UBUNTU-CVE-2026-19742: Low severity ubuntu/zephyr vulnerability

The IPv6 stack derived the long-lived secret key used to compute RFC 7217 stable interface identifiers (genstableiid() in subsys/net/ip/ipv6.c) and RFC 8981 temporary interface identifiers (gentemporaryiid() in subsys/net/ip/ipv6pe.c) with sysrandget(). That API is documented as not cryptographically secure, while both RFCs require this key to be an unpredictable random value of at least 128 bits — the unpredictability of every generated IID rests entirely on it. Depending on the platform, sysrandget() resolves either to a hardware entropy device or, on boards whose entropy driver sets CONFIGENTROPYNEEDSPRNG, to the non-cryptographic xoshiro128++ generator in subsys/random/randomxoshiro128.c, which keeps a single global 128-bit state shared with every other sysrandget() consumer. Many of those consumers put their output directly on the wire (ephemeral source ports, DNS query IDs, CoAP message IDs and tokens, DHCPv6 transaction IDs, WebSocket masking keys), and the xoshiro state transition is invertible, so an observer of a device's traffic can recover the generator state and reconstruct the IID secret key. Additionally, with the entropy-device backend sysrandget() silently falls back to kcycleget32() samples when the entropy driver fails, and returns no error, so the key could be derived from the cycle counter without the caller noticing. With the key known, stable IIDs become directly computable for any prefix (the remaining HMAC inputs — prefix, interface index, interface name, DAD counter — are guessable), and temporary IIDs become linkable after a cheap search over the mixed-in kuptimeget32() value. This defeats the properties both features exist to provide: an attacker can track a device across networks and prefixes, and can predict its address inside a /64 without scanning, removing the usual barrier to directly targeting the host. There is no memory-safety, integrity, or availability impact. The fix generates the key with syscsrandget() and aborts IID generation with -EIO when no cryptographic randomness is available, instead of silently deriving addresses from a predictable key. Both CONFIGNETIPV6IIDSTABLE and CONFIGNETIPV6PE are experimental and disabled by default; builds using CONFIGTESTRANDOMGENERATOR remain predictable by design, as syscsrandget() is deliberately routed back to the test generator there.