mptcp: clear 'kern' flag from fallback sockets

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: mptcp: clear 'kern' flag from fallback sockets The mptcp ULP extension relies on sk->sk_sock_kern being set correctly: It prevents setsockopt(fd, IPPROTO_TCP, TCP_ULP, "mptcp", 6); from working for plain tcp sockets (any userspace-exposed socket). But in case of fallback, accept() can return a plain tcp sk. In such case, sk is still tagged as 'kernel' and setsockopt will work. This will crash the kernel, The subflow extension has a NULL ctx->conn mptcp socket: BUG: KASAN: null-ptr-deref in subflow_data_ready+0x181/0x2b0 Call Trace: tcp_data_ready+0xf8/0x370 [..]

References

• https://git.kernel.org/stable/c/451f1eded7f56e93aaf52eb547ba97742d9c0e97
• https://git.kernel.org/stable/c/c26ac0ea3a91c210cf90452e625dc441adf3e549
• https://git.kernel.org/stable/c/d6692b3b97bdc165d150f4c1505751a323a80717