mptcp: only fallback due to inf mapping if allowed

Any fallback should happen only if allowed, so only if this variable is
set: msk->allow_infinite_fallback. This boolean will be set to false
once MPTCP-specific operations acting on the whole MPTCP connection vs
the initial path have been done, e.g. a second path has been created, or
an MPTCP re-injection -- yes, possible even with a single subflow.

In other words, the !msk->allow_infinite_fallback condition should be
placed first. If it is no longer possible to do a fallback, there should
not be any bypass, e.g. when receiving an infinite mapping.

Now, regarding the conditions to do a fallback, the RFC mentions [1]
that if the checksum is used, a fallback is only possible when "it is
known that all unacknowledged data in flight is contiguous (which will
usually be the case with a single subflow)". In other words, if the
checksum is used, a fallback is possible when the other peer sent an
infinite mapping indicating the flow has been altered.

Note that the issue is present since a merge commit, where both
subflow_can_fallback() and the previous extra condition with
'subflow->map_data_len' got introduced.

Fixes: d7e6f58 ("Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net")
Link: https://datatracker.ietf.org/doc/html/rfc8684#section-3.7-11 [1]
Signed-off-by: Matthieu Baerts (NGI0) <[email protected]>

Author: matttbe

net/mptcp/subflow.c

@@ -1298,16 +1298,20 @@ static void subflow_sched_work_if_closed(struct mptcp_sock *msk, struct sock *ss
 		mptcp_schedule_work(sk);
 }
 
-static bool subflow_can_fallback(struct mptcp_subflow_context *subflow)
+static bool subflow_can_fallback(struct mptcp_subflow_context *subflow,
+				 enum mapping_status status)
 {
 	struct mptcp_sock *msk = mptcp_sk(subflow->conn);
 
-	if (subflow->mp_join)
+	/* If not allowed (additional paths, MPTCP reinjections): no fallback */
+	if (!READ_ONCE(msk->allow_infinite_fallback))
 		return false;
-	else if (READ_ONCE(msk->csum_enabled))
+
+	/* More strict with csum: fallback in 2 cases: inf map or never valid */
+	if (status != MAPPING_INFINITE && READ_ONCE(msk->csum_enabled))
 		return !subflow->valid_csum_seen;
-	else
-		return READ_ONCE(msk->allow_infinite_fallback);
+
+	return true;
 }
 
 static void mptcp_subflow_fail(struct mptcp_sock *msk, struct sock *ssk)
@@ -1406,7 +1410,7 @@ static bool subflow_check_data_avail(struct sock *ssk)
 			return true;
 		}
 
-		if (!subflow_can_fallback(subflow) && status != MAPPING_INFINITE) {
+		if (!subflow_can_fallback(subflow, status)) {
 			/* fatal protocol error, close the socket.
 			 * subflow_error_report() will introduce the appropriate barriers
 			 */