CVE-2024-53122: Concurrent Execution using Shared Resource with Improper Synchronization

In the Linux kernel, the following vulnerability has been resolved:

mptcp: cope racing subflow creation in mptcp_rcv_space_adjust

Additional active subflows - i.e. created by the in kernel path manager - are included into the subflow list before starting the 3whs.

A racing recvmsg() spooling data received on an already established subflow would unconditionally call tcp_cleanup_rbuf() on all the current subflows, potentially hitting a divide by zero error on the newly created ones.

Explicitly check that the subflow is in a suitable state before invoking tcp_cleanup_rbuf().