SCTP PMTU discovery fixes

PATCH Superseded Marcelo Ricardo Leitner 2 participants 6 messages Jan 3, 2018 – Jan 4, 2018

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Networking Network drivers SCTP protocol

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Marcelo Ricardo Leitner

author maintainer

2018-01-03 22:59 UTC #0 lore raw

v2 cover 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           | 21 +++++++++++++++------
 net/sctp/transport.c       | 29 +++++++++++++++++++----------
 3 files changed, 35 insertions(+), 17 deletions(-)

Signature · 1 line

2.14.3

Marcelo Ricardo Leitner

author maintainer

2018-01-03 22:59 UTC #1 lore raw

PATCH v2 1/2 sctp: do not retransmit upon FragNeeded if PMTU discovery is disabled

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.

Signed-off-by Marcelo Ricardo Leitner <[email protected]>

---
 net/sctp/input.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

net/sctp/input.c +11 -6

index 621b5ca3fd1c17c3d7ef7bb1c7677ab98cebbe77..4a8e76f4834c90de9398455862423e598b8354a7 100644
@@ -399,13 +399,18 @@ 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);
+
+	/* Update association pmtu. */
+	sctp_assoc_sync_pmtu(asoc);
 
 	/* Retransmit with the new pmtu setting.
 	 * Normally, if PMTU discovery is disabled, an ICMP Fragmentation

Signature · 1 line

2.14.3

Marcelo Ricardo Leitner

author maintainer

2018-01-03 22:59 UTC #5 lore raw

PATCH v2 2/2 sctp: fix the handling of ICMP Frag Needed for too small MTUs

In reply to Marcelo Ricardo Leitner

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_pmtuU6) and IPv6 (IPV6_MIN_MTU80) 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.
- change the warn to use pr_warn_ratelimited, to protect in case there are several transports using such path.

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

Reported-by syzbot <[email protected]>

Signed-off-by Marcelo Ricardo Leitner <[email protected]>

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

include/net/sctp/structs.h +1 -1

index 2f8f93da5dc2660f4db37c04f8a434809b3120a1..9a5ccf03a59b1e0a4820e2a978c66f1df8a9a82f 100644
@@ -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);

net/sctp/input.c +6 -2

index 4a8e76f4834c90de9398455862423e598b8354a7..3cddbecabaa2db410971c66d3256c466c7f5289f 100644
@@ -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);

net/sctp/transport.c +19 ~~-10~~

index 1e5a22430cf56e40a6f323081beb97836b506384..47f82bd794d915188bad037463c2aa14175a55ef 100644
@@ -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

Signature · 1 line

2.14.3

Xin Long

maintainer

2018-01-04 04:52 UTC #2 lore raw

Re: [PATCH net v2 1/2] sctp: do not retransmit upon FragNeeded if PMTU discovery is disabled

In reply to Marcelo Ricardo Leitner

Quoted 45 lines · Marcelo Ricardo Leitner

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

Signed-off-by Marcelo Ricardo Leitner <[email protected]>

---
 net/sctp/input.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

net/sctp/input.c +11 -7

index 621b5ca3fd1c17c3d7ef7bb1c7677ab98cebbe77..4a8e76f4834c90de9398455862423e598b8354a7 100644
@@ -399,13 +399,18 @@ 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);
+
+       /* Update association pmtu. */
+       sctp_assoc_sync_pmtu(asoc);
 
        /* Retransmit with the new pmtu setting.
         * Normally, if PMTU discovery is disabled, an ICMP Fragmentation
--

Quoted 2 lines

2.14.3

commit 52ccb8e90c0ace233b8b740f2fc5de0dbd706b27

Author: Frank Filz <[email protected]>
Date:   Thu Dec 22 11:36:46 2005 -0800

[SCTP]: Update SCTP_PEER_ADDR_PARAMS socket option to the latest api draft.

It seemed intended to move sctp_retransmit out of 'if (SPP_PMTUD_ENABLE) {}' on the above commit with some notes:

/* 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. */

But this patch is equivalent to move it back into 'if (SPP_PMTUD_ENABLE) {}'. will there be no regression caused?

Marcelo Ricardo Leitner

author maintainer

2018-01-04 11:23 UTC #3 lore raw

Re: [PATCH net v2 1/2] sctp: do not retransmit upon FragNeeded if PMTU discovery is disabled

Quoted 51 lines · Xin Long

Quoted 45 lines · Marcelo Ricardo Leitner

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

Signed-off-by Marcelo Ricardo Leitner <[email protected]>

---
 net/sctp/input.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

net/sctp/input.c +11 -7

index 621b5ca3fd1c17c3d7ef7bb1c7677ab98cebbe77..4a8e76f4834c90de9398455862423e598b8354a7 100644
@@ -399,13 +399,18 @@ 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);
+
+       /* Update association pmtu. */
+       sctp_assoc_sync_pmtu(asoc);
 
        /* Retransmit with the new pmtu setting.
         * Normally, if PMTU discovery is disabled, an ICMP Fragmentation
--

Quoted 2 lines

2.14.3

commit 52ccb8e90c0ace233b8b740f2fc5de0dbd706b27

Quoted 7 lines

Author: Frank Filz <[email protected]>
Date:   Thu Dec 22 11:36:46 2005 -0800
[SCTP]: Update SCTP_PEER_ADDR_PARAMS socket option to the latest api draft.
It seemed intended to move sctp_retransmit out of 'if (SPP_PMTUD_ENABLE) {}' on the above commit with some notes:

Good point.

Quoted 10 lines

/* 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. */
But this patch is equivalent to move it back into 'if (SPP_PMTUD_ENABLE) {}'. will there be no regression caused?

I don't think this comment has been effective because the function starts with:

void sctp_icmp_frag_needed(struct sock *sk, struct sctp_association *asoc,
struct sctp_transport *t, __u32 pmtu)

{
        if (!t || (t->pathmtu <= pmtu))
                return;

So if the application managed to adjust pmtu after sending some data, t->pathmtu will fit this check and nothing would be done anyway.

commit 91bd6b1e030266cf87d3f567b49f0fa60a7318ba

Author: Wei Yongjun <[email protected]>
Date:   Thu Oct 23 00:59:52 2008 -0700

sctp: Drop ICMP packet too big message with MTU larger than current PMTU

I guess I should have removed this comment too. WDYT? I'll prepare a v3 meanwhile.

Xin Long

maintainer

2018-01-04 13:29 UTC #4 lore raw

Re: [PATCH net v2 1/2] sctp: do not retransmit upon FragNeeded if PMTU discovery is disabled

Quoted 85 lines · Marcelo Ricardo Leitner

Quoted 51 lines · Xin Long
Quoted 45 lines · Marcelo Ricardo Leitner
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.
Signed-off-by Marcelo Ricardo Leitner <[email protected]>
---
 net/sctp/input.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)
net/sctp/input.c +11 -7
index 621b5ca3fd1c17c3d7ef7bb1c7677ab98cebbe77..4a8e76f4834c90de9398455862423e598b8354a7 100644
@@ -399,13 +399,18 @@ 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);
+
+       /* Update association pmtu. */
+       sctp_assoc_sync_pmtu(asoc);
 
        /* Retransmit with the new pmtu setting.
         * Normally, if PMTU discovery is disabled, an ICMP Fragmentation
--
Quoted 2 lines
2.14.3
commit 52ccb8e90c0ace233b8b740f2fc5de0dbd706b27
Quoted 7 lines
Author: Frank Filz <[email protected]>
Date:   Thu Dec 22 11:36:46 2005 -0800
[SCTP]: Update SCTP_PEER_ADDR_PARAMS socket option to the latest api draft.
It seemed intended to move sctp_retransmit out of 'if (SPP_PMTUD_ENABLE) {}' on the above commit with some notes:
Good point.
Quoted 10 lines
/* 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. */
But this patch is equivalent to move it back into 'if (SPP_PMTUD_ENABLE) {}'. will there be no regression caused?
I don't think this comment has been effective because the function starts with:
void sctp_icmp_frag_needed(struct sock *sk, struct sctp_association *asoc,
struct sctp_transport *t, __u32 pmtu)
{
        if (!t || (t->pathmtu <= pmtu))
                return;
So if the application managed to adjust pmtu after sending some data, t->pathmtu will fit this check and nothing would be done anyway.

Quoted 8 lines

commit 91bd6b1e030266cf87d3f567b49f0fa60a7318ba
Author: Wei Yongjun <[email protected]>
Date:   Thu Oct 23 00:59:52 2008 -0700
sctp: Drop ICMP packet too big message with MTU larger than current PMTU
I guess I should have removed this comment too. WDYT?

Yes, I agree. Next time the retransmit could also fragment it, and it actually rarely happens, so no need to do it that immediately.

Quoted 1 line

I'll prepare a v3 meanwhile.