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@iem.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]), ", --