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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
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>
1 parent 5dfd038
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  • c0c9b13
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  • test
  • /
  • enginetest.c
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enginetest.c
/*
 * Copyright 2000-2016 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 <string.h>
#include <openssl/e_os2.h>

#ifdef OPENSSL_NO_ENGINE
int main(int argc, char *argv[])
{
    printf("No ENGINE support\n");
    return (0);
}
#else
# include <openssl/buffer.h>
# include <openssl/crypto.h>
# include <openssl/engine.h>
# include <openssl/err.h>

static void display_engine_list(void)
{
    ENGINE *h;
    int loop;

    h = ENGINE_get_first();
    loop = 0;
    printf("listing available engine types\n");
    while (h) {
        printf("engine %i, id = \"%s\", name = \"%s\"\n",
               loop++, ENGINE_get_id(h), ENGINE_get_name(h));
        h = ENGINE_get_next(h);
    }
    printf("end of list\n");
    /*
     * ENGINE_get_first() increases the struct_ref counter, so we must call
     * ENGINE_free() to decrease it again
     */
    ENGINE_free(h);
}

int main(int argc, char *argv[])
{
    ENGINE *block[512];
    char buf[256];
    const char *id, *name, *p;
    ENGINE *ptr;
    int loop;
    int to_return = 1;
    ENGINE *new_h1 = NULL;
    ENGINE *new_h2 = NULL;
    ENGINE *new_h3 = NULL;
    ENGINE *new_h4 = NULL;

    p = getenv("OPENSSL_DEBUG_MEMORY");
    if (p != NULL && strcmp(p, "on") == 0)
        CRYPTO_set_mem_debug(1);

    memset(block, 0, sizeof(block));
    if (((new_h1 = ENGINE_new()) == NULL) ||
        !ENGINE_set_id(new_h1, "test_id0") ||
        !ENGINE_set_name(new_h1, "First test item") ||
        ((new_h2 = ENGINE_new()) == NULL) ||
        !ENGINE_set_id(new_h2, "test_id1") ||
        !ENGINE_set_name(new_h2, "Second test item") ||
        ((new_h3 = ENGINE_new()) == NULL) ||
        !ENGINE_set_id(new_h3, "test_id2") ||
        !ENGINE_set_name(new_h3, "Third test item") ||
        ((new_h4 = ENGINE_new()) == NULL) ||
        !ENGINE_set_id(new_h4, "test_id3") ||
        !ENGINE_set_name(new_h4, "Fourth test item")) {
        printf("Couldn't set up test ENGINE structures\n");
        goto end;
    }
    printf("\nenginetest beginning\n\n");
    display_engine_list();
    if (!ENGINE_add(new_h1)) {
        printf("Add failed!\n");
        goto end;
    }
    display_engine_list();
    ptr = ENGINE_get_first();
    if (!ENGINE_remove(ptr)) {
        printf("Remove failed!\n");
        goto end;
    }
    ENGINE_free(ptr);
    display_engine_list();
    if (!ENGINE_add(new_h3) || !ENGINE_add(new_h2)) {
        printf("Add failed!\n");
        goto end;
    }
    display_engine_list();
    if (!ENGINE_remove(new_h2)) {
        printf("Remove failed!\n");
        goto end;
    }
    display_engine_list();
    if (!ENGINE_add(new_h4)) {
        printf("Add failed!\n");
        goto end;
    }
    display_engine_list();
    if (ENGINE_add(new_h3)) {
        printf("Add *should* have failed but didn't!\n");
        goto end;
    } else
        printf("Add that should fail did.\n");
    ERR_clear_error();
    if (ENGINE_remove(new_h2)) {
        printf("Remove *should* have failed but didn't!\n");
        goto end;
    } else
        printf("Remove that should fail did.\n");
    ERR_clear_error();
    if (!ENGINE_remove(new_h3)) {
        printf("Remove failed!\n");
        goto end;
    }
    display_engine_list();
    if (!ENGINE_remove(new_h4)) {
        printf("Remove failed!\n");
        goto end;
    }
    display_engine_list();
    /*
     * Depending on whether there's any hardware support compiled in, this
     * remove may be destined to fail.
     */
    ptr = ENGINE_get_first();
    if (ptr)
        if (!ENGINE_remove(ptr))
            printf("Remove failed!i - probably no hardware "
                   "support present.\n");
    ENGINE_free(ptr);
    display_engine_list();
    if (!ENGINE_add(new_h1) || !ENGINE_remove(new_h1)) {
        printf("Couldn't add and remove to an empty list!\n");
        goto end;
    } else
        printf("Successfully added and removed to an empty list!\n");
    printf("About to beef up the engine-type list\n");
    for (loop = 0; loop < 512; loop++) {
        sprintf(buf, "id%i", loop);
        id = OPENSSL_strdup(buf);
        sprintf(buf, "Fake engine type %i", loop);
        name = OPENSSL_strdup(buf);
        if (((block[loop] = ENGINE_new()) == NULL) ||
            !ENGINE_set_id(block[loop], id) ||
            !ENGINE_set_name(block[loop], name)) {
            printf("Couldn't create block of ENGINE structures.\n"
                   "I'll probably also core-dump now, damn.\n");
            goto end;
        }
    }
    for (loop = 0; loop < 512; loop++) {
        if (!ENGINE_add(block[loop])) {
            printf("\nAdding stopped at %i, (%s,%s)\n",
                   loop, ENGINE_get_id(block[loop]),
                   ENGINE_get_name(block[loop]));
            goto cleanup_loop;
        } else
            printf(".");
        fflush(stdout);
    }
 cleanup_loop:
    printf("\nAbout to empty the engine-type list\n");
    while ((ptr = ENGINE_get_first()) != NULL) {
        if (!ENGINE_remove(ptr)) {
            printf("\nRemove failed!\n");
            goto end;
        }
        ENGINE_free(ptr);
        printf(".");
        fflush(stdout);
    }
    for (loop = 0; loop < 512; loop++) {
        OPENSSL_free((void *)ENGINE_get_id(block[loop]));
        OPENSSL_free((void *)ENGINE_get_name(block[loop]));
    }
    printf("\nTests completed happily\n");
    to_return = 0;
 end:
    if (to_return)
        ERR_print_errors_fp(stderr);
    ENGINE_free(new_h1);
    ENGINE_free(new_h2);
    ENGINE_free(new_h3);
    ENGINE_free(new_h4);
    for (loop = 0; loop < 512; loop++)
        ENGINE_free(block[loop]);

#ifndef OPENSSL_NO_CRYPTO_MDEBUG
    if (CRYPTO_mem_leaks_fp(stderr) <= 0)
        to_return = 1;
#endif
    return to_return;
}
#endif
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