Commit message: Prevent handshake with unseeded PRNG

Fix security issue where under certain conditions a client can complete a
handshake with an unseeded PRNG. The conditions are:
- Client is on a platform where the PRNG has not been seeded, and the
user has not seeded manually
- A protocol specific client method version has been used (i.e. not
SSL_client_methodv23)
- A ciphersuite is used that does not require additional random data
from the PRNG beyond the initial ClientHello client random
(e.g. PSK-RC4-SHA)

If the handshake succeeds then the client random that has been used will
have been generated from a PRNG with insufficient entropy and therefore
the output may be predictable.

For example using the following command with an unseeded openssl will
succeed on an unpatched platform:

openssl s_client -psk 1a2b3c4d -tls1_2 -cipher PSK-RC4-SHA

CVE-2015-0285

Reviewed-by: Richard Levitte <levitte@openssl.org>
CVE: CVE-2015-0285
CWE: CWE-310
Vulnerability description: The ssl3_client_hello function in s3_clnt.c in OpenSSL 1.0.2 before 1.0.2a does not ensure that the PRNG is seeded before proceeding with a handshake, which makes it easier for remote attackers to defeat cryptographic protection mechanisms by sniffing the network and then conducting a brute-force attack.

Project: openssl

int ssl3_client_hello(SSL *s)
{
    unsigned char *buf;
    unsigned char *p, *d;
    int i;
    unsigned long l;
    int al = 0;
#ifndef OPENSSL_NO_COMP
    int j;
    SSL_COMP *comp;
#endif

    buf = (unsigned char *)s->init_buf->data;
    if (s->state == SSL3_ST_CW_CLNT_HELLO_A) {
        SSL_SESSION *sess = s->session;
        if ((sess == NULL) ||
            (sess->ssl_version != s->version) ||
            !sess->session_id_length || (sess->not_resumable)) {
            if (!ssl_get_new_session(s, 0))
                goto err;
        }
        if (s->method->version == DTLS_ANY_VERSION) {
            /* Determine which DTLS version to use */
            int options = s->options;
            /* If DTLS 1.2 disabled correct the version number */
            if (options & SSL_OP_NO_DTLSv1_2) {
                if (tls1_suiteb(s)) {
                    SSLerr(SSL_F_SSL3_CLIENT_HELLO,
                           SSL_R_ONLY_DTLS_1_2_ALLOWED_IN_SUITEB_MODE);
                    goto err;
                }
                /*
                 * Disabling all versions is silly: return an error.
                 */
                if (options & SSL_OP_NO_DTLSv1) {
                    SSLerr(SSL_F_SSL3_CLIENT_HELLO, SSL_R_WRONG_SSL_VERSION);
                    goto err;
                }
                /*
                 * Update method so we don't use any DTLS 1.2 features.
                 */
                s->method = DTLSv1_client_method();
                s->version = DTLS1_VERSION;
            } else {
                /*
                 * We only support one version: update method
                 */
                if (options & SSL_OP_NO_DTLSv1)
                    s->method = DTLSv1_2_client_method();
                s->version = DTLS1_2_VERSION;
            }
            s->client_version = s->version;
        }
        /* else use the pre-loaded session */

        p = s->s3->client_random;

        /*
         * for DTLS if client_random is initialized, reuse it, we are
         * required to use same upon reply to HelloVerify
         */
        if (SSL_IS_DTLS(s)) {
            size_t idx;
            i = 1;
            for (idx = 0; idx < sizeof(s->s3->client_random); idx++) {
                if (p[idx]) {
                    i = 0;
                    break;
                }
            }
         } else
             i = 1;
 
        if (i)
            ssl_fill_hello_random(s, 0, p, sizeof(s->s3->client_random));
 
         /* Do the message type and length last */
         d = p = ssl_handshake_start(s);
        /*-
         * version indicates the negotiated version: for example from
         * an SSLv2/v3 compatible client hello). The client_version
         * field is the maximum version we permit and it is also
         * used in RSA encrypted premaster secrets. Some servers can
         * choke if we initially report a higher version then
         * renegotiate to a lower one in the premaster secret. This
         * didn't happen with TLS 1.0 as most servers supported it
         * but it can with TLS 1.1 or later if the server only supports
         * 1.0.
         *
         * Possible scenario with previous logic:
         *      1. Client hello indicates TLS 1.2
         *      2. Server hello says TLS 1.0
         *      3. RSA encrypted premaster secret uses 1.2.
         *      4. Handhaked proceeds using TLS 1.0.
         *      5. Server sends hello request to renegotiate.
         *      6. Client hello indicates TLS v1.0 as we now
         *         know that is maximum server supports.
         *      7. Server chokes on RSA encrypted premaster secret
         *         containing version 1.0.
         *
         * For interoperability it should be OK to always use the
         * maximum version we support in client hello and then rely
         * on the checking of version to ensure the servers isn't
         * being inconsistent: for example initially negotiating with
         * TLS 1.0 and renegotiating with TLS 1.2. We do this by using
         * client_version in client hello and not resetting it to
         * the negotiated version.
         */
        *(p++) = s->client_version >> 8;
        *(p++) = s->client_version & 0xff;

        /* Random stuff */
        memcpy(p, s->s3->client_random, SSL3_RANDOM_SIZE);
        p += SSL3_RANDOM_SIZE;

        /* Session ID */
        if (s->new_session)
            i = 0;
        else
            i = s->session->session_id_length;
        *(p++) = i;
        if (i != 0) {
            if (i > (int)sizeof(s->session->session_id)) {
                SSLerr(SSL_F_SSL3_CLIENT_HELLO, ERR_R_INTERNAL_ERROR);
                goto err;
            }
            memcpy(p, s->session->session_id, i);
            p += i;
        }

        /* cookie stuff for DTLS */
        if (SSL_IS_DTLS(s)) {
            if (s->d1->cookie_len > sizeof(s->d1->cookie)) {
                SSLerr(SSL_F_SSL3_CLIENT_HELLO, ERR_R_INTERNAL_ERROR);
                goto err;
            }
            *(p++) = s->d1->cookie_len;
            memcpy(p, s->d1->cookie, s->d1->cookie_len);
            p += s->d1->cookie_len;
        }

        /* Ciphers supported */
        i = ssl_cipher_list_to_bytes(s, SSL_get_ciphers(s), &(p[2]), 0);
        if (i == 0) {
            SSLerr(SSL_F_SSL3_CLIENT_HELLO, SSL_R_NO_CIPHERS_AVAILABLE);
            goto err;
        }
#ifdef OPENSSL_MAX_TLS1_2_CIPHER_LENGTH
        /*
         * Some servers hang if client hello > 256 bytes as hack workaround
         * chop number of supported ciphers to keep it well below this if we
         * use TLS v1.2
         */
        if (TLS1_get_version(s) >= TLS1_2_VERSION
            && i > OPENSSL_MAX_TLS1_2_CIPHER_LENGTH)
            i = OPENSSL_MAX_TLS1_2_CIPHER_LENGTH & ~1;
#endif
        s2n(i, p);
        p += i;

        /* COMPRESSION */
#ifdef OPENSSL_NO_COMP
        *(p++) = 1;
#else

        if (!ssl_allow_compression(s) || !s->ctx->comp_methods)
            j = 0;
        else
            j = sk_SSL_COMP_num(s->ctx->comp_methods);
        *(p++) = 1 + j;
        for (i = 0; i < j; i++) {
            comp = sk_SSL_COMP_value(s->ctx->comp_methods, i);
            *(p++) = comp->id;
        }
#endif
        *(p++) = 0;             /* Add the NULL method */

#ifndef OPENSSL_NO_TLSEXT
        /* TLS extensions */
        if (ssl_prepare_clienthello_tlsext(s) <= 0) {
            SSLerr(SSL_F_SSL3_CLIENT_HELLO, SSL_R_CLIENTHELLO_TLSEXT);
            goto err;
        }
        if ((p =
             ssl_add_clienthello_tlsext(s, p, buf + SSL3_RT_MAX_PLAIN_LENGTH,
                                        &al)) == NULL) {
            ssl3_send_alert(s, SSL3_AL_FATAL, al);
            SSLerr(SSL_F_SSL3_CLIENT_HELLO, ERR_R_INTERNAL_ERROR);
            goto err;
        }
#endif

        l = p - d;
        ssl_set_handshake_header(s, SSL3_MT_CLIENT_HELLO, l);
        s->state = SSL3_ST_CW_CLNT_HELLO_B;
    }

    /* SSL3_ST_CW_CLNT_HELLO_B */
    return ssl_do_write(s);
 err:
    return (-1);
}