netdev - [PATCH net] sctp: fix an issue that plpmtu can never go to complete state

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Date: Tue, 16 May 2023 21:22:02 -0400
From: Xin Long <lucien.xin@...il.com>
To: network dev <netdev@...r.kernel.org>,
	linux-sctp@...r.kernel.org
Cc: davem@...emloft.net,
	kuba@...nel.org,
	Eric Dumazet <edumazet@...gle.com>,
	Paolo Abeni <pabeni@...hat.com>,
	Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Subject: [PATCH net] sctp: fix an issue that plpmtu can never go to complete state

When doing plpmtu probe, the probe size is growing every time when it
receives the ACK during the Search state until the probe fails. When
the failure occurs, pl.probe_high is set and it goes to the Complete
state.

However, if the link pmtu is huge, like 65535 in loopback_dev, the probe
eventually keeps using SCTP_MAX_PLPMTU as the probe size and never fails.
Because of that, pl.probe_high can not be set, and the plpmtu probe can
never go to the Complete state.

Fix it by setting pl.probe_high to SCTP_MAX_PLPMTU when the probe size
grows to SCTP_MAX_PLPMTU in sctp_transport_pl_recv(). Also, increase
the probe size only when the next is less than SCTP_MAX_PLPMTU.

Fixes: b87641aff9e7 ("sctp: do state transition when a probe succeeds on HB ACK recv path")
Signed-off-by: Xin Long <lucien.xin@...il.com>
---
 net/sctp/transport.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/net/sctp/transport.c b/net/sctp/transport.c
index 2f66a2006517..b0ccfaa4c1d1 100644
--- a/net/sctp/transport.c
+++ b/net/sctp/transport.c
@@ -324,9 +324,11 @@ bool sctp_transport_pl_recv(struct sctp_transport *t)
 		t->pl.probe_size += SCTP_PL_BIG_STEP;
 	} else if (t->pl.state == SCTP_PL_SEARCH) {
 		if (!t->pl.probe_high) {
-			t->pl.probe_size = min(t->pl.probe_size + SCTP_PL_BIG_STEP,
-					       SCTP_MAX_PLPMTU);
-			return false;
+			if (t->pl.probe_size + SCTP_PL_BIG_STEP < SCTP_MAX_PLPMTU) {
+				t->pl.probe_size += SCTP_PL_BIG_STEP;
+				return false;
+			}
+			t->pl.probe_high = SCTP_MAX_PLPMTU;
 		}
 		t->pl.probe_size += SCTP_PL_MIN_STEP;
 		if (t->pl.probe_size >= t->pl.probe_high) {
@@ -341,7 +343,8 @@ bool sctp_transport_pl_recv(struct sctp_transport *t)
 	} else if (t->pl.state == SCTP_PL_COMPLETE) {
 		/* Raise probe_size again after 30 * interval in Search Complete */
 		t->pl.state = SCTP_PL_SEARCH; /* Search Complete -> Search */
-		t->pl.probe_size += SCTP_PL_MIN_STEP;
+		if (t->pl.probe_size + SCTP_PL_MIN_STEP < SCTP_MAX_PLPMTU)
+			t->pl.probe_size += SCTP_PL_MIN_STEP;
 	}
 
 	return t->pl.state == SCTP_PL_COMPLETE;
-- 
2.39.1