From: Daniel Borkmann <dborkman@redhat.com>

Date: Thu, 9 Oct 2014 22:55:32 +0200

Subject: [PATCH] net: sctp: fix panic on duplicate ASCONF chunks

Upstream commit b69040d8e39f20d5215a03502a8e8b4c6ab78395 CVE-2014-3687

When receiving a e.g. semi-good formed connection scan in the

form of ...

  -------------- INIT[ASCONF; ASCONF_ACK] ------------->

  <----------- INIT-ACK[ASCONF; ASCONF_ACK] ------------

  -------------------- COOKIE-ECHO -------------------->

  <-------------------- COOKIE-ACK ---------------------

  ---------------- ASCONF_a; ASCONF_b ----------------->

... where ASCONF_a equals ASCONF_b chunk (at least both serials

need to be equal), we panic an SCTP server!

The problem is that good-formed ASCONF chunks that we reply with

ASCONF_ACK chunks are cached per serial. Thus, when we receive a

same ASCONF chunk twice (e.g. through a lost ASCONF_ACK), we do

not need to process them again on the server side (that was the

idea, also proposed in the RFC). Instead, we know it was cached

and we just resend the cached chunk instead. So far, so good.

Where things get nasty is in SCTP's side effect interpreter, that

is, sctp_cmd_interpreter():

While incoming ASCONF_a (chunk = event_arg) is being marked

!end_of_packet and !singleton, and we have an association context,

we do not flush the outqueue the first time after processing the

ASCONF_ACK singleton chunk via SCTP_CMD_REPLY. Instead, we keep it

queued up, although we set local_cork to 1. Commit 2e3216cd54b1

changed the precedence, so that as long as we get bundled, incoming

chunks we try possible bundling on outgoing queue as well. Before

this commit, we would just flush the output queue.

Now, while ASCONF_a's ASCONF_ACK sits in the corked outq, we

continue to process the same ASCONF_b chunk from the packet. As

we have cached the previous ASCONF_ACK, we find it, grab it and

do another SCTP_CMD_REPLY command on it. So, effectively, we rip

the chunk->list pointers and requeue the same ASCONF_ACK chunk

another time. Since we process ASCONF_b, it's correctly marked

with end_of_packet and we enforce an uncork, and thus flush, thus

crashing the kernel.

Fix it by testing if the ASCONF_ACK is currently pending and if

that is the case, do not requeue it. When flushing the output

queue we may relink the chunk for preparing an outgoing packet,

but eventually unlink it when it's copied into the skb right

before transmission.

Joint work with Vlad Yasevich.

Fixes: 2e3216cd54b1 ("sctp: Follow security requirement of responding with 1 packet")

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>

Signed-off-by: Vlad Yasevich <vyasevich@gmail.com>

Signed-off-by: David S. Miller <davem@davemloft.net>

---

 include/net/sctp/sctp.h | 5 +++++

 net/sctp/associola.c    | 2 ++

 2 files changed, 7 insertions(+)

diff --git a/include/net/sctp/sctp.h b/include/net/sctp/sctp.h

index 9fbd856e6713..856f01cb51dd 100644

--- a/include/net/sctp/sctp.h

+++ b/include/net/sctp/sctp.h

@@ -426,6 +426,11 @@ static inline void sctp_assoc_pending_pmtu(struct sock *sk, struct sctp_associat

 	asoc->pmtu_pending = 0;

 }

+static inline bool sctp_chunk_pending(const struct sctp_chunk *chunk)

+{

+	return !list_empty(&chunk->list);

+}

+

 /* Walk through a list of TLV parameters.  Don't trust the

  * individual parameter lengths and instead depend on

  * the chunk length to indicate when to stop.  Make sure

diff --git a/net/sctp/associola.c b/net/sctp/associola.c

index a88b8524846e..f791edd64d6c 100644

--- a/net/sctp/associola.c

+++ b/net/sctp/associola.c

@@ -1668,6 +1668,8 @@ struct sctp_chunk *sctp_assoc_lookup_asconf_ack(

 	 * ack chunk whose serial number matches that of the request.

 	 */

 	list_for_each_entry(ack, &asoc->asconf_ack_list, transmitted_list) {

+		if (sctp_chunk_pending(ack))

+			continue;

 		if (ack->subh.addip_hdr->serial == serial) {

 			sctp_chunk_hold(ack);

 			return ack;

-- 

1.9.3