Commit b6a57912854e ("HID: logitech-dj: Prevent REPORT_ID_DJ_SHORT
related user initiated OOB write") added validation for the DJ short
output report, but the error path dereferences rep->field[0] even when
rep->maxfield is zero.

Commit 8b9a097eb2fc ("HID: logitech-dj: fix wrong detection of bad
DJ_SHORT output report") made the check conditional on rep being present,
but a crafted descriptor can still create report ID 0x20 with only padding
output items. hid-core registers the report, ignores the padding field,
and leaves rep->maxfield as zero.

In that case the validation enters the rep->maxfield < 1 branch and then
dereferences rep->field[0]->report_count while printing the error message,
causing a NULL pointer dereference during probe. This is reproducible with
uhid by emulating a Logitech receiver with a padding-only DJ short output
report:

  BUG: KASAN: null-ptr-deref in logi_dj_probe+0xb1/0x754 [hid_logitech_dj]
  Read of size 4 at addr 0000000000000028 by task kworker/4:1/129
  ...
  Call Trace:
   logi_dj_probe+0xb1/0x754 [hid_logitech_dj]
   hid_device_probe+0x329/0x3f0 [hid]
   really_probe+0x162/0x570
   __device_attach+0x137/0x2c0
   bus_probe_device+0x38/0xc0
   device_add+0xa56/0xce0
   hid_add_device+0x19c/0x280 [hid]
   uhid_device_add_worker+0x2c/0xb0 [uhid]

Reject the zero-field report before printing the field report_count.

Fixes: b6a57912854e ("HID: logitech-dj: Prevent REPORT_ID_DJ_SHORT related user initiated OOB write")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: HyeongJun An <redacted>
---
 drivers/hid/hid-logitech-dj.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/hid/hid-logitech-dj.c b/drivers/hid/hid-logitech-dj.c
index 381e4dc5aba7..9c574ab8b60b 100644
--- a/drivers/hid/hid-logitech-dj.c
+++ b/drivers/hid/hid-logitech-dj.c

@@ -1907,8 +1907,13 @@ static int logi_dj_probe(struct hid_device *hdev,
 	output_report_enum = &hdev->report_enum[HID_OUTPUT_REPORT];
 	rep = output_report_enum->report_id_hash[REPORT_ID_DJ_SHORT];
 
-	if (rep && (rep->maxfield < 1 ||
-		    rep->field[0]->report_count != DJREPORT_SHORT_LENGTH - 1)) {
+	if (rep && rep->maxfield < 1) {
+		hid_err(hdev, "Expected size of DJ short report is %d, but got 0",
+			DJREPORT_SHORT_LENGTH - 1);
+		return -EINVAL;
+	}
+
+	if (rep && rep->field[0]->report_count != DJREPORT_SHORT_LENGTH - 1) {
 		hid_err(hdev, "Expected size of DJ short report is %d, but got %d",
 			DJREPORT_SHORT_LENGTH - 1, rep->field[0]->report_count);
 		return -EINVAL;

-- 
2.43.0