Patch "sctp: fix the handling of ICMP Frag Needed for too small MTUs" has been added to the 4.14-stable tree

This is a note to let you know that I've just added the patch titled

sctp: fix the handling of ICMP Frag Needed for too small MTUs

to the 4.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
sctp-fix-the-handling-of-icmp-frag-needed-for-too-small-mtus.patch
and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Marcelo Ricardo Leitner <[email protected]>
Date: Fri, 5 Jan 2018 11:17:18 -0200
Subject: sctp: fix the handling of ICMP Frag Needed for too small MTUs

From: Marcelo Ricardo Leitner <[email protected]>

[ Upstream commit b6c5734db07079c9410147b32407f2366d584e6c ]

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).

As IPv4 (ip_rt_min_pmtu=556) and IPv6 (IPV6_MIN_MTU=1280) minimum MTU are higher than that, sctp_transport_update_pmtu() is changed to re-fetch the PMTU that got set after our request, and with that, detect if there was an actual change or not.

The fix, thus, skips the immediate retransmission if the received ICMP resulted in no change, in the hope that SCTP will select another path.

Note: The value being used for the minimum MTU (512, SCTP_DEFAULT_MINSEGMENT) is not right and instead it should be (576, SCTP_MIN_PMTU), but such change belongs to another patch.

Changes from v1:
- do not disable PMTU discovery, in the light of commit 06ad391919b2 ("[SCTP] Don't disable PMTU discovery when mtu is small") and as suggested by Xin Long.
- changed the way to break the rtx loop by detecting if the icmp resulted in a change or not
Changes from v2:
none

See-also: https://lkml.org/lkml/2017/12/22/811

---
 include/net/sctp/structs.h |    2 +-
 net/sctp/input.c           |    8 ++++++--
 net/sctp/transport.c       |   29 +++++++++++++++++++----------
 3 files changed, 26 insertions(+), 13 deletions(-)

@@ -955,7 +955,7 @@ void sctp_transport_burst_limited(struct
 void sctp_transport_burst_reset(struct sctp_transport *);
 unsigned long sctp_transport_timeout(struct sctp_transport *);
 void sctp_transport_reset(struct sctp_transport *t);
-void sctp_transport_update_pmtu(struct sctp_transport *t, u32 pmtu);
+bool sctp_transport_update_pmtu(struct sctp_transport *t, u32 pmtu);
 void sctp_transport_immediate_rtx(struct sctp_transport *);
 void sctp_transport_dst_release(struct sctp_transport *t);
 void sctp_transport_dst_confirm(struct sctp_transport *t);

@@ -406,8 +406,12 @@ void sctp_icmp_frag_needed(struct sock *
 		 */
 		return;
 
-	/* Update transports view of the MTU */
-	sctp_transport_update_pmtu(t, pmtu);
+	/* Update transports view of the MTU. Return if no update was needed.
+	 * If an update wasn't needed/possible, it also doesn't make sense to
+	 * try to retransmit now.
+	 */
+	if (!sctp_transport_update_pmtu(t, pmtu))
+		return;
 
 	/* Update association pmtu. */
 	sctp_assoc_sync_pmtu(asoc);

@@ -251,28 +251,37 @@ void sctp_transport_pmtu(struct sctp_tra
 		transport->pathmtu = SCTP_DEFAULT_MAXSEGMENT;
 }
 
-void sctp_transport_update_pmtu(struct sctp_transport *t, u32 pmtu)
+bool sctp_transport_update_pmtu(struct sctp_transport *t, u32 pmtu)
 {
 	struct dst_entry *dst = sctp_transport_dst_check(t);
+	bool change = true;
 
 	if (unlikely(pmtu < SCTP_DEFAULT_MINSEGMENT)) {
-		pr_warn("%s: Reported pmtu %d too low, using default minimum of %d\n",
-			__func__, pmtu, SCTP_DEFAULT_MINSEGMENT);
-		/* Use default minimum segment size and disable
-		 * pmtu discovery on this transport.
-		 */
-		t->pathmtu = SCTP_DEFAULT_MINSEGMENT;
-	} else {
-		t->pathmtu = pmtu;
+		pr_warn_ratelimited("%s: Reported pmtu %d too low, using default minimum of %d\n",
+				    __func__, pmtu, SCTP_DEFAULT_MINSEGMENT);
+		/* Use default minimum segment instead */
+		pmtu = SCTP_DEFAULT_MINSEGMENT;
 	}
+	pmtu = SCTP_TRUNC4(pmtu);
 
 	if (dst) {
 		dst->ops->update_pmtu(dst, t->asoc->base.sk, NULL, pmtu);
 		dst = sctp_transport_dst_check(t);
 	}
 
-	if (!dst)
+	if (!dst) {
 		t->af_specific->get_dst(t, &t->saddr, &t->fl, t->asoc->base.sk);
+		dst = t->dst;
+	}
+
+	if (dst) {
+		/* Re-fetch, as under layers may have a higher minimum size */
+		pmtu = SCTP_TRUNC4(dst_mtu(dst));
+		change = t->pathmtu != pmtu;
+	}
+	t->pathmtu = pmtu;
+
+	return change;
 }
 
 /* Caches the dst entry and source address for a transport's destination

Patches currently in stable-queue which might be from [email protected] are

queue-4.14/sctp-do-not-retransmit-upon-fragneeded-if-pmtu-discovery-is-disabled.patch queue-4.14/sctp-fix-the-handling-of-icmp-frag-needed-for-too-small-mtus.patch