From: Matt Caswell Date: Thu, 30 Jun 2016 12:17:08 +0000 (+0100) Subject: Fix DTLS buffered message DoS attack X-Git-Tag: OpenSSL_1_1_0~99 X-Git-Url: https://git.openssl.org/?p=openssl.git;a=commitdiff_plain;h=f5c7f5dfbaf0d2f7d946d0fe86f08e6bcb36ed0d Fix DTLS buffered message DoS attack DTLS can handle out of order record delivery. Additionally since handshake messages can be bigger than will fit into a single packet, the messages can be fragmented across multiple records (as with normal TLS). That means that the messages can arrive mixed up, and we have to reassemble them. We keep a queue of buffered messages that are "from the future", i.e. messages we're not ready to deal with yet but have arrived early. The messages held there may not be full yet - they could be one or more fragments that are still in the process of being reassembled. The code assumes that we will eventually complete the reassembly and when that occurs the complete message is removed from the queue at the point that we need to use it. However, DTLS is also tolerant of packet loss. To get around that DTLS messages can be retransmitted. If we receive a full (non-fragmented) message from the peer after previously having received a fragment of that message, then we ignore the message in the queue and just use the non-fragmented version. At that point the queued message will never get removed. Additionally the peer could send "future" messages that we never get to in order to complete the handshake. Each message has a sequence number (starting from 0). We will accept a message fragment for the current message sequence number, or for any sequence up to 10 into the future. However if the Finished message has a sequence number of 2, anything greater than that in the queue is just left there. So, in those two ways we can end up with "orphaned" data in the queue that will never get removed - except when the connection is closed. At that point all the queues are flushed. An attacker could seek to exploit this by filling up the queues with lots of large messages that are never going to be used in order to attempt a DoS by memory exhaustion. I will assume that we are only concerned with servers here. It does not seem reasonable to be concerned about a memory exhaustion attack on a client. They are unlikely to process enough connections for this to be an issue. A "long" handshake with many messages might be 5 messages long (in the incoming direction), e.g. ClientHello, Certificate, ClientKeyExchange, CertificateVerify, Finished. So this would be message sequence numbers 0 to 4. Additionally we can buffer up to 10 messages in the future. Therefore the maximum number of messages that an attacker could send that could get orphaned would typically be 15. The maximum size that a DTLS message is allowed to be is defined by max_cert_list, which by default is 100k. Therefore the maximum amount of "orphaned" memory per connection is 1500k. Message sequence numbers get reset after the Finished message, so renegotiation will not extend the maximum number of messages that can be orphaned per connection. As noted above, the queues do get cleared when the connection is closed. Therefore in order to mount an effective attack, an attacker would have to open many simultaneous connections. Issue reported by Quan Luo. CVE-2016-2179 Reviewed-by: Richard Levitte --- diff --git a/ssl/d1_lib.c b/ssl/d1_lib.c index 7fb9b96d1d..0ada7edf77 100644 --- a/ssl/d1_lib.c +++ b/ssl/d1_lib.c @@ -114,6 +114,12 @@ int dtls1_new(SSL *s) } static void dtls1_clear_queues(SSL *s) +{ + dtls1_clear_received_buffer(s); + dtls1_clear_sent_buffer(s); +} + +void dtls1_clear_received_buffer(SSL *s) { pitem *item = NULL; hm_fragment *frag = NULL; @@ -123,6 +129,12 @@ static void dtls1_clear_queues(SSL *s) dtls1_hm_fragment_free(frag); pitem_free(item); } +} + +void dtls1_clear_sent_buffer(SSL *s) +{ + pitem *item = NULL; + hm_fragment *frag = NULL; while ((item = pqueue_pop(s->d1->sent_messages)) != NULL) { frag = (hm_fragment *)item->data; @@ -131,6 +143,7 @@ static void dtls1_clear_queues(SSL *s) } } + void dtls1_free(SSL *s) { DTLS_RECORD_LAYER_free(&s->rlayer); @@ -325,7 +338,7 @@ void dtls1_stop_timer(SSL *s) BIO_ctrl(SSL_get_rbio(s), BIO_CTRL_DGRAM_SET_NEXT_TIMEOUT, 0, &(s->d1->next_timeout)); /* Clear retransmission buffer */ - dtls1_clear_record_buffer(s); + dtls1_clear_sent_buffer(s); } int dtls1_check_timeout_num(SSL *s) diff --git a/ssl/ssl_locl.h b/ssl/ssl_locl.h index 6fba98325a..4079b31637 100644 --- a/ssl/ssl_locl.h +++ b/ssl/ssl_locl.h @@ -1923,7 +1923,8 @@ __owur int dtls1_buffer_message(SSL *s, int ccs); __owur int dtls1_retransmit_message(SSL *s, unsigned short seq, int *found); __owur int dtls1_get_queue_priority(unsigned short seq, int is_ccs); int dtls1_retransmit_buffered_messages(SSL *s); -void dtls1_clear_record_buffer(SSL *s); +void dtls1_clear_received_buffer(SSL *s); +void dtls1_clear_sent_buffer(SSL *s); void dtls1_get_message_header(unsigned char *data, struct hm_header_st *msg_hdr); __owur long dtls1_default_timeout(void); diff --git a/ssl/statem/statem_dtls.c b/ssl/statem/statem_dtls.c index 46843ce0fc..de2de09796 100644 --- a/ssl/statem/statem_dtls.c +++ b/ssl/statem/statem_dtls.c @@ -442,11 +442,23 @@ static int dtls1_retrieve_buffered_fragment(SSL *s, int *ok) int al; *ok = 0; - item = pqueue_peek(s->d1->buffered_messages); - if (item == NULL) - return 0; - frag = (hm_fragment *)item->data; + do { + item = pqueue_peek(s->d1->buffered_messages); + if (item == NULL) + return 0; + + frag = (hm_fragment *)item->data; + + if (frag->msg_header.seq < s->d1->handshake_read_seq) { + /* This is a stale message that has been buffered so clear it */ + pqueue_pop(s->d1->buffered_messages); + dtls1_hm_fragment_free(frag); + pitem_free(item); + item = NULL; + frag = NULL; + } + } while (item == NULL); /* Don't return if reassembly still in progress */ if (frag->reassembly != NULL) @@ -1117,18 +1129,6 @@ int dtls1_retransmit_message(SSL *s, unsigned short seq, int *found) return ret; } -/* call this function when the buffered messages are no longer needed */ -void dtls1_clear_record_buffer(SSL *s) -{ - pitem *item; - - for (item = pqueue_pop(s->d1->sent_messages); - item != NULL; item = pqueue_pop(s->d1->sent_messages)) { - dtls1_hm_fragment_free((hm_fragment *)item->data); - pitem_free(item); - } -} - void dtls1_set_message_header(SSL *s, unsigned char mt, unsigned long len, unsigned long frag_off, unsigned long frag_len) diff --git a/ssl/statem/statem_lib.c b/ssl/statem/statem_lib.c index fac9684ce2..19b75a7ac7 100644 --- a/ssl/statem/statem_lib.c +++ b/ssl/statem/statem_lib.c @@ -331,6 +331,7 @@ WORK_STATE tls_finish_handshake(SSL *s, WORK_STATE wst) s->d1->handshake_read_seq = 0; s->d1->handshake_write_seq = 0; s->d1->next_handshake_write_seq = 0; + dtls1_clear_received_buffer(s); } } diff --git a/ssl/statem/statem_srvr.c b/ssl/statem/statem_srvr.c index ae1c65d9ae..95dcc9b60b 100644 --- a/ssl/statem/statem_srvr.c +++ b/ssl/statem/statem_srvr.c @@ -433,13 +433,13 @@ WORK_STATE ossl_statem_server_pre_work(SSL *s, WORK_STATE wst) case TLS_ST_SW_HELLO_REQ: s->shutdown = 0; if (SSL_IS_DTLS(s)) - dtls1_clear_record_buffer(s); + dtls1_clear_sent_buffer(s); break; case DTLS_ST_SW_HELLO_VERIFY_REQUEST: s->shutdown = 0; if (SSL_IS_DTLS(s)) { - dtls1_clear_record_buffer(s); + dtls1_clear_sent_buffer(s); /* We don't buffer this message so don't use the timer */ st->use_timer = 0; }