CVE-2026-89554mptcp: fix uninitialized local_id in syncookie MP_JOIN reconstruction

Official description

In the Linux kernel, the following vulnerability has been resolved: mptcp: fix uninitialized local_id in syncookie MP_JOIN reconstruction mptcp_token_join_cookie_init_state() restores remote_nonce, local_nonce, backup, join_id, token and msk from the saved cookie entry when rebuilding the request socket for a MP_JOIN 4th-ACK handled under SYN cookies, but it does not restore local_id, even though the SYN path saved it. subflow_ulp_clone() then reads that uninitialized field and stores it as the joined subflow's address-ID. Because the request-sock slab is SLAB_TYPESAFE_BY_RCU 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 MP_JOIN SYNs. This corrupts the path manager's id-based subflow bookkeeping for the connection. Restore subflow_req->local_id from the cookie entry, as done for the other fields.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/4534e0eead692bb069ad1e6a5a245fc2e2078300
• https://git.kernel.org/stable/c/63cacb05e51a1c6e1349d2e593d00fd6ef43def4
• https://git.kernel.org/stable/c/954b5ea836eb118d188975ced803448537ab8479
• https://git.kernel.org/stable/c/9df36a4846375a9b75bf42d77bfed216b0f366f9
• https://git.kernel.org/stable/c/64f2c5dd49b956a542c8c02b8dad5d262a462bba
• https://git.kernel.org/stable/c/51887ccd88791ddaa8755a7c614fda031ecf7982
• https://git.kernel.org/stable/c/bf19d166337c6488b39cb03eeeffb30a941d6326
• https://git.kernel.org/stable/c/b878dfdd12d7a5b8722a78d35e313506140ca3d9