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
exptest.c
/*
* Copyright 1995-2016 The OpenSSL Project Authors. All Rights Reserved.
*
* Licensed under the OpenSSL license (the "License"). You may not use
* this file except in compliance with the License. You can obtain a copy
* in the file LICENSE in the source distribution or at
* https://www.openssl.org/source/license.html
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "../e_os.h"
#include <openssl/bio.h>
#include <openssl/bn.h>
#include <openssl/rand.h>
#include <openssl/err.h>
#define NUM_BITS (BN_BITS2 * 4)
static const char rnd_seed[] =
"string to make the random number generator think it has entropy";
/*
* Test that r == 0 in test_exp_mod_zero(). Returns one on success,
* returns zero and prints debug output otherwise.
*/
static int a_is_zero_mod_one(const char *method, const BIGNUM *r,
const BIGNUM *a) {
if (!BN_is_zero(r)) {
fprintf(stderr, "%s failed:\n", method);
fprintf(stderr, "a ** 0 mod 1 = r (should be 0)\n");
fprintf(stderr, "a = ");
BN_print_fp(stderr, a);
fprintf(stderr, "\nr = ");
BN_print_fp(stderr, r);
fprintf(stderr, "\n");
return 0;
}
return 1;
}
/*
* test_exp_mod_zero tests that x**0 mod 1 == 0. It returns zero on success.
*/
static int test_exp_mod_zero()
{
BIGNUM *a = NULL, *p = NULL, *m = NULL;
BIGNUM *r = NULL;
BN_ULONG one_word = 1;
BN_CTX *ctx = BN_CTX_new();
int ret = 1, failed = 0;
m = BN_new();
if (!m)
goto err;
BN_one(m);
a = BN_new();
if (!a)
goto err;
BN_one(a);
p = BN_new();
if (!p)
goto err;
BN_zero(p);
r = BN_new();
if (!r)
goto err;
if (!BN_rand(a, 1024, BN_RAND_TOP_ONE, BN_RAND_BOTTOM_ANY))
goto err;
if (!BN_mod_exp(r, a, p, m, ctx))
goto err;
if (!a_is_zero_mod_one("BN_mod_exp", r, a))
failed = 1;
if (!BN_mod_exp_recp(r, a, p, m, ctx))
goto err;
if (!a_is_zero_mod_one("BN_mod_exp_recp", r, a))
failed = 1;
if (!BN_mod_exp_simple(r, a, p, m, ctx))
goto err;
if (!a_is_zero_mod_one("BN_mod_exp_simple", r, a))
failed = 1;
if (!BN_mod_exp_mont(r, a, p, m, ctx, NULL))
goto err;
if (!a_is_zero_mod_one("BN_mod_exp_mont", r, a))
failed = 1;
if (!BN_mod_exp_mont_consttime(r, a, p, m, ctx, NULL)) {
goto err;
}
if (!a_is_zero_mod_one("BN_mod_exp_mont_consttime", r, a))
failed = 1;
/*
* A different codepath exists for single word multiplication
* in non-constant-time only.
*/
if (!BN_mod_exp_mont_word(r, one_word, p, m, ctx, NULL))
goto err;
if (!BN_is_zero(r)) {
fprintf(stderr, "BN_mod_exp_mont_word failed:\n");
fprintf(stderr, "1 ** 0 mod 1 = r (should be 0)\n");
fprintf(stderr, "r = ");
BN_print_fp(stderr, r);
fprintf(stderr, "\n");
return 0;
}
ret = failed;
err:
BN_free(r);
BN_free(a);
BN_free(p);
BN_free(m);
BN_CTX_free(ctx);
return ret;
}
int main(int argc, char *argv[])
{
BN_CTX *ctx;
BIO *out = NULL;
int i, ret;
unsigned char c;
BIGNUM *r_mont, *r_mont_const, *r_recp, *r_simple, *a, *b, *m;
RAND_seed(rnd_seed, sizeof rnd_seed); /* or BN_rand may fail, and we
* don't even check its return
* value (which we should) */
ctx = BN_CTX_new();
if (ctx == NULL)
EXIT(1);
r_mont = BN_new();
r_mont_const = BN_new();
r_recp = BN_new();
r_simple = BN_new();
a = BN_new();
b = BN_new();
m = BN_new();
if ((r_mont == NULL) || (r_recp == NULL) || (a == NULL) || (b == NULL))
goto err;
out = BIO_new(BIO_s_file());
if (out == NULL)
EXIT(1);
BIO_set_fp(out, stdout, BIO_NOCLOSE | BIO_FP_TEXT);
for (i = 0; i < 200; i++) {
RAND_bytes(&c, 1);
c = (c % BN_BITS) - BN_BITS2;
BN_rand(a, NUM_BITS + c, BN_RAND_TOP_ONE, BN_RAND_BOTTOM_ANY);
RAND_bytes(&c, 1);
c = (c % BN_BITS) - BN_BITS2;
BN_rand(b, NUM_BITS + c, BN_RAND_TOP_ONE, BN_RAND_BOTTOM_ANY);
RAND_bytes(&c, 1);
c = (c % BN_BITS) - BN_BITS2;
BN_rand(m, NUM_BITS + c, BN_RAND_TOP_ONE, BN_RAND_BOTTOM_ODD);
BN_mod(a, a, m, ctx);
BN_mod(b, b, m, ctx);
ret = BN_mod_exp_mont(r_mont, a, b, m, ctx, NULL);
if (ret <= 0) {
printf("BN_mod_exp_mont() problems\n");
ERR_print_errors(out);
EXIT(1);
}
ret = BN_mod_exp_recp(r_recp, a, b, m, ctx);
if (ret <= 0) {
printf("BN_mod_exp_recp() problems\n");
ERR_print_errors(out);
EXIT(1);
}
ret = BN_mod_exp_simple(r_simple, a, b, m, ctx);
if (ret <= 0) {
printf("BN_mod_exp_simple() problems\n");
ERR_print_errors(out);
EXIT(1);
}
ret = BN_mod_exp_mont_consttime(r_mont_const, a, b, m, ctx, NULL);
if (ret <= 0) {
printf("BN_mod_exp_mont_consttime() problems\n");
ERR_print_errors(out);
EXIT(1);
}
if (BN_cmp(r_simple, r_mont) == 0
&& BN_cmp(r_simple, r_recp) == 0
&& BN_cmp(r_simple, r_mont_const) == 0) {
printf(".");
fflush(stdout);
} else {
if (BN_cmp(r_simple, r_mont) != 0)
printf("\nsimple and mont results differ\n");
if (BN_cmp(r_simple, r_mont_const) != 0)
printf("\nsimple and mont const time results differ\n");
if (BN_cmp(r_simple, r_recp) != 0)
printf("\nsimple and recp results differ\n");
printf("a (%3d) = ", BN_num_bits(a));
BN_print(out, a);
printf("\nb (%3d) = ", BN_num_bits(b));
BN_print(out, b);
printf("\nm (%3d) = ", BN_num_bits(m));
BN_print(out, m);
printf("\nsimple =");
BN_print(out, r_simple);
printf("\nrecp =");
BN_print(out, r_recp);
printf("\nmont =");
BN_print(out, r_mont);
printf("\nmont_ct =");
BN_print(out, r_mont_const);
printf("\n");
EXIT(1);
}
}
BN_free(r_mont);
BN_free(r_mont_const);
BN_free(r_recp);
BN_free(r_simple);
BN_free(a);
BN_free(b);
BN_free(m);
BN_CTX_free(ctx);
if (test_exp_mod_zero() != 0)
goto err;
#ifndef OPENSSL_NO_CRYPTO_MDEBUG
if (CRYPTO_mem_leaks(out) <= 0)
goto err;
#endif
BIO_free(out);
printf("\n");
printf("done\n");
EXIT(0);
err:
ERR_print_errors(out);
EXIT(1);
}
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