CVE-2026-10655: Use-after-free race in SNTP async client when closing the socket while the socket service is still polling it

The asynchronous SNTP client in Zephyr (subsys/net/lib/sntp/sntp.c, sntpcloseasync) closed the UDP socket file descriptor directly from the calling thread immediately after detaching it from the network socket service, without synchronizing with the socket-service poll thread.

The socket service thread polls each socket via zvfspoll, which (in zsockpollpreparectx) registers a kpollevent pointing into the socket's netcontext (&ctx->recvq) and then blocks in kpoll without holding a reference or lock. netcontext objects are allocated from a fixed pool (contexts[CONFIGNETMAXCONTEXTS]) and reused after close.

When sntpcloseasync is invoked from a different thread than the poll thread (in the in-tree consumer subsys/net/lib/config/initclocksntp.c, the SNTP timeout handler runs on the system workqueue while the socket service thread is blocked in poll on the same fd), the close frees and may reuse the netcontext while the poll thread still has a poller node linked into the freed object, resulting in a use-after-free / object confusion of kernel poll structures.

The SNTP timeout path is the normal no-response failure mode, so a network peer or off-path attacker who drops or delays the SNTP/NTP response can drive the racing close repeatedly (and periodically with NETCONFIGSNTPINITRESYNC). The most likely consequence is a crash of the networking thread (denial of service), with potential memory corruption when the freed context slot is reallocated.

The fix defers the close to the socket service thread itself via netsocketserviceclose (NETSOCKETSERVICECLOSESOCKETS), so the same thread that polls performs the close, eliminating the race. Affected releases: v4.2.0 through v4.4.0.