[PATCH] HID: wacom: fix NULL pointer dereference in wacom_intuos_pad()

[Jinmo Yang <jinmo44.yang@gmail.com>]

wacom_intuos_pad() dereferences wacom->pad_input without a NULL check.
When a Wacom tablet is created via UHID with parameters that route
through wacom_bpt_irq() -> wacom_intuos_irq() -> wacom_intuos_pad(),
but probe did not allocate pad_input, the call to
wacom_report_numbered_buttons() passes a NULL input_dev, causing a
general protection fault in input_get_drvdata().

Add a NULL check for pad_input at the top of wacom_intuos_pad() to
bail out early when the pad input device was not set up.

The bug was found by syzkaller and confirmed on a Pixel 9 Pro
(Android 16, kernel 6.1.124) where it causes an immediate kernel
panic and reboot via /dev/uhid without requiring root privileges:

  KP: Oops: Fatal exception: comm:wacom_27qhdt
  Reboot reason: 0xbaba - Kernel PANIC

Reproducer (unprivileged):
  open("/dev/uhid", O_RDWR)
  write(fd, UHID_CREATE2{vendor=0x056a, product=0x0020})
  write(fd, UHID_INPUT2{report_id=0x0c, size=10})

Fixes: c7f0522a1ad1 ("HID: wacom: Slim down wacom_intuos_pad processing")
Signed-off-by: Jinmo Yang <jinmo44.yang@gmail.com>
---
 drivers/hid/wacom_wac.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/hid/wacom_wac.c b/drivers/hid/wacom_wac.c
index da1f0ea85..251ddda3e 100644
--- a/drivers/hid/wacom_wac.c
+++ b/drivers/hid/wacom_wac.c
@@ -515,7 +515,6 @@ static int wacom_intuos_pad(struct wacom_wac *wacom)
 	struct wacom_features *features = &wacom->features;
 	unsigned char *data = wacom->data;
 	struct input_dev *input = wacom->pad_input;
-	int i;
 	int buttons = 0, nbuttons = features->numbered_buttons;
 	int keys = 0, nkeys = 0;
 	int ring1 = 0, ring2 = 0;
@@ -523,6 +522,10 @@ static int wacom_intuos_pad(struct wacom_wac *wacom)
 	bool prox = false;
 	bool wrench = false, keyboard = false, mute_touch = false, menu = false,
 	     info = false;
+	int i;
+
+	if (!input)
+		return 0;
 
 	/* pad packets. Works as a second tool and is always in prox */
 	if (!(data[0] == WACOM_REPORT_INTUOSPAD || data[0] == WACOM_REPORT_INTUOS5PAD ||
-- 
2.53.0