[PATCH] gpiolib: cdev: fix null pointer dereference in linereq_free()

[Kent Gibson <warthog618@gmail.com>]

This patch fixes a kernel NULL pointer dereference that is reported by
gpio kselftests.

linereq_free() can be called as part of the cleanup of a failed request,
at which time the desc for a line may not have been determined, so it
is unsafe to dereference without a check.

Add a check prior to dereferencing the line desc.

Fixes: 2068339a6c35 ("gpiolib: cdev: Add hardware timestamp clock type")
Signed-off-by: Kent Gibson <warthog618@gmail.com>
---

I suspect the edge_detector_stop() and gpiod_free() could also be moved
inside the same desc check but, as we are late in the rc cycle, I don't
want to push my luck and have kept to the minimum change required to
address the bug.

 drivers/gpio/gpiolib-cdev.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/gpio/gpiolib-cdev.c b/drivers/gpio/gpiolib-cdev.c
index f5aa5f93342a..0c9a63becfef 100644
--- a/drivers/gpio/gpiolib-cdev.c
+++ b/drivers/gpio/gpiolib-cdev.c
@@ -1460,11 +1460,12 @@ static ssize_t linereq_read(struct file *file,
 static void linereq_free(struct linereq *lr)
 {
 	unsigned int i;
-	bool hte;
+	bool hte = false;
 
 	for (i = 0; i < lr->num_lines; i++) {
-		hte = !!test_bit(FLAG_EVENT_CLOCK_HTE,
-				 &lr->lines[i].desc->flags);
+		if (lr->lines[i].desc)
+			hte = !!test_bit(FLAG_EVENT_CLOCK_HTE,
+					 &lr->lines[i].desc->flags);
 		edge_detector_stop(&lr->lines[i], hte);
 		if (lr->lines[i].desc)
 			gpiod_free(lr->lines[i].desc);
-- 
2.37.0