UBUNTU-CVE-2026-89554: High severity ubuntu/linux-hwe-edge vulnerability

In the Linux kernel, the following vulnerability has been resolved: mptcp: fix uninitialized localid in syncookie MPJOIN reconstruction mptcptokenjoincookieinitstate() restores remotenonce, localnonce, backup, joinid, token and msk from the saved cookie entry when rebuilding the request socket for a MPJOIN 4th-ACK handled under SYN cookies, but it does not restore localid, even though the SYN path saved it. subflowulpclone() then reads that uninitialized field and stores it as the joined subflow's address-ID. Because the request-sock slab is SLABTYPESAFEBYRCU and not zeroed on allocation, the value is the stale byte of a previously freed request socket, which an off-path peer can influence by sending concurrent MPJOIN SYNs. This corrupts the path manager's id-based subflow bookkeeping for the connection. Restore subflowreq->localid from the cookie entry, as done for the other fields.