tcp: check mptcp-level constraints for backlog coalescing

The MPTCP protocol can acquire the subflow-level socket lock and
cause the tcp backlog usage. When inserting new skbs into the
backlog, the stack will try to coalesce them.

Currently, we have no check in place to ensure that such coalescing
will respect the MPTCP-level DSS, and that may cause data stream
corruption, as reported by Christoph.

Address the issue by adding the relevant admission check for coalescing
in tcp_add_backlog().

Note the issue is not easy to reproduce, as the MPTCP protocol tries
hard to avoid acquiring the subflow-level socket lock.

Fixes: 648ef4b ("mptcp: Implement MPTCP receive path")
Reported-by: Christoph Paasch <cpaasch@apple.com>
Closes: #420
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <martineau@kernel.org>

net/ipv4/tcp_ipv4.c

@@ -1869,6 +1869,7 @@ bool tcp_add_backlog(struct sock *sk, struct sk_buff *skb,
 #ifdef CONFIG_TLS_DEVICE
 	    tail->decrypted != skb->decrypted ||
 #endif
+	    !mptcp_skb_can_collapse(tail, skb) ||
 	    thtail->doff != th->doff ||
 	    memcmp(thtail + 1, th + 1, hdrlen - sizeof(*th)))
 		goto no_coalesce;