[PATCH v2] iio: cros_ec: fix an use-after-free in cros_ec_sensors_push_data()

[Tzung-Bi Shih <tzungbi@kernel.org>]

cros_ec_sensors_push_data() reads `indio_dev->active_scan_mask` and
calls iio_push_to_buffers_with_timestamp() without making sure the
`indio_dev` stays in buffer mode.  There is a race if `indio_dev` exits
buffer mode right before cros_ec_sensors_push_data() accesses them.

An use-after-free on `indio_dev->active_scan_mask` was observed.  The
call trace:
[...]
 _find_next_bit
 cros_ec_sensors_push_data
 cros_ec_sensorhub_event
 blocking_notifier_call_chain
 cros_ec_irq_thread

It was caused by a race condition: one thread just freed
`active_scan_mask` at [1]; while another thread tried to access the
memory at [2].

Fix it by calling iio_device_claim_buffer_mode() to ensure the
`indio_dev` can't exit buffer mode during cros_ec_sensors_push_data().

[1]: https://elixir.bootlin.com/linux/v6.5/source/drivers/iio/industrialio-buffer.c#L1189
[2]: https://elixir.bootlin.com/linux/v6.5/source/drivers/iio/common/cros_ec_sensors/cros_ec_sensors_core.c#L198

Cc: stable@vger.kernel.org
Fixes: aa984f1ba4a4 ("iio: cros_ec: Register to cros_ec_sensorhub when EC supports FIFO")
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
---
Changes from v1(https://patchwork.kernel.org/project/linux-iio/patch/20230828094339.1248472-1-tzungbi@kernel.org/):
- Use iio_device_{claim|release}_buffer_mode() instead of accessing `mlock`.

 drivers/iio/common/cros_ec_sensors/cros_ec_sensors_core.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/iio/common/cros_ec_sensors/cros_ec_sensors_core.c b/drivers/iio/common/cros_ec_sensors/cros_ec_sensors_core.c
index b72d39fc2434..6bfe5d6847e7 100644
--- a/drivers/iio/common/cros_ec_sensors/cros_ec_sensors_core.c
+++ b/drivers/iio/common/cros_ec_sensors/cros_ec_sensors_core.c
@@ -190,8 +190,11 @@ int cros_ec_sensors_push_data(struct iio_dev *indio_dev,
 	/*
 	 * Ignore samples if the buffer is not set: it is needed if the ODR is
 	 * set but the buffer is not enabled yet.
+	 *
+	 * Note: iio_device_claim_buffer_mode() returns -EBUSY if the buffer
+	 * is not enabled.
 	 */
-	if (!iio_buffer_enabled(indio_dev))
+	if (iio_device_claim_buffer_mode(indio_dev) < 0)
 		return 0;
 
 	out = (s16 *)st->samples;
@@ -210,6 +213,7 @@ int cros_ec_sensors_push_data(struct iio_dev *indio_dev,
 	iio_push_to_buffers_with_timestamp(indio_dev, st->samples,
 					   timestamp + delta);
 
+	iio_device_release_buffer_mode(indio_dev);
 	return 0;
 }
 EXPORT_SYMBOL_GPL(cros_ec_sensors_push_data);
-- 
2.42.0.rc2.253.gd59a3bf2b4-goog