CVE-2025-38557: HID: apple: validate feature-report field count to prevent NULL pointer dereference

In the Linux kernel, the following vulnerability has been resolved:

HID: apple: validate feature-report field count to prevent NULL pointer dereference

A malicious HID device with quirk APPLEMAGICBACKLIGHT can trigger a NULL pointer dereference whilst the power feature-report is toggled and sent to the device in applemagicbacklightreportset(). The power feature-report is expected to have two data fields, but if the descriptor declares one field then accessing field[1] and dereferencing it in applemagicbacklightreportset() becomes invalid since field[1] will be NULL.

An example of a minimal descriptor which can cause the crash is something like the following where the report with ID 3 (power report) only references a single 1-byte field. When hid core parses the descriptor it will encounter the final feature tag, allocate a hidreport (all members of field[] will be zeroed out), create field structure and populate it, increasing the maxfield to 1. The subsequent field[1] access and dereference causes the crash.

Usage Page (Vendor Defined 0xFF00) Usage (0x0F) Collection (Application) Report ID (1) Usage (0x01) Logical Minimum (0) Logical Maximum (255) Report Size (8) Report Count (1) Feature (Data,Var,Abs)

Usage (0x02) Logical Maximum (32767) Report Size (16) Report Count (1) Feature (Data,Var,Abs)

Report ID (3) Usage (0x03) Logical Minimum (0) Logical Maximum (1) Report Size (8) Report Count (1) Feature (Data,Var,Abs) End Collection

Here we see the KASAN splat when the kernel dereferences the NULL pointer and crashes:

[ 15.164723] Oops: general protection fault, probably for non-canonical address 0xdffffc0000000006: 0000 [#1] SMP KASAN NOPTI [ 15.165691] KASAN: null-ptr-deref in range [0x0000000000000030-0x0000000000000037] [ 15.165691] CPU: 0 UID: 0 PID: 10 Comm: kworker/0:1 Not tainted 6.15.0 #31 PREEMPT(voluntary) [ 15.165691] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014 [ 15.165691] RIP: 0010:applemagicbacklightreportset+0xbf/0x210 [ 15.165691] Call Trace: [ 15.165691] <TASK> [ 15.165691] appleprobe+0x571/0xa20 [ 15.165691] hiddeviceprobe+0x2e2/0x6f0 [ 15.165691] reallyprobe+0x1ca/0x5c0 [ 15.165691] driverprobedevice+0x24f/0x310 [ 15.165691] driverprobedevice+0x4a/0xd0 [ 15.165691] deviceattachdriver+0x169/0x220 [ 15.165691] busforeachdrv+0x118/0x1b0 [ 15.165691] deviceattach+0x1d5/0x380 [ 15.165691] deviceinitialprobe+0x12/0x20 [ 15.165691] busprobedevice+0x13d/0x180 [ 15.165691] deviceadd+0xd87/0x1510 [...]

To fix this issue we should validate the number of fields that the backlight and power reports have and if they do not have the required number of fields then bail.