Revision f5c7f5dfbaf0d2f7d946d0fe86f08e6bcb36ed0d authored by Matt Caswell on 30 June 2016, 12:17:08 UTC, committed by Matt Caswell on 22 August 2016, 09:53:55 UTC
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>
1 parent 5dfd038
asynciotest.c
/*
* Copyright 2016 The OpenSSL Project Authors. All Rights Reserved.
*
* Licensed under the OpenSSL licenses, (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
* https://www.openssl.org/source/license.html
* or in the file LICENSE in the source distribution.
*/
#include <string.h>
#include <openssl/ssl.h>
#include <openssl/bio.h>
#include <openssl/err.h>
#include "../ssl/packet_locl.h"
#include "ssltestlib.h"
/* Should we fragment records or not? 0 = no, !0 = yes*/
static int fragment = 0;
static int async_new(BIO *bi);
static int async_free(BIO *a);
static int async_read(BIO *b, char *out, int outl);
static int async_write(BIO *b, const char *in, int inl);
static long async_ctrl(BIO *b, int cmd, long num, void *ptr);
static int async_gets(BIO *bp, char *buf, int size);
static int async_puts(BIO *bp, const char *str);
/* Choose a sufficiently large type likely to be unused for this custom BIO */
# define BIO_TYPE_ASYNC_FILTER (0x80 | BIO_TYPE_FILTER)
static BIO_METHOD *methods_async = NULL;
struct async_ctrs {
unsigned int rctr;
unsigned int wctr;
};
static const BIO_METHOD *bio_f_async_filter()
{
if (methods_async == NULL) {
methods_async = BIO_meth_new(BIO_TYPE_ASYNC_FILTER, "Async filter");
if ( methods_async == NULL
|| !BIO_meth_set_write(methods_async, async_write)
|| !BIO_meth_set_read(methods_async, async_read)
|| !BIO_meth_set_puts(methods_async, async_puts)
|| !BIO_meth_set_gets(methods_async, async_gets)
|| !BIO_meth_set_ctrl(methods_async, async_ctrl)
|| !BIO_meth_set_create(methods_async, async_new)
|| !BIO_meth_set_destroy(methods_async, async_free))
return NULL;
}
return methods_async;
}
static int async_new(BIO *bio)
{
struct async_ctrs *ctrs;
ctrs = OPENSSL_zalloc(sizeof(struct async_ctrs));
if (ctrs == NULL)
return 0;
BIO_set_data(bio, ctrs);
BIO_set_init(bio, 1);
return 1;
}
static int async_free(BIO *bio)
{
struct async_ctrs *ctrs;
if (bio == NULL)
return 0;
ctrs = BIO_get_data(bio);
OPENSSL_free(ctrs);
BIO_set_data(bio, NULL);
BIO_set_init(bio, 0);
return 1;
}
static int async_read(BIO *bio, char *out, int outl)
{
struct async_ctrs *ctrs;
int ret = 0;
BIO *next = BIO_next(bio);
if (outl <= 0)
return 0;
if (next == NULL)
return 0;
ctrs = BIO_get_data(bio);
BIO_clear_retry_flags(bio);
if (ctrs->rctr > 0) {
ret = BIO_read(next, out, 1);
if (ret <= 0 && BIO_should_read(next))
BIO_set_retry_read(bio);
ctrs->rctr = 0;
} else {
ctrs->rctr++;
BIO_set_retry_read(bio);
}
return ret;
}
#define MIN_RECORD_LEN 6
#define CONTENTTYPEPOS 0
#define VERSIONHIPOS 1
#define VERSIONLOPOS 2
#define DATAPOS 5
static int async_write(BIO *bio, const char *in, int inl)
{
struct async_ctrs *ctrs;
int ret = 0;
size_t written = 0;
BIO *next = BIO_next(bio);
if (inl <= 0)
return 0;
if (next == NULL)
return 0;
ctrs = BIO_get_data(bio);
BIO_clear_retry_flags(bio);
if (ctrs->wctr > 0) {
ctrs->wctr = 0;
if (fragment) {
PACKET pkt;
if (!PACKET_buf_init(&pkt, (const unsigned char *)in, inl))
abort();
while (PACKET_remaining(&pkt) > 0) {
PACKET payload;
unsigned int contenttype, versionhi, versionlo, data;
if ( !PACKET_get_1(&pkt, &contenttype)
|| !PACKET_get_1(&pkt, &versionhi)
|| !PACKET_get_1(&pkt, &versionlo)
|| !PACKET_get_length_prefixed_2(&pkt, &payload))
abort();
/* Pretend we wrote out the record header */
written += SSL3_RT_HEADER_LENGTH;
while (PACKET_get_1(&payload, &data)) {
/* Create a new one byte long record for each byte in the
* record in the input buffer
*/
char smallrec[MIN_RECORD_LEN] = {
0, /* Content type */
0, /* Version hi */
0, /* Version lo */
0, /* Length hi */
1, /* Length lo */
0 /* Data */
};
smallrec[CONTENTTYPEPOS] = contenttype;
smallrec[VERSIONHIPOS] = versionhi;
smallrec[VERSIONLOPOS] = versionlo;
smallrec[DATAPOS] = data;
ret = BIO_write(next, smallrec, MIN_RECORD_LEN);
if (ret <= 0)
abort();
written++;
}
/*
* We can't fragment anything after the CCS, otherwise we
* get a bad record MAC
*/
if (contenttype == SSL3_RT_CHANGE_CIPHER_SPEC) {
fragment = 0;
break;
}
}
}
/* Write any data we have left after fragmenting */
ret = 0;
if ((int)written < inl) {
ret = BIO_write(next, in + written , inl - written);
}
if (ret <= 0 && BIO_should_write(next))
BIO_set_retry_write(bio);
else
ret += written;
} else {
ctrs->wctr++;
BIO_set_retry_write(bio);
}
return ret;
}
static long async_ctrl(BIO *bio, int cmd, long num, void *ptr)
{
long ret;
BIO *next = BIO_next(bio);
if (next == NULL)
return 0;
switch (cmd) {
case BIO_CTRL_DUP:
ret = 0L;
break;
default:
ret = BIO_ctrl(next, cmd, num, ptr);
break;
}
return ret;
}
static int async_gets(BIO *bio, char *buf, int size)
{
/* We don't support this - not needed anyway */
return -1;
}
static int async_puts(BIO *bio, const char *str)
{
return async_write(bio, str, strlen(str));
}
int main(int argc, char *argv[])
{
SSL_CTX *serverctx = NULL, *clientctx = NULL;
SSL *serverssl = NULL, *clientssl = NULL;
BIO *s_to_c_fbio = NULL, *c_to_s_fbio = NULL;
int test, err = 1;
CRYPTO_set_mem_debug(1);
CRYPTO_mem_ctrl(CRYPTO_MEM_CHECK_ON);
if (argc != 3) {
printf("Invalid argument count\n");
goto end;
}
if (!create_ssl_ctx_pair(TLS_server_method(), TLS_client_method(),
&serverctx, &clientctx, argv[1], argv[2])) {
printf("Failed to create SSL_CTX pair\n");
goto end;
}
/*
* We do 2 test runs. The first time around we just do a normal handshake
* with lots of async io going on. The second time around we also break up
* all records so that the content is only one byte length (up until the
* CCS)
*/
for (test = 1; test < 3; test++) {
if (test == 2)
fragment = 1;
s_to_c_fbio = BIO_new(bio_f_async_filter());
c_to_s_fbio = BIO_new(bio_f_async_filter());
if (s_to_c_fbio == NULL || c_to_s_fbio == NULL) {
printf("Failed to create filter BIOs\n");
BIO_free(s_to_c_fbio);
BIO_free(c_to_s_fbio);
goto end;
}
/* BIOs get freed on error */
if (!create_ssl_objects(serverctx, clientctx, &serverssl, &clientssl,
s_to_c_fbio, c_to_s_fbio)) {
printf("Test %d failed: Create SSL objects failed\n", test);
goto end;
}
if (!create_ssl_connection(serverssl, clientssl)) {
printf("Test %d failed: Create SSL connection failed\n", test);
goto end;
}
/* Also frees the BIOs */
SSL_free(clientssl);
SSL_free(serverssl);
clientssl = serverssl = NULL;
}
printf("Test success\n");
err = 0;
end:
if (err)
ERR_print_errors_fp(stderr);
SSL_free(clientssl);
SSL_free(serverssl);
SSL_CTX_free(clientctx);
SSL_CTX_free(serverctx);
# ifndef OPENSSL_NO_CRYPTO_MDEBUG
CRYPTO_mem_leaks_fp(stderr);
# endif
return err;
}
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