CVE-2026-10639: Use-after-free reading `net_pkt_iface()` of a sent ICMPv4 echo-reply packet in `icmpv4_handle_echo_request()`

In Zephyr's native IPv4 stack, icmpv4handleechorequest() in subsys/net/ip/icmpv4.c builds an echo-reply packet (reply), hands it to nettrysenddata(), and then, on success, calls netstatsupdateicmpsent(netpktiface(reply)). nettrysenddata() transfers ownership of reply to the TX path (netiftryqueuetx -> netiftx -> L2/driver send, or the asynchronous netiftxthread), which can unref it to refcount 0 and return the struct netpkt to its slab (netpktunref -> kmemslabfree) before the stats line runs. netcore.c documents this exact contract ('the pkt might contain garbage already ... do not use pkt after that call').

The post-send netpktiface(reply) therefore reads reply->iface out of a freed (and possibly already reallocated) netpkt, a use-after-free read; with CONFIGNETSTATISTICSPERINTERFACE the stats macro additionally increments a counter through that value, i.e. a dereference/write through a stale or recycled-slot pointer.

The path is reached unauthenticated by any remote host that pings the device (neticmpv4input -> neticmpcallipv4handlers -> icmpv4handleechorequest) and is gated on CONFIGNETSTATISTICSICMP. Impact is a probabilistic read of recycled packet memory plus a possible wild-pointer write under a timing race, leading most likely to corrupted interface statistics or a remotely triggerable crash (DoS).

The defect was introduced in 2019 (v1.14) and is present through v4.4.0. The companion change in neticmpv4senderror() is not a use-after-free because it reads netpktiface(orig), the caller-owned received packet, which stays alive across the send. The fix caches the interface pointer from the live received packet before sending and uses it for the post-send stats updates.