[PATCH net v3 0/2] SCTP PMTU discovery fixes

This patchset fixes 2 issues with PMTU discovery that can lead to flood
of retransmissions.
The first patch fixes the issue for when PMTUD is disabled by the
application, while the second fixes it for when its enabled.

Please consider these to stable.

Thanks,

Marcelo Ricardo Leitner (2):
  sctp: do not retransmit upon FragNeeded if PMTU discovery is disabled
  sctp: fix the handling of ICMP Frag Needed for too small MTUs

 include/net/sctp/structs.h |  2 +-
 net/sctp/input.c           | 28 ++++++++++++++++------------
 net/sctp/transport.c       | 29 +++++++++++++++++++----------
 3 files changed, 36 insertions(+), 23 deletions(-)

-- 
2.14.3

Currently, if PMTU discovery is disabled on a given transport, but the
configured value is higher than the actual PMTU, it is likely that we
will get some icmp Frag Needed. The issue is, if PMTU discovery is
disabled, we won't update the information and will issue a
retransmission immediately, which may very well trigger another ICMP,
and another retransmission, leading to a loop.

The fix is to simply not trigger immediate retransmissions if PMTU
discovery is disabled on the given transport.

Changes from v2:
- updated stale comment, noticed by Xin Long

Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
---
 net/sctp/input.c | 24 ++++++++++++------------
 1 file changed, 12 insertions(+), 12 deletions(-)

diff --git a/net/sctp/input.c b/net/sctp/input.c
index 621b5ca3fd1c17c3d7ef7bb1c7677ab98cebbe77..9320661cc41da0b280f69f379128ab7d062e5528 100644
--- a/net/sctp/input.c
+++ b/net/sctp/input.c

@@ -399,20 +399,20 @@ void sctp_icmp_frag_needed(struct sock *sk, struct sctp_association *asoc,
 		return;
 	}
 
-	if (t->param_flags & SPP_PMTUD_ENABLE) {
-		/* Update transports view of the MTU */
-		sctp_transport_update_pmtu(t, pmtu);
+	if (!(t->param_flags & SPP_PMTUD_ENABLE))
+		/* We can't allow retransmitting in such case, as the
+		 * retransmission would be sized just as before, and thus we
+		 * would get another icmp, and retransmit again.
+		 */
+		return;
 
-		/* Update association pmtu. */
-		sctp_assoc_sync_pmtu(asoc);
-	}
+	/* Update transports view of the MTU */
+	sctp_transport_update_pmtu(t, pmtu);
 
-	/* Retransmit with the new pmtu setting.
-	 * Normally, if PMTU discovery is disabled, an ICMP Fragmentation
-	 * 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.
-	 */
+	/* Update association pmtu. */
+	sctp_assoc_sync_pmtu(asoc);
+
+	/* Retransmit with the new pmtu setting. */
 	sctp_retransmit(&asoc->outqueue, t, SCTP_RTXR_PMTUD);
 }
 

-- 
2.14.3

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
Reported-by: syzbot <redacted>
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
---
 include/net/sctp/structs.h |  2 +-
 net/sctp/input.c           |  8 ++++++--
 net/sctp/transport.c       | 29 +++++++++++++++++++----------
 3 files changed, 26 insertions(+), 13 deletions(-)

diff --git a/include/net/sctp/structs.h b/include/net/sctp/structs.h
index 2f8f93da5dc2660f4db37c04f8a434809b3120a1..9a5ccf03a59b1e0a4820e2a978c66f1df8a9a82f 100644
--- a/include/net/sctp/structs.h
+++ b/include/net/sctp/structs.h

@@ -966,7 +966,7 @@ void sctp_transport_burst_limited(struct sctp_transport *);
 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);

diff --git a/net/sctp/input.c b/net/sctp/input.c
index 9320661cc41da0b280f69f379128ab7d062e5528..141c9c466ec172ff67930cf38eb02a49e01a12ab 100644
--- a/net/sctp/input.c
+++ b/net/sctp/input.c

@@ -406,8 +406,12 @@ void sctp_icmp_frag_needed(struct sock *sk, struct sctp_association *asoc,
 		 */
 		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);

diff --git a/net/sctp/transport.c b/net/sctp/transport.c
index 1e5a22430cf56e40a6f323081beb97836b506384..47f82bd794d915188bad037463c2aa14175a55ef 100644
--- a/net/sctp/transport.c
+++ b/net/sctp/transport.c

@@ -248,28 +248,37 @@ void sctp_transport_pmtu(struct sctp_transport *transport, struct sock *sk)
 		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

-- 
2.14.3

On Fri, Jan 05, 2018 at 11:17:16AM -0200, Marcelo Ricardo Leitner wrote:

This patchset fixes 2 issues with PMTU discovery that can lead to flood
of retransmissions.
The first patch fixes the issue for when PMTUD is disabled by the
application, while the second fixes it for when its enabled.

Please consider these to stable.

Thanks,

Marcelo Ricardo Leitner (2):
  sctp: do not retransmit upon FragNeeded if PMTU discovery is disabled
  sctp: fix the handling of ICMP Frag Needed for too small MTUs

 include/net/sctp/structs.h |  2 +-
 net/sctp/input.c           | 28 ++++++++++++++++------------
 net/sctp/transport.c       | 29 +++++++++++++++++++----------
 3 files changed, 36 insertions(+), 23 deletions(-)

-- 
2.14.3

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Series
Acked-by: Neil Horman <nhorman@tuxdriver.com>

On Fri, Jan 5, 2018 at 9:17 PM, Marcelo Ricardo Leitner
[off-list ref] wrote:

This patchset fixes 2 issues with PMTU discovery that can lead to flood
of retransmissions.
The first patch fixes the issue for when PMTUD is disabled by the
application, while the second fixes it for when its enabled.

Please consider these to stable.

Thanks,

Marcelo Ricardo Leitner (2):
  sctp: do not retransmit upon FragNeeded if PMTU discovery is disabled
  sctp: fix the handling of ICMP Frag Needed for too small MTUs

 include/net/sctp/structs.h |  2 +-
 net/sctp/input.c           | 28 ++++++++++++++++------------
 net/sctp/transport.c       | 29 +++++++++++++++++++----------
 3 files changed, 36 insertions(+), 23 deletions(-)

--
2.14.3

Reviewed-by: Xin Long <lucien.xin@gmail.com>

From: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Date: Fri,  5 Jan 2018 11:17:16 -0200

This patchset fixes 2 issues with PMTU discovery that can lead to flood
of retransmissions.
The first patch fixes the issue for when PMTUD is disabled by the
application, while the second fixes it for when its enabled.

Please consider these to stable.

Applied and queued up for -stable, thank you.