Date:	Wed, 15 Sep 2010 10:01:56 +0200
From:	Thomas Dreibholz <dreibh@....uni-due.de>
To:	netdev@...r.kernel.org, linux-sctp@...r.kernel.org
Cc:	Martin Becke <martin.becke@...-due.de>
Subject: [PATCH] net: SCTP remote/local Denial of Service vulnerability description and fix

sctp_outq_flush() in net/sctp/outqueue.c may call sctp_packet_reset() on a 
packet structure which has already been filled with chunks. sctp_packet_reset() 
will not take care of the chunks in its list and only reset the packet length. 
After that, the SCTP code assumes the packet to be re-initialized and adds 
further chunks to the structure. The length will be wrong. When actually 
trying to transmit the packet, the packet sk_buff structure may be exceeded 
within sctp_packet_transmit(), resulting in skb_over_panic() => denial of 
service.

Such a DoS can be triggered by a malicious remote SCTP instance, as follows:
- The remote endpoint has to use two paths (i.e. 2 IP addresses); easy to 
achieve using an IPv4 address and an IPv6 address -> path A and B.
- The remote user has to trigger the transmission of a HEARTBEAT_ACK on path 
A. This is trivial, by sending a HEARTBEAT chunk. sctp_outq_flush() will call 
sctp_packet_config() for the packet on path A. The HEARTBEAT_ACK will be added 
to this packet.
- The remote user has to trigger a DATA chunk retransmission on path B. This 
is trivial, since it only has to send appropriate SACK chunks. 
sctp_outq_flush() notices that the retransmission is on a different path and 
calls sctp_packet_config() for the packet on path B. The DATA chunk to be 
retransmitted is added to this packet.
- The local instance has to send another DATA chunk on path A. This depends on 
the application, but should be easy to trigger from a remote instance. 
sctp_outq_flush() notices that the path has changed again, and calls 
sctp_packet_config() for the packet on path A. This resets the size of the 
HEARTBEAT_ACK chunk, but the chunk remains in the packet. If 
size(HEARTBEAT_ACK) + size(DATA) > MTU - overhead, the next call to 
sctp_packet_transmit() causes the kernel panic => DoS.
In a similar way, the problem can also be triggered by a local user, having 
only the permission to establish SCTP associations. Of course, the problem can 
also occur during normal network operation, just by a retransmission at the 
wrong time.

The patch below against 2.6.36-rc4 (git repository) fixes sctp_outq_flush() by 
ensuring that the packet on each path is initialized only once. Furthermore, a 
BUG_ON() statement ensures that further problems by calling 
sctp_packet_reset() on packets with chunks are detected directly.

Signed-off-by: Thomas Dreibholz <dreibh@....uni-due.de>
---
diff --git a/net/sctp/output.c b/net/sctp/output.c
index a646681..744e667 100644
--- a/net/sctp/output.c
+++ b/net/sctp/output.c
@@ -72,6 +72,7 @@ static sctp_xmit_t sctp_packet_will_fit(struct sctp_packet 
*packet,

 static void sctp_packet_reset(struct sctp_packet *packet)
 {
+        BUG_ON(!list_empty(&packet->chunk_list));
 	packet->size = packet->overhead;
 	packet->has_cookie_echo = 0;
 	packet->has_sack = 0;
diff --git a/net/sctp/outqueue.c b/net/sctp/outqueue.c
index c04b2eb..69296c8 100644
--- a/net/sctp/outqueue.c
+++ b/net/sctp/outqueue.c
@@ -799,13 +799,13 @@ static int sctp_outq_flush(struct sctp_outq *q, int 
rtx_timeout)
 		 */
 		if (new_transport != transport) {
 			transport = new_transport;
+			packet = &transport->packet;
 			if (list_empty(&transport->send_ready)) {
 				list_add_tail(&transport->send_ready,
 					      &transport_list);
+				sctp_packet_config(packet, vtag,
+					      asoc->peer.ecn_capable);
 			}
-			packet = &transport->packet;
-			sctp_packet_config(packet, vtag,
-					   asoc->peer.ecn_capable);
 		}

 		switch (chunk->chunk_hdr->type) {
@@ -900,15 +900,14 @@ static int sctp_outq_flush(struct sctp_outq *q, int 
rtx_timeout)
 			/* Switch transports & prepare the packet.  */

 			transport = asoc->peer.retran_path;
+			packet = &transport->packet;

 			if (list_empty(&transport->send_ready)) {
 				list_add_tail(&transport->send_ready,
 					      &transport_list);
+				sctp_packet_config(packet, vtag,
+						   asoc->peer.ecn_capable);
 			}
-
-			packet = &transport->packet;
-			sctp_packet_config(packet, vtag,
-					   asoc->peer.ecn_capable);
 		retran:
 			error = sctp_outq_flush_rtx(q, packet,
 						    rtx_timeout, &start_timer);
@@ -970,6 +969,7 @@ static int sctp_outq_flush(struct sctp_outq *q, int 
rtx_timeout)
 			/* Change packets if necessary.  */
 			if (new_transport != transport) {
 				transport = new_transport;
+				packet = &transport->packet;

 				/* Schedule to have this transport's
 				 * packet flushed.
@@ -977,15 +977,14 @@ static int sctp_outq_flush(struct sctp_outq *q, int 
rtx_timeout)
 				if (list_empty(&transport->send_ready)) {
 					list_add_tail(&transport->send_ready,
 						      &transport_list);
-				}
+					sctp_packet_config(packet, vtag,
+							   asoc->peer.ecn_capable);

-				packet = &transport->packet;
-				sctp_packet_config(packet, vtag,
-						   asoc->peer.ecn_capable);
-				/* We've switched transports, so apply the
-				 * Burst limit to the new transport.
-				 */
-				sctp_transport_burst_limited(transport);
+					/* We've switched transports, so apply the
+					 * Burst limit to the new transport.
+					 */
+					sctp_transport_burst_limited(transport);
+				}
 			}

 			SCTP_DEBUG_PRINTK("sctp_outq_flush(%p, %p[%s]), ",
--
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