[PATCH v4] iio: health: max30102: fix NULL dereference in interrupt handler

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
  Call trace:
   __bitmap_weight+0x64/0x98 (P)
   max30102_interrupt_handler+0x48/0x160 [max30102]

Call max30102_fifo_count() at the top of the handler and return early
unless it reports a FIFO sample is ready. Because FIFO_RDY is the only
interrupt source enabled in max30102_chip_init(), an invocation of
max30102_interrupt_handler() without the FIFO_RDY interrupt status bit
set carries no data to read and can return before touching
active_scan_mask. A negative return from max30102_fifo_count()
indicates a failed interrupt status read and is treated the same way.

Fixes: 90579b69e94b ("iio: health: max30102: Add MAX30105 support")
Suggested-by: Jonathan Cameron <[email protected]>
Signed-off-by: Marco Chen <[email protected]>
---
Changes in v4:
- Trim the backtrace to only relevant frames and unwrap the first line
  as suggested by Andy.

max30102_fifo_count() still has the odd name and int return you mentioned
in v2, so I will send a follow-up patch once this lands.

Tested on a MAX30102 on Raspberry Pi 4 over I2C.

v1: https://lore.kernel.org/linux-iio/[email protected]/
v2: https://lore.kernel.org/linux-iio/[email protected]/
v3: https://lore.kernel.org/linux-iio/[email protected]/

 drivers/iio/health/max30102.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/drivers/iio/health/max30102.c b/drivers/iio/health/max30102.c
index c37316c86f14..aee96167f01e 100644
--- a/drivers/iio/health/max30102.c
+++ b/drivers/iio/health/max30102.c
@@ -290,9 +290,15 @@ 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));
-	int ret, cnt = 0;
+	unsigned int measurements;
+	int ret, cnt;
+
+	cnt = max30102_fifo_count(data);
+	if (cnt <= 0)
+		return IRQ_HANDLED;
+
+	measurements = bitmap_weight(indio_dev->active_scan_mask,
+				     iio_get_masklength(indio_dev));
 
 	mutex_lock(&data->lock);
 
-- 
2.55.0