[PATCH] HID: saitek: don't dereference a NULL hidinput in saitek_event()

hid_process_event() calls the driver's ->event() for every usage of a
report, including fields that hidinput did not map to any input device,
e.g. a constant input item. For those fields field->hidinput is NULL,
and saitek_event() loads field->hidinput->input unconditionally:

  KASAN: null-ptr-deref in range [0x0000000000000018-0x000000000000001f]
  RIP: 0010:saitek_event+0x51/0x2b0 drivers/hid/hid-saitek.c:154
   hid_process_event drivers/hid/hid-core.c:1572
   hid_report_raw_event drivers/hid/hid-core.c:2109
   hid_input_report drivers/hid/hid-core.c:2180
   hid_irq_in drivers/hid/usbhid/hid-core.c

A device that claims to be a Saitek MMO7 and uses such a descriptor
triggers it with a single input report.

Bail out early when the field has no input device, as the other HID
drivers implementing ->event() already do.

Fixes: 37c492c8f6a9 ("HID: quirk for Saitek RAT7 and MMO7 mices' mode button")
Reported-by: syzbot+eba2b3201bd25a97cddf@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=eba2b3201bd25a97cddf
Cc: stable@vger.kernel.org
Signed-off-by: Nguyen Ngoc Thang <redacted>
---
 drivers/hid/hid-saitek.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/hid/hid-saitek.c b/drivers/hid/hid-saitek.c
index 6fe7c087c594..2a9e07a4fc77 100644
--- a/drivers/hid/hid-saitek.c
+++ b/drivers/hid/hid-saitek.c

@@ -151,7 +151,12 @@ static int saitek_event(struct hid_device *hdev, struct hid_field *field,
 		struct hid_usage *usage, __s32 value)
 {
 	struct saitek_sc *ssc = hid_get_drvdata(hdev);
-	struct input_dev *input = field->hidinput->input;
+	struct input_dev *input;
+
+	if (!field->hidinput)
+		return 0;
+
+	input = field->hidinput->input;
 
 	if (usage->type == EV_KEY && value &&
 			(((ssc->quirks & SAITEK_RELEASE_MODE_RAT7) &&

-- 
2.43.0