[PATCH 4.19 79/91] staging: comedi: amplc_pci230: fix null pointer deref on interrupt

[Greg Kroah-Hartman]

From: Ian Abbott <abbotti@xxxxxxxxx>

commit 7379e6baeddf580d01feca650ec1ad508b6ea8ee upstream.

The interrupt handler `pci230_interrupt()` causes a null pointer
dereference for a PCI260 card.  There is no analog output subdevice for
a PCI260.  The `dev->write_subdev` subdevice pointer and therefore the
`s_ao` subdevice pointer variable will be `NULL` for a PCI260.  The
following call near the end of the interrupt handler results in the null
pointer dereference for a PCI260:

	comedi_handle_events(dev, s_ao);

Fix it by only calling the above function if `s_ao` is valid.

Note that the other uses of `s_ao` in the calls
`pci230_handle_ao_nofifo(dev, s_ao);` and `pci230_handle_ao_fifo(dev,
s_ao);` will never be reached for a PCI260, so they are safe.

Fixes: 39064f23284c ("staging: comedi: amplc_pci230: use comedi_handle_events()")
Cc: <stable@xxxxxxxxxxxxxxx> # v3.19+
Signed-off-by: Ian Abbott <abbotti@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 drivers/staging/comedi/drivers/amplc_pci230.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/drivers/staging/comedi/drivers/amplc_pci230.c
+++ b/drivers/staging/comedi/drivers/amplc_pci230.c
@@ -2330,7 +2330,8 @@ static irqreturn_t pci230_interrupt(int
 	devpriv->intr_running = false;
 	spin_unlock_irqrestore(&devpriv->isr_spinlock, irqflags);
 
-	comedi_handle_events(dev, s_ao);
+	if (s_ao)
+		comedi_handle_events(dev, s_ao);
 	comedi_handle_events(dev, s_ai);
 
 	return IRQ_HANDLED;