the FIFO reset before `anon_inode_getfd()`. At that point the event fd is
marked busy, but the new fd has not been installed yet, so userspace cannot
access it while the FIFO is reset.

Fixes: b91accafbb ("iio:event: Fix and cleanup locking")
Reported-by: Codex:gpt-5.5
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/industrialio-event.c

@@ -207,6 +207,8 @@ static int iio_event_getfd(struct iio_dev *indio_dev)
 		goto unlock;
 	}
 
+	kfifo_reset_out(&ev_int->det_events);
+
 	iio_device_get(indio_dev);
 
 	fd = anon_inode_getfd("iio:event", &iio_event_chrdev_fileops,
@@ -214,10 +216,7 @@ static int iio_event_getfd(struct iio_dev *indio_dev)
 	if (fd < 0) {
 		clear_bit(IIO_BUSY_BIT_POS, &ev_int->flags);
 		iio_device_put(indio_dev);
-	} else {
-		kfifo_reset_out(&ev_int->det_events);
 	}
-
 unlock:
 	mutex_unlock(&iio_dev_opaque->mlock);
 	return fd;