[PATCH] staging: comedi: dt282x: fix a null pointer deref on interrupt

Ian Abbott abbotti at mev.co.uk
Wed Jun 26 13:18:04 UTC 2019

The interrupt handler `dt282x_interrupt()` causes a null pointer
dereference for those supported boards that have no analog output
support.  For these boards, `dev->write_subdev` will be `NULL` and
therefore the `s_ao` subdevice pointer variable will be `NULL`.  In that
case, the following call near the end of the interrupt handler results
in a null pointer dereference:

	comedi_handle_events(dev, s_ao);

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

(There are other uses of `s_ao` by the interrupt handler that may or may
not be reached depending on values of hardware registers.  Trust that
they are reliable for now.)

commit 4f6f009b204f ("staging: comedi: dt282x: use comedi_handle_events()")
propagates an earlier error from
commit f21c74fa4cfe ("staging: comedi: dt282x: use cfc_handle_events()").

Fixes: 4f6f009b204f ("staging: comedi: dt282x: use comedi_handle_events()")
Cc: <stable at vger.kernel.org> # v3.19+
Signed-off-by: Ian Abbott <abbotti at mev.co.uk>
Note: A similar patch will be required for stable v3.16.x, but that
calls cfc_handle_events() instead of comedi_handle_events().
 drivers/staging/comedi/drivers/dt282x.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/staging/comedi/drivers/dt282x.c b/drivers/staging/comedi/drivers/dt282x.c
index 3be927f1d3a9..e15e33ed94ae 100644
--- a/drivers/staging/comedi/drivers/dt282x.c
+++ b/drivers/staging/comedi/drivers/dt282x.c
@@ -557,7 +557,8 @@ static irqreturn_t dt282x_interrupt(int irq, void *d)
 	comedi_handle_events(dev, s);
-	comedi_handle_events(dev, s_ao);
+	if (s_ao)
+		comedi_handle_events(dev, s_ao);
 	return IRQ_RETVAL(handled);

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