[PATCH v2] Input: apple_z2 - bound the device-reported finger count

[Dmitry Torokhov]

Hi Bryam,

On Sat, Jun 13, 2026 at 08:22:51PM -0500, Bryam Vargas via B4 Relay wrote:
> From: Bryam Vargas <hexlabsecurity at proton.me>
> 
> apple_z2_parse_touches() takes the finger count from the touch
> controller's report and loops over that many fixed-size finger records
> without ever checking the count against the length of the report:
> 
> 	nfingers = msg[APPLE_Z2_NUM_FINGERS_OFFSET];
> 	fingers = (struct apple_z2_finger *)(msg + APPLE_Z2_FINGERS_OFFSET);
> 	for (i = 0; i < nfingers; i++)
> 		/* read fingers[i] ... */
> 
> msg points into the fixed 4000-byte z2->rx_buf and nfingers is a single
> device-supplied byte, so it can be as large as 255.  A malicious,
> malfunctioning or counterfeit controller (or an interposer on the SPI
> bus) can report a large finger count in a short packet, making the loop
> read up to 255 * sizeof(struct apple_z2_finger) bytes starting 24 bytes
> into msg -- far past the 4000-byte buffer.  This is a controller-driven
> heap out-of-bounds read, and the finger fields that are read (position,
> pressure, touch and tool dimensions) are forwarded to userspace as input
> events, leaking adjacent kernel memory.
> 
> Bound the device-reported count to the number of finger records the
> report actually carries.

As Sashiko mentioned, if we do not trust hardware to send valid data we
should also validate that packet size supplied by the device is
reasonable.

Also I wonder why would we want to report some of fingers in case when
device sends bogus number of contacts? I'd drop such packet (maybe
logging ratelimited or "once" message).

You can ignore Sahiko's comment about __free(kfree) not handling error
pointers.

Thanks.

-- 
Dmitry