magicmouse_raw_event() and magicmouse_event() dereference msc->input
(e.g. input->id.product, and via magicmouse_emit_touch() and
magicmouse_emit_buttons()) without checking it for NULL. hid-input only
sets msc->input when the device exposes a usable input device.

magicmouse_probe() guards against this with an "input not registered"
check that fails the probe when msc->input is NULL -- but the USB Magic
Mouse 2 / Magic Trackpad 2 path returns 0 before reaching that check.
A device that binds this driver on that path (for example a malicious
one spoofing an Apple VID/PID) with a report descriptor that does not
produce an input device therefore ends up bound with msc->input == NULL.
A subsequent input report then dereferences the NULL pointer in the
->raw_event / ->event callbacks and panics the kernel.

Move the msc->input check ahead of the early return so it covers every
bind path. Legitimate devices register an input during hid_hw_start()
and are unaffected.

Fixes: 0b91b4e4dae6 ("HID: magicmouse: Report battery level over USB")
Link: https://lore.kernel.org/linux-input/20260714102540.3EB2E1F000E9@smtp.kernel.org/ (local)
Cc: stable@vger.kernel.org
Signed-off-by: Jose Villaseñor Montfort <redacted>
---
Surfaced by an automated review of Alec Hall's parallel battery series
(the Link: above), independent of that work. This is a sibling hardening
fix to "HID: magicmouse: prevent unbounded recursion in
magicmouse_raw_event()" [1], which touches the same driver.

I went with fixing the probe path (rejecting a bind without an input)
rather than adding per-callback "if (!msc->input) return 0;" guards,
since a single check at probe covers both ->raw_event and ->event and
addresses the root asymmetry. Happy to switch to per-callback guards if
reviewers prefer that.

[1] https://lore.kernel.org/linux-input/20260715053526.574725-1-pepemontfort@gmail.com/ (local)

 drivers/hid/hid-magicmouse.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/drivers/hid/hid-magicmouse.c b/drivers/hid/hid-magicmouse.c
index 97562765a..bd6a12e40 100644
--- a/drivers/hid/hid-magicmouse.c
+++ b/drivers/hid/hid-magicmouse.c

@@ -923,17 +923,25 @@ static int magicmouse_probe(struct hid_device *hdev,
 		magicmouse_fetch_battery(hdev);
 	}
 
-	if (is_usb_magicmouse2(id->vendor, id->product) ||
-	    (is_usb_magictrackpad2(id->vendor, id->product) &&
-	     hdev->type != HID_TYPE_USBMOUSE))
-		return 0;
-
+	/*
+	 * The ->raw_event and ->event callbacks dereference msc->input, which
+	 * hid-input only populates when the device exposes a usable input.
+	 * Reject a device that bound without one -- including on the USB Magic
+	 * Mouse 2 / Trackpad 2 path that returns early below -- so a device
+	 * (e.g. one spoofing an Apple VID/PID) cannot drive those callbacks
+	 * into a NULL pointer dereference.
+	 */
 	if (!msc->input) {
 		hid_err(hdev, "magicmouse input not registered\n");
 		ret = -ENOMEM;
 		goto err_stop_hw;
 	}
 
+	if (is_usb_magicmouse2(id->vendor, id->product) ||
+	    (is_usb_magictrackpad2(id->vendor, id->product) &&
+	     hdev->type != HID_TYPE_USBMOUSE))
+		return 0;
+
 	switch (id->product) {
 	case USB_DEVICE_ID_APPLE_MAGICMOUSE:
 		report = hid_register_report(hdev, HID_INPUT_REPORT, MOUSE_REPORT_ID, 0);

-- 
2.55.0