[PATCH v1] iio: health: max30102: fix NULL dereference in interrupt handler

[PATCH v1] iio: health: max30102: fix NULL dereference in interrupt handler

[Marco Chen]

The interrupt is requested in max30102_probe() and stays enabled
for the lifetime of the device, but indio_dev->active_scan_mask is only
valid while a buffer is enabled. When an interrupt arrives while no
buffer is enabled, the handler dereferences the NULL active_scan_mask:

  Unable to handle kernel NULL pointer dereference at virtual address 
0000000000000000
  pc : __bitmap_weight+0x64/0x98
  lr : max30102_interrupt_handler+0x48/0x160 [max30102]
  Call trace:
   __bitmap_weight+0x64/0x98 (P)
   max30102_interrupt_handler+0x48/0x160 [max30102]
   irq_thread_fn+0x28/0xa8
   irq_thread+0x184/0x30c
   kthread+0x118/0x124
   ret_from_fork+0x10/0x20

Return early when no buffer is enabled. The interrupt status register is
read before returning, since reading it deasserts the chip's active-low
interrupt pin. Otherwise the pin would stay asserted and no further
edges would be delivered.

Fixes: 90579b69e94b ("iio: health: max30102: Add MAX30105 support")
Signed-off-by: Marco Chen <marcochen.dev@gmail.com>
---
I ran into this while interfacing with the MAX30102 over I2C on a
Raspberry Pi 4 running the IIO subsystem tree testing branch and
learning the IIO sysfs interface for the first time. When physically
rearranging INT pin wiring with no buffer enabled, the kernel oopsed.
This happened because max30102_interrupt_handler() attempted to
dereference active_scan_mask, which is NULL because no buffer is
enabled. With this patch, the same situation no longer oopses
and the buffered capture was tested to work normally afterward.

Some things I am unsure about though:
- Is IRQ_HANDLED or IRQ_NONE preferred here? I chose IRQ_HANDLED 
  because of the status register read to deassert the INT pin, but 
  I am not 100% confident on this choice.
- Should the regmap_read() return value be checked? I did not add a 
  check because I do not see a useful recovery path from this I2C failure, 
  but I can add a check in a v2 if it is better.
Thank you.

drivers/iio/health/max30102.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/drivers/iio/health/max30102.c b/drivers/iio/health/max30102.c
index c37316c86f14..c30b029ba8aa 100644
--- a/drivers/iio/health/max30102.c
+++ b/drivers/iio/health/max30102.c
@@ -290,10 +290,23 @@ static irqreturn_t max30102_interrupt_handler(int irq, void *private)
 {
 	struct iio_dev *indio_dev = private;
 	struct max30102_data *data = iio_priv(indio_dev);
-	unsigned int measurements = bitmap_weight(indio_dev->active_scan_mask,
-						  iio_get_masklength(indio_dev));
+	unsigned int measurements, val;
 	int ret, cnt = 0;
 
+	if (!indio_dev->active_scan_mask) {
+		/*
+		 * No buffer is enabled so there is nothing to read. Read the
+		 * status register anyway to deassert the max30102's interrupt
+		 * pin; otherwise it would stay asserted and further edges
+		 * would not be delivered.
+		 */
+		regmap_read(data->regmap, MAX30102_REG_INT_STATUS, &val);
+		return IRQ_HANDLED;
+	}
+
+	measurements = bitmap_weight(indio_dev->active_scan_mask,
+				     iio_get_masklength(indio_dev));
+
 	mutex_lock(&data->lock);
 
 	while (cnt || (cnt = max30102_fifo_count(data)) > 0) {
-- 
2.55.0