In the Linux kernel, the following vulnerability has been resolved:
mptcp: ensure snd_una is properly initialized on connect
This is strictly related to commit fb7a0d334894 ("mptcp: ensure sndnxt is properly initialized on connect"). It turns out that syzkaller can trigger the retransmit after fallback and before processing any other incoming packet - so that snduna is still left uninitialized.
Address the issue explicitly initializing snduna together with sndnxt and write_seq.