CVE-2025-38552

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

mptcp: plug races between subflow fail and subflow creation

We have races similar to the one addressed by the previous patch between subflow failing and additional subflow creation. They are just harder to trigger.

The solution is similar. Use a separate flag to track the condition 'socket state prevent any additional subflow creation' protected by the fallback lock.

The socket fallback makes such flag true, and also receiving or sending an MP_FAIL option.

The field 'allow_infinite_fallback' is now always touched under the relevant lock, we can drop the ONCE annotation on write.

This vulnerability is rated as Moderate severity because exploitation requires a local attacker to win precise timing races during Multipath TCP (MPTCP) connection state transitions, limiting the blast radius primarily to a denial of service without direct privilege escalation. Exploitation relies on concurrent subflow handling while the connection undergoes fallback or error signaling, which is difficult to reliably synchronize. Systems that do not utilize MPTCP or have the protocol disabled via kernel configuration are not exposed to this issue.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.