HID: logitech-dj: Fix maxfield check in DJ short report validation

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 <sammiee5311@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>

diff --git a/drivers/hid/hid-logitech-dj.c b/drivers/hid/hid-logitech-dj.c
index 381e4dc5aba7aa6bc5182efe9c67fe69688b8421..9c574ab8b60b3789b17cbe7e4f377f49f9f93181 100644 (file)
--- 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;