[PATCH] HID: logitech-hidpp: fix NULL pointer dereference in hidpp_get_report_length()

Add validation for report->maxfield and report->field[0] before
dereferencing to prevent NULL pointer dereference.

The HID report descriptor is provided by the USB device firmware via
USB control transfer (GET_DESCRIPTOR). A malicious device can craft
a descriptor that defines an OUTPUT report without any usages
(padding fields). When the HID subsystem parses such a descriptor:

1. hid_add_field() calls hid_register_report() to create the report
   object and stores it in report_id_hash[id]
2. Since parser->local.usage_index is 0, hid_add_field() returns early
   without calling hid_register_field() to add any fields
3. Result: report exists with maxfield=0 and field[0]=NULL

When hidpp_probe() is called for a device matching this driver:
  - hidpp_validate_device() calls hidpp_get_report_length()
  - hidpp_get_report_length() retrieves the report from hash (not NULL)
  - It then dereferences report->field[0]->report_count
  - Since field[0] is NULL, this triggers a kernel NULL pointer
    dereference

Data flow from attacker to crash:
  Malicious USB Device
       |
       v (USB GET_DESCRIPTOR control transfer)
  hid_get_class_descriptor() -- reads HID report descriptor from device
       |
       v
  hid_parse_report() -- stores descriptor in hid->dev_rdesc
       |
       v
  hid_open_report() -> hid_add_field()
       |                    |
       |                    v
       |              hid_register_report() -- creates report, maxfield=0
       |                    |
       |                    v
       |              returns early if usage_index==0, no field added
       |
       v
  hidpp_validate_device() -> hidpp_get_report_length()
       |
       v
  report->field[0]->report_count -- NULL pointer dereference!

This is triggerable by an attacker with physical access using a
malicious USB device (e.g., BadUSB, programmable USB development
boards).

Fixes: d71b18f7c7999 ("HID: logitech-hidpp: do not hardcode very long report length")
Signed-off-by: Kery Qi <redacted>
---
 drivers/hid/hid-logitech-hidpp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/hid/hid-logitech-hidpp.c b/drivers/hid/hid-logitech-hidpp.c
index d5011a5d0890..02ddbd658e89 100644
--- a/drivers/hid/hid-logitech-hidpp.c
+++ b/drivers/hid/hid-logitech-hidpp.c

@@ -4314,7 +4314,7 @@ static int hidpp_get_report_length(struct hid_device *hdev, int id)
 
 	re = &(hdev->report_enum[HID_OUTPUT_REPORT]);
 	report = re->report_id_hash[id];
-	if (!report)
+	if (!report || report->maxfield < 1 || !report->field[0])
 		return 0;
 
 	return report->field[0]->report_count + 1;

-- 
2.34.1