int ssl3_send_client_verify(SSL *s)
{
    unsigned char *p;
    unsigned char data[MD5_DIGEST_LENGTH + SHA_DIGEST_LENGTH];
    EVP_PKEY *pkey;
    EVP_PKEY_CTX *pctx = NULL;
    EVP_MD_CTX mctx;
    unsigned u = 0;
    unsigned long n;
    int j;

    EVP_MD_CTX_init(&mctx);

    if (s->state == SSL3_ST_CW_CERT_VRFY_A) {
        p = ssl_handshake_start(s);
        pkey = s->cert->key->privatekey;
/* Create context from key and test if sha1 is allowed as digest */
        pctx = EVP_PKEY_CTX_new(pkey, NULL);
        EVP_PKEY_sign_init(pctx);
        if (EVP_PKEY_CTX_set_signature_md(pctx, EVP_sha1()) > 0) {
            if (!SSL_USE_SIGALGS(s))
                s->method->ssl3_enc->cert_verify_mac(s,
                                                     NID_sha1,
                                                     &(data
                                                       [MD5_DIGEST_LENGTH]));
        } else {
            ERR_clear_error();
        }
        /*
         * For TLS v1.2 send signature algorithm and signature using agreed
         * digest and cached handshake records.
         */
        if (SSL_USE_SIGALGS(s)) {
            long hdatalen = 0;
            void *hdata;
            const EVP_MD *md = s->s3->tmp.md[s->cert->key - s->cert->pkeys];
            hdatalen = BIO_get_mem_data(s->s3->handshake_buffer, &hdata);
            if (hdatalen <= 0 || !tls12_get_sigandhash(p, pkey, md)) {
                SSLerr(SSL_F_SSL3_SEND_CLIENT_VERIFY, ERR_R_INTERNAL_ERROR);
                goto err;
            }
            p += 2;
#ifdef SSL_DEBUG
            fprintf(stderr, "Using TLS 1.2 with client alg %s\n",
                    EVP_MD_name(md));
#endif
            if (!EVP_SignInit_ex(&mctx, md, NULL)
                || !EVP_SignUpdate(&mctx, hdata, hdatalen)
                || !EVP_SignFinal(&mctx, p + 2, &u, pkey)) {
                SSLerr(SSL_F_SSL3_SEND_CLIENT_VERIFY, ERR_R_EVP_LIB);
                goto err;
            }
            s2n(u, p);
            n = u + 4;
            /*
             * For extended master secret we've already digested cached
             * records.
             */
            if (s->session->flags & SSL_SESS_FLAG_EXTMS) {
                BIO_free(s->s3->handshake_buffer);
                s->s3->handshake_buffer = NULL;
                s->s3->flags &= ~TLS1_FLAGS_KEEP_HANDSHAKE;
            } else if (!ssl3_digest_cached_records(s))
                goto err;
        } else
#ifndef OPENSSL_NO_RSA
        if (pkey->type == EVP_PKEY_RSA) {
            s->method->ssl3_enc->cert_verify_mac(s, NID_md5, &(data[0]));
            if (RSA_sign(NID_md5_sha1, data,
                         MD5_DIGEST_LENGTH + SHA_DIGEST_LENGTH,
                         &(p[2]), &u, pkey->pkey.rsa) <= 0) {
                SSLerr(SSL_F_SSL3_SEND_CLIENT_VERIFY, ERR_R_RSA_LIB);
                goto err;
            }
            s2n(u, p);
            n = u + 2;
        } else
#endif
#ifndef OPENSSL_NO_DSA
        if (pkey->type == EVP_PKEY_DSA) {
            if (!DSA_sign(pkey->save_type,
                          &(data[MD5_DIGEST_LENGTH]),
                          SHA_DIGEST_LENGTH, &(p[2]),
                          (unsigned int *)&j, pkey->pkey.dsa)) {
                SSLerr(SSL_F_SSL3_SEND_CLIENT_VERIFY, ERR_R_DSA_LIB);
                goto err;
            }
            s2n(j, p);
            n = j + 2;
        } else
#endif
#ifndef OPENSSL_NO_EC
        if (pkey->type == EVP_PKEY_EC) {
            if (!ECDSA_sign(pkey->save_type,
                            &(data[MD5_DIGEST_LENGTH]),
                            SHA_DIGEST_LENGTH, &(p[2]),
                            (unsigned int *)&j, pkey->pkey.ec)) {
                SSLerr(SSL_F_SSL3_SEND_CLIENT_VERIFY, ERR_R_ECDSA_LIB);
                goto err;
            }
            s2n(j, p);
            n = j + 2;
        } else
#endif
        if (pkey->type == NID_id_GostR3410_94
                || pkey->type == NID_id_GostR3410_2001) {
            unsigned char signbuf[64];
            int i;
            size_t sigsize = 64;
            s->method->ssl3_enc->cert_verify_mac(s,
                                                 NID_id_GostR3411_94, data);
            if (EVP_PKEY_sign(pctx, signbuf, &sigsize, data, 32) <= 0) {
                SSLerr(SSL_F_SSL3_SEND_CLIENT_VERIFY, ERR_R_INTERNAL_ERROR);
                goto err;
            }
            for (i = 63, j = 0; i >= 0; j++, i--) {
                p[2 + j] = signbuf[i];
            }
            s2n(j, p);
            n = j + 2;
        } else {
            SSLerr(SSL_F_SSL3_SEND_CLIENT_VERIFY, ERR_R_INTERNAL_ERROR);
            goto err;
        }
        if (!ssl_set_handshake_header(s, SSL3_MT_CERTIFICATE_VERIFY, n)) {
            SSLerr(SSL_F_SSL3_SEND_CLIENT_VERIFY, ERR_R_INTERNAL_ERROR);
            goto err;
        }
        s->state = SSL3_ST_CW_CERT_VRFY_B;
    }
    EVP_MD_CTX_cleanup(&mctx);
    EVP_PKEY_CTX_free(pctx);
    return ssl_do_write(s);
 err:
    EVP_MD_CTX_cleanup(&mctx);
    EVP_PKEY_CTX_free(pctx);
    s->state = SSL_ST_ERR;
    return (-1);
}

The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.