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SUSE:SLE-12-SP2:GA
compat-openssl098.20929
openssl-CVE-2016-2179.patch
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File openssl-CVE-2016-2179.patch of Package compat-openssl098.20929
commit 00a4c1421407b6ac796688871b0a49a179c694d9 Author: Matt Caswell <matt@openssl.org> Date: Thu Jun 30 13:17:08 2016 +0100 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 <levitte@openssl.org> commit cfd40fd39e69f5e3c654ae8fbf9acb1d2a051144 Author: Matt Caswell <matt@openssl.org> Date: Thu Jun 30 15:06:27 2016 +0100 Prevent DTLS Finished message injection Follow on from CVE-2016-2179 The investigation and analysis of CVE-2016-2179 highlighted a related flaw. This commit fixes a security "near miss" in the buffered message handling code. Ultimately this is not currently believed to be exploitable due to the reasons outlined below, and therefore there is no CVE for this on its own. The issue this commit fixes is a MITM attack where the attacker can inject a Finished message into the handshake. In the description below it is assumed that the attacker injects the Finished message for the server to receive it. The attack could work equally well the other way around (i.e where the client receives the injected Finished message). The MITM requires the following capabilities: - The ability to manipulate the MTU that the client selects such that it is small enough for the client to fragment Finished messages. - The ability to selectively drop and modify records sent from the client - The ability to inject its own records and send them to the server The MITM forces the client to select a small MTU such that the client will fragment the Finished message. Ideally for the attacker the first fragment will contain all but the last byte of the Finished message, with the second fragment containing the final byte. During the handshake and prior to the client sending the CCS the MITM injects a plaintext Finished message fragment to the server containing all but the final byte of the Finished message. The message sequence number should be the one expected to be used for the real Finished message. OpenSSL will recognise that the received fragment is for the future and will buffer it for later use. After the client sends the CCS it then sends its own Finished message in two fragments. The MITM causes the first of these fragments to be dropped. The OpenSSL server will then receive the second of the fragments and reassemble the complete Finished message consisting of the MITM fragment and the final byte from the real client. The advantage to the attacker in injecting a Finished message is that this provides the capability to modify other handshake messages (e.g. the ClientHello) undetected. A difficulty for the attacker is knowing in advance what impact any of those changes might have on the final byte of the handshake hash that is going to be sent in the "real" Finished message. In the worst case for the attacker this means that only 1 in 256 of such injection attempts will succeed. It may be possible in some situations for the attacker to improve this such that all attempts succeed. For example if the handshake includes client authentication then the final message flight sent by the client will include a Certificate. Certificates are ASN.1 objects where the signed portion is DER encoded. The non-signed portion could be BER encoded and so the attacker could re-encode the certificate such that the hash for the whole handshake comes to a different value. The certificate re-encoding would not be detectable because only the non-signed portion is changed. As this is the final flight of messages sent from the client the attacker knows what the complete hanshake hash value will be that the client will send - and therefore knows what the final byte will be. Through a process of trial and error the attacker can re-encode the certificate until the modified handhshake also has a hash with the same final byte. This means that when the Finished message is verified by the server it will be correct in all cases. In practice the MITM would need to be able to perform the same attack against both the client and the server. If the attack is only performed against the server (say) then the server will not detect the modified handshake, but the client will and will abort the connection. Fortunately, although OpenSSL is vulnerable to Finished message injection, it is not vulnerable if *both* client and server are OpenSSL. The reason is that OpenSSL has a hard "floor" for a minimum MTU size that it will never go below. This minimum means that a Finished message will never be sent in a fragmented form and therefore the MITM does not have one of its pre-requisites. Therefore this could only be exploited if using OpenSSL and some other DTLS peer that had its own and separate Finished message injection flaw. The fix is to ensure buffered messages are cleared on epoch change. Reviewed-by: Richard Levitte <levitte@openssl.org> Index: openssl-0.9.8j/ssl/d1_both.c =================================================================== --- openssl-0.9.8j.orig/ssl/d1_both.c 2016-09-06 16:53:27.987444880 +0200 +++ openssl-0.9.8j/ssl/d1_both.c 2016-09-06 17:06:32.496241369 +0200 @@ -433,7 +433,7 @@ long dtls1_get_message(SSL *s, int st1, * have been received by the client before clearing this * buffer. this can simply be done by waiting for the * first data segment, but is there a better way? */ - dtls1_clear_record_buffer(s); + dtls1_clear_sent_buffer(s); s->init_msg = s->init_buf->data + DTLS1_HM_HEADER_LENGTH; return s->init_num; @@ -510,12 +510,22 @@ dtls1_retrieve_buffered_fragment(SSL *s, hm_fragment *frag; 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); if ( s->d1->handshake_read_seq == frag->msg_header.seq) { @@ -1128,21 +1138,6 @@ dtls1_retransmit_message(SSL *s, unsigne 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); - } - } - - unsigned char * dtls1_set_message_header(SSL *s, unsigned char *p, unsigned char mt, unsigned long len, unsigned long frag_off, unsigned long frag_len) Index: openssl-0.9.8j/ssl/d1_clnt.c =================================================================== --- openssl-0.9.8j.orig/ssl/d1_clnt.c 2016-09-06 16:53:27.991444947 +0200 +++ openssl-0.9.8j/ssl/d1_clnt.c 2016-09-06 16:53:28.295449963 +0200 @@ -545,6 +545,7 @@ int dtls1_connect(SSL *s) /* done with handshaking */ s->d1->handshake_read_seq = 0; + dtls1_clear_received_buffer(s); goto end; /* break; */ Index: openssl-0.9.8j/ssl/d1_lib.c =================================================================== --- openssl-0.9.8j.orig/ssl/d1_lib.c 2016-09-06 16:53:28.287449831 +0200 +++ openssl-0.9.8j/ssl/d1_lib.c 2016-09-06 16:53:28.295449963 +0200 @@ -153,6 +153,15 @@ static void dtls1_clear_queues(SSL *s) pitem_free(item); } + dtls1_clear_received_buffer(s); + dtls1_clear_sent_buffer(s); +} + +void dtls1_clear_received_buffer(SSL *s) +{ + pitem *item = NULL; + hm_fragment *frag = NULL; + while( (item = pqueue_pop(s->d1->buffered_messages)) != NULL) { frag = (hm_fragment *)item->data; @@ -160,6 +169,12 @@ static void dtls1_clear_queues(SSL *s) OPENSSL_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) { Index: openssl-0.9.8j/ssl/ssl_locl.h =================================================================== --- openssl-0.9.8j.orig/ssl/ssl_locl.h 2016-09-06 16:53:28.135447322 +0200 +++ openssl-0.9.8j/ssl/ssl_locl.h 2016-09-06 16:53:28.295449963 +0200 @@ -872,7 +872,8 @@ int dtls1_read_failed(SSL *s, int code); int dtls1_buffer_message(SSL *s, int ccs); int dtls1_retransmit_message(SSL *s, unsigned short seq, unsigned long frag_off, int *found); -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); void dtls1_get_ccs_header(unsigned char *data, struct ccs_header_st *ccs_hdr); void dtls1_reset_seq_numbers(SSL *s, int rw); Index: openssl-0.9.8j/ssl/d1_pkt.c =================================================================== --- openssl-0.9.8j.orig/ssl/d1_pkt.c 2016-09-06 16:53:28.055446003 +0200 +++ openssl-0.9.8j/ssl/d1_pkt.c 2016-09-06 16:54:04.160041447 +0200 @@ -1783,6 +1783,11 @@ dtls1_reset_seq_numbers(SSL *s, int rw) memset(&(s->d1->next_bitmap), 0x00, sizeof(DTLS1_BITMAP)); pq_64bit_init(&(s->d1->next_bitmap.map)); pq_64bit_init(&(s->d1->next_bitmap.max_seq_num)); + /* + * We must not use any buffered messages received from the previous + * epoch + */ + dtls1_clear_received_buffer(s); } else {
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