[PATCH v2] HID: magicmouse: avoid NULL pointer deref when there is no input device

["Jose Villaseñor Montfort" <pepemontfort@gmail.com>]

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() has 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 it. Real hardware ends up in
that state: a USB Magic Trackpad 2 exposes four HID interfaces and only
the first two register an input, and a USB-C Magic Trackpad (05ac:0324)
exposes three with the same split. The remaining interfaces are
vendor-defined, are claimed as hiddev/hidraw only, and stay bound to
this driver with msc->input == NULL.

Both callbacks are reachable in that state. ->raw_event is called for
every incoming report, and hid_process_event() calls ->event without
requiring HID_CLAIMED_INPUT -- hid_match_usage() matches everything here
because this driver has no usage_table. An input report on one of those
interfaces therefore dereferences a NULL pointer and panics the kernel.
A device that binds this driver by spoofing an Apple VID/PID, with a
report descriptor that does not produce an input device, reaches the
same state.

Bail out of both callbacks when msc->input is NULL and leave the report
to the generic HID paths, which is what those interfaces get today.
Rejecting the bind in magicmouse_probe() instead would unbind interfaces
that a healthy device legitimately exposes and drop their hidraw nodes.

Fixes: 0b91b4e4dae6 ("HID: magicmouse: Report battery level over USB")
Suggested-by: Alec Hall <signshop.alec@gmail.com>
Link: https://lore.kernel.org/linux-input/20260714102540.3EB2E1F000E9@smtp.kernel.org/
Link: https://lore.kernel.org/linux-input/20260728072554.47069-1-signshop.alec@gmail.com/
Cc: stable@vger.kernel.org
Signed-off-by: Jose Villaseñor Montfort <pepemontfort@gmail.com>
---
Changes in v2:
- Switch from rejecting the bind in magicmouse_probe() to guarding both
  callbacks. Alec Hall tested v1 on a USB Magic Trackpad 2 and found that
  it unbinds interfaces 2 and 3: they are vendor-defined, have no input
  device, and have been claimed by this driver since 0b91b4e4dae6, so
  every cable plug logged two -ENOMEM probe failures and their hidraw
  nodes went away. Nothing user-facing broke (touch and battery live on
  interfaces 0/1), but silently unbinding a healthy device's interfaces
  is not a change to make in a stable-tagged fix. Report:
  https://lore.kernel.org/linux-input/20260728072554.47069-1-signshop.alec@gmail.com/
- I confirmed the same shape on a USB-C Magic Trackpad (05ac:0324) here,
  on an unpatched 7.1.5: three HID interfaces, all three bound to
  magicmouse, and the last one -- vendor-defined, bInterfaceSubClass 0,
  two endpoints -- has a hidraw node but no input device. So the exact
  interface count varies by model, but at least one input-less interface
  bound to this driver is normal on genuine hardware.
- Reworded the commit message accordingly. Those interfaces show that a
  bind with msc->input == NULL is a state real hardware reaches, not only
  a spoofed device; and hid_process_event() calls ->event regardless of
  HID_CLAIMED_INPUT (hid_match_usage() matches everything, this driver
  has no usage_table), so ->event is reachable on them too.
- v1: https://lore.kernel.org/linux-input/20260715195853.1302765-1-pepemontfort@gmail.com/

This is a sibling hardening fix to "HID: magicmouse: prevent unbounded
recursion in magicmouse_raw_event()" [1], which touches the same driver.
If that one is applied first, this needs a trivial rebase: the guard
moves into __magicmouse_raw_event().

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

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

diff --git a/drivers/hid/hid-magicmouse.c b/drivers/hid/hid-magicmouse.c
index 802a3479e..7164ced59 100644
--- a/drivers/hid/hid-magicmouse.c
+++ b/drivers/hid/hid-magicmouse.c
@@ -390,6 +390,14 @@ static int magicmouse_raw_event(struct hid_device *hdev,
 	struct input_dev *input = msc->input;
 	int x = 0, y = 0, ii, clicks = 0, npoints;
 
+	/*
+	 * This driver also binds interfaces that hid-input does not create an
+	 * input device for, so msc->input can legitimately be NULL here. Leave
+	 * their reports to the generic HID paths instead of dereferencing it.
+	 */
+	if (!input)
+		return 0;
+
 	/* Protect against zero sized recursive calls from DOUBLE_REPORT_ID */
 	if (size < 1)
 		return 0;
@@ -538,6 +546,11 @@ static int magicmouse_event(struct hid_device *hdev, struct hid_field *field,
 		struct hid_usage *usage, __s32 value)
 {
 	struct magicmouse_sc *msc = hid_get_drvdata(hdev);
+
+	/* See the comment in magicmouse_raw_event(). */
+	if (!msc->input)
+		return 0;
+
 	if ((msc->input->id.product == USB_DEVICE_ID_APPLE_MAGICMOUSE2 ||
 	     msc->input->id.product == USB_DEVICE_ID_APPLE_MAGICMOUSE2_USBC) &&
 	    field->report->id == MOUSE2_REPORT_ID) {

base-commit: b7556c8e713c88596046a906c7c4385218d44736
-- 
2.55.0