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Date: Wed, 15 Sep 2010 10:02:16 -0400
From: Vlad Yasevich <vladislav.yasevich@...com>
To: Thomas Dreibholz <dreibh@....uni-due.de>
CC: netdev@...r.kernel.org, linux-sctp@...r.kernel.org,
Martin Becke <martin.becke@...-due.de>
Subject: Re: [PATCH] net: SCTP remote/local Denial of Service vulnerability
description and fix
On 09/15/2010 04:01 AM, Thomas Dreibholz wrote:
> 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.
>
Actually I have a much easier solution. When calling packet_config, we should not
be resetting the packet contents.
Packet contents need to be reset when a new packet is created or when the packet has
been sent. We explicitly reset it after sending in sctp_packet_transmit. We also reset it
in sctp_packet_init(). So, code such as:
sctp_packet_init()
sctp_packet_config()
already guarantees that at config time we have a clear packet.
So, simply removing a reset call from sctp_packet_config() solves this issue.
I've attached the patch that corrects this.
-vlad
> 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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View attachment "0001-sctp-Do-not-reset-the-packet-during-sctp_packet_conf.patch" of type "text/x-patch" (1120 bytes)
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