CVE-2026-80585mptcp: fastopen: only mark MPTFO subflows with SYN data

Official description

In the Linux kernel, the following vulnerability has been resolved: mptcp: fastopen: only mark MPTFO subflows with SYN data Passive TCP Fast Open accepts a valid-cookie SYN even when it carries no data. In that case the child socket's receive queue is intentionally left empty. mptcp_fastopen_subflow_synack_set_params() set is_mptfo before checking for queued SYN data. That made data-less TFO SYNs hit a WARN and, if the warning was non-fatal, left stale MPTFO state behind. The stale flag could later trigger a state-confusion bug in check_fully_established(). Only mark the subflow as MPTFO after confirming that an SKB was queued. Return quietly when the receive queue is empty. Note that mptcp_subflow_context's is_mptfo field is now not just about subflows where the TFO was present, but about MPTFO subflow that consumed SYN data. Only having a valid cookie but not carrying data is not really "doing TFO".

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/f75f174edc865738522e514d042cf5627f084859
• https://git.kernel.org/stable/c/fca7e444c04689fe4cc6b56f2725f804a4bb1ef9
• https://git.kernel.org/stable/c/75e564b2ced1cc3d9a8904c7d2d2bb448fffb8b5
• https://git.kernel.org/stable/c/72b4a0c51a4b550d40301d60a366b429b8c8e78d
• https://git.kernel.org/stable/c/e00b63056fb4f261455b3e5df5268a1f8ce47a87