CVE-2026-12234: TOCTOU double-fetch in `zsock_sendmsg`/`recvmsg` userspace verifiers allows kernel-heap out-of-bounds write

The userspace syscall verifiers zvrfyzsocksendmsg() and zvrfyzsockrecvmsg() in subsys/net/lib/sockets/sockets.c snapshot the caller-supplied struct netmsghdr into a kernel-side copy with kusermodefromcopy(), but then re-read the still-live user struct for subsequent decisions. The kernel iovec shadow buffer is sized from one read of msg->msgiovlen, while the population loop is bounded by a second, live read of the same field.

Because msg points into ordinary user memory, a cooperating second thread in the same memory domain can inflate msg->msgiovlen in the window between the sizing read and the loop test (a classic double-fetch / TOCTOU). The population loop then iterates past the number of netiovec slots actually allocated, writing attacker-influenced iovbase/iovlen values beyond the end of the kernel-heap shadow buffer. The recvmsg verifier has the same defect on both its inbound and result write-back loops.

The code is reachable from an unprivileged user thread whenever CONFIGUSERSPACE is enabled and the zsocksendmsg/zsockrecvmsg syscalls are available. A successful race corrupts kernel-managed heap memory across the user-to-kernel privilege boundary, yielding a local privilege-escalation primitive or, at minimum, a kernel-fault denial of service. The fix copies the header once and derives every size, bound, and gate from the snapshot, copying each iovec entry atomically so its base and length can no longer be raced apart.