CVE-2026-10653

The Zephyr netbuf library (lib/netbuf/buf.c) manipulated both of its reference counts -- the per-header buf->ref and the per-data-block refcount at the start of each variable/heap data allocation -- with plain non-atomic C operators (buf->ref++, if (--buf->ref > 0), if (--(*refcount))).

The API is documented as self-synchronizing: callers may share one buffer across threads (e.g. via kfifo) and each holder independently calls netbuf_unref() with no surrounding lock. Under true concurrency (SMP, or single-core preemption between the non-atomic load and store while another context unrefs the same buffer), two holders can both observe the same prior reference value and both conclude they are the last reference.

For heap/variable-data pools (mempooldataunref/heapdataunref, used by zbus message subscribers, the IP stack RX/TX buffers when CONFIGNETBUFFIXEDDATASIZE=n, capture, wireguard, ISO-TP and usbip) this produces a double kheapfree()/k_free() of the same block -- heap-metadata corruption and a use-after-free on the heap-hardening poison pattern.

For the per-header refcount the buffer is returned to the pool free LIFO twice for any pool type (including fixed-data pools used by Bluetooth and networking), corrupting the free list so a later allocation hands the same buffer to two owners.

The fix converts both refcounts to atomicinc/atomicdec (overlaying buf->ref in an atomict-sized union and changing the data-block refcount from uint8t to atomic_t).

Impact is gated on genuine concurrency and on an application architecture that shares one buffer among multiple independent unref'ers; the trigger is a refcount/timing race rather than packet content, so an external attacker has at most weak indirect influence over the race window. Affects all Zephyr releases through v4.4.0.