Re: [PATCH] iio: accel: fxls8962af: Fix use after free in fxls8962af_fifo_flush

[David Lechner <dlechner@baylibre.com>]

On 5/24/25 5:34 AM, Sean Nyekjaer wrote:
> fxls8962af_fifo_flush() uses indio_dev->active_scan_mask (with
> iio_for_each_active_channel()) without making sure the indio_dev
> stays in buffer mode.
> There is a race if indio_dev exits buffer mode in the middle of the
> interrupt that flushes the fifo. Fix this by calling
> iio_device_claim_buffer_mode() to ensure indio_dev can't exit buffer
> mode during the flush.
> 
> Unable to handle kernel NULL pointer dereference at virtual address 00000000 when read
> [...]
> _find_first_bit_le from fxls8962af_fifo_flush+0x17c/0x290
> fxls8962af_fifo_flush from fxls8962af_interrupt+0x80/0x178
> fxls8962af_interrupt from irq_thread_fn+0x1c/0x7c
> irq_thread_fn from irq_thread+0x110/0x1f4
> irq_thread from kthread+0xe0/0xfc
> kthread from ret_from_fork+0x14/0x2c
> 
> Fixes: 79e3a5bdd9ef ("iio: accel: fxls8962af: add hw buffered sampling")
> Cc: stable@vger.kernel.org
> Signed-off-by: Sean Nyekjaer <sean@geanix.com>
> ---
>  drivers/iio/accel/fxls8962af-core.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/iio/accel/fxls8962af-core.c b/drivers/iio/accel/fxls8962af-core.c
> index 6d23da3e7aa22c61f2d9348bb91d70cc5719a732..7db83ebeea823173d79bf8ff484add16f575edfc 100644
> --- a/drivers/iio/accel/fxls8962af-core.c
> +++ b/drivers/iio/accel/fxls8962af-core.c
> @@ -973,6 +973,9 @@ static int fxls8962af_fifo_flush(struct iio_dev *indio_dev)
>  	if (ret)
>  		return ret;
>  
> +	if (iio_device_claim_buffer_mode(indio_dev) < 0)
> +		return 0;
> +

I see one other driver with a check like this, so I suppose there is some precedent,
but I wonder if this could be fixed instead with:

---
diff --git a/drivers/iio/accel/fxls8962af-core.c b/drivers/iio/accel/fxls8962af-core.c
index 9aa02f599ae9..2034f4407dd7 100644
--- a/drivers/iio/accel/fxls8962af-core.c
+++ b/drivers/iio/accel/fxls8962af-core.c
@@ -866,6 +866,8 @@ static int fxls8962af_buffer_predisable(struct iio_dev *indio_dev)
 	if (ret)
 		return ret;
 
+	synchronize_irq(data->irq);
+
 	ret = __fxls8962af_fifo_set_mode(data, false);
 
 	if (data->enable_event)
---

This should ensure that if the interrupt handler is running while trying to
disable the buffer, that the interrupt handler completes before returning
from fxls8962af_buffer_predisable(). And no more interrupts should occur
because the interrupt was disabled before waiting for completion.

>  	/* Demux hw FIFO into kfifo. */
>  	for (i = 0; i < count; i++) {
>  		int j, bit;
> @@ -989,6 +992,8 @@ static int fxls8962af_fifo_flush(struct iio_dev *indio_dev)
>  		tstamp += sample_period;
>  	}
>  
> +	iio_device_release_buffer_mode(indio_dev);
> +
>  	return count;
>  }
>  
> 
> ---
> base-commit: 5c3fcb36c92443a9a037683626a2e43d8825f783
> change-id: 20250524-fxlsrace-f4d20e29fb29
> 
> Best regards,

In addition to the race you have identified, I wonder if there is also a race
in fxls8962af_suspend(). This one wouldn't cause a crash, but would cause an
immediate wake.

fxls8962af_suspend() calls enable_irq_wake(data->irq); before disabling the
interrupt by calling fxls8962af_buffer_predisable(indio_dev);

It seems like the order should be reversed.