CVE-2023-53839

In the Linux kernel, the following vulnerability has been resolved: dccp: fix data-race around dp->dccps_mss_cache dccp_sendmsg() reads dp->dccps_mss_cache before locking the socket. Same thing in do_dccp_getsockopt(). Add READ_ONCE()/WRITE_ONCE() annotations, and change dccp_sendmsg() to check again dccps_mss_cache after socket is locked.