Stack out-of-bounds write in Linux kernel BPF devmap

CVE-2026-23359 is a Linux kernel memory-safety flaw in the BPF devmap/XDP redirect path. The vulnerable logic is in get_upper_ifindexes(), which iterates over upper network devices and stores their interface indices into a caller-provided array without enforcing array bounds. Callers assumed the number of upper devices would not exceed MAX_NEST_DEV and allocated excluded_devices[1+MAX_NEST_DEV] on the stack, but that assumption is invalid because a device can have more than MAX_NEST_DEV upper devices, for example many macvlans. When more entries are written than the stack buffer can hold, a stack out-of-bounds write occurs. The fix adds a maximum-size parameter to get_upper_ifindexes(); if too many upper devices are present, the kernel now returns -EOVERFLOW and aborts the redirect.