CVE-2026-10648: NULL-pointer dereference in MCUmgr serial/console SMP transport on buffer-pool exhaustion

mcumgrserialprocessfrag() in subsys/mgmt/mcumgr/transport/src/serialutil.c calls netbufreset() on the result of smppacketalloc() before checking it for NULL. smppacketalloc() uses netbufalloc(KNOWAIT) against the shared MCUmgr packet pool (CONFIGMCUMGRTRANSPORTNETBUFCOUNT, default 4), which returns NULL when the pool is exhausted. In default builds the ASSERTNOMSG in netbufreset is a no-op, so netbufsimplereset writes through the NULL pointer (buf->len = 0; buf->data = buf->buf), causing a fault/crash.

The fragment data reaches this code from attacker-controlled bytes on the MCUmgr serial/UART/shell-console transports (smpuart.c, smprawuart.c, smpshell.c), and a fresh buffer is allocated at the start of essentially every new packet. An attacker on the serial/console link can flood the transport to drive the 4-entry buffer pool to exhaustion and induce the NULL dereference, crashing the device (denial of service).

The defect was introduced after the original MCUmgr rework and shipped in Zephyr v4.4.0. The fix moves the NULL check ahead of netbufreset.