netdev - [PATCH net] sctp: fix handling of ICMP Frag Needed for too small MTUs

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Message-Id: <bc48ba9bc70796eb309a28b83f7fcf986ad13211.1514928667.git.marcelo.leitner@gmail.com>
Date:   Tue,  2 Jan 2018 19:44:38 -0200
From:   Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
To:     netdev@...r.kernel.org
Cc:     linux-sctp@...r.kernel.org, Xin Long <lucien.xin@...il.com>,
        Vlad Yasevich <vyasevich@...il.com>,
        Neil Horman <nhorman@...driver.com>
Subject: [PATCH net] sctp: fix handling of ICMP Frag Needed for too small MTUs

syzbot reported a hang involving SCTP, on which it kept flooding dmesg
with the message:
[  246.742374] sctp: sctp_transport_update_pmtu: Reported pmtu 508 too
low, using default minimum of 512

That happened because whenever SCTP hits an ICMP Frag Needed, it tries
to adjust to the new MTU and triggers an immediate retransmission. But
it didn't consider the fact that MTUs smaller than the SCTP minimum MTU
allowed (512) would not cause the PMTU to change, and issued the
retransmission anyway (thus leading to another ICMP Frag Needed, and so
on).

The fix is to disable Path MTU discovery for such transport and to skip
the retransmission in such cases. By doing this, SCTP will do the
backoff retransmissions as needed and will likely switch to another
transport if available.

See-also: https://lkml.org/lkml/2017/12/22/811
Reported-by: syzbot <syzkaller@...glegroups.com>
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
---
 net/sctp/input.c     | 5 ++++-
 net/sctp/transport.c | 2 ++
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/net/sctp/input.c b/net/sctp/input.c
index 621b5ca3fd1c17c3d7ef7bb1c7677ab98cebbe77..a24658c6f181e03d85f12dbe929c8bb4abaefcbd 100644
--- a/net/sctp/input.c
+++ b/net/sctp/input.c
@@ -412,8 +412,11 @@ void sctp_icmp_frag_needed(struct sock *sk, struct sctp_association *asoc,
 	 * Needed will never be sent, but if a message was sent before
 	 * PMTU discovery was disabled that was larger than the PMTU, it
 	 * would not be fragmented, so it must be re-transmitted fragmented.
+	 * If the new PMTU is invalid, we will keep getting ICMP Frag
+	 * Needed. In this case, simply avoid the retransmit.
 	 */
-	sctp_retransmit(&asoc->outqueue, t, SCTP_RTXR_PMTUD);
+	if (pmtu >= SCTP_DEFAULT_MINSEGMENT)
+		sctp_retransmit(&asoc->outqueue, t, SCTP_RTXR_PMTUD);
 }
 
 void sctp_icmp_redirect(struct sock *sk, struct sctp_transport *t,
diff --git a/net/sctp/transport.c b/net/sctp/transport.c
index 1e5a22430cf56e40a6f323081beb97836b506384..fbd9fe25764d4d98f93c60a48eccefd9cc6b4165 100644
--- a/net/sctp/transport.c
+++ b/net/sctp/transport.c
@@ -259,6 +259,8 @@ void sctp_transport_update_pmtu(struct sctp_transport *t, u32 pmtu)
 		 * pmtu discovery on this transport.
 		 */
 		t->pathmtu = SCTP_DEFAULT_MINSEGMENT;
+		t->param_flags = (t->param_flags & ~SPP_PMTUD) |
+				 SPP_PMTUD_DISABLE;
 	} else {
 		t->pathmtu = pmtu;
 	}
-- 
2.14.3