CVE-2026-63896 — usb: gadget: composite: fix integer underflow in WebUSB GET_URL handling

Affected: Linux

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

usb: gadget: composite: fix integer underflow in WebUSB GET_URL handling

The WebUSB GET_URL handler in composite_setup() narrows landing_page_length to fit the host-supplied wLength using

landing_page_length = w_length - WEBUSB_URL_DESCRIPTOR_HEADER_LENGTH + landing_page_offset;

If wLength is smaller than WEBUSB_URL_DESCRIPTOR_HEADER_LENGTH the unsigned subtraction wraps, and the subsequent

memcpy(url_descriptor->URL, cdev->landing_page + landing_page_offset, landing_page_length - landing_page_offset);

ends up copying close to UINT_MAX bytes from cdev->landing_page into cdev->req->buf. KASAN reports a slab-out-of-bounds in composite_setup on the kmalloc-2k gadget_info allocation, and FORTIFY_SOURCE traps the memcpy as a 4294967293-byte field-spanning write into url_descriptor->URL (size 252).

A USB host can reach this from a single SETUP packet against any gadget that has webusb/use=1 and a landingPage configured.

Handle the small-wLength case before the math: when the host requested fewer bytes than the URL descriptor header, only the header is meaningful and no URL bytes need to be copied. Setting landing_page_length to landing_page_offset makes the existing memcpy a no-op and leaves the descriptor returned to the host unchanged for all larger wLength values.

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CVE-2026-63896 — usb: gadget: composite: fix integer underflow in WebUSB GET_URL handling