int ssl3_get_cert_verify(SSL *s)
{
    EVP_PKEY *pkey = NULL;
    unsigned char *p;
    int al, ok, ret = 0;
    long n;
    int type = 0, i, j;
    X509 *peer;
    const EVP_MD *md = NULL;
    EVP_MD_CTX mctx;
    EVP_MD_CTX_init(&mctx);

    /*
     * We should only process a CertificateVerify message if we have received
     * a Certificate from the client. If so then |s->session->peer| will be non
     * NULL. In some instances a CertificateVerify message is not required even
     * if the peer has sent a Certificate (e.g. such as in the case of static
     * DH). In that case the ClientKeyExchange processing will skip the
     * CertificateVerify state so we should not arrive here.
     */
    if (s->session->peer == NULL) {
        ret = 1;
        goto end;
    }

    n = s->method->ssl_get_message(s,
                                   SSL3_ST_SR_CERT_VRFY_A,
                                   SSL3_ST_SR_CERT_VRFY_B,
                                   SSL3_MT_CERTIFICATE_VERIFY,
                                   SSL3_RT_MAX_PLAIN_LENGTH, &ok);

    if (!ok)
        return ((int)n);

    peer = s->session->peer;
    pkey = X509_get_pubkey(peer);
    type = X509_certificate_type(peer, pkey);

    if (!(type & EVP_PKT_SIGN)) {
        SSLerr(SSL_F_SSL3_GET_CERT_VERIFY,
               SSL_R_SIGNATURE_FOR_NON_SIGNING_CERTIFICATE);
        al = SSL_AD_ILLEGAL_PARAMETER;
        goto f_err;
    }

    /* we now have a signature that we need to verify */
    p = (unsigned char *)s->init_msg;
    /* Check for broken implementations of GOST ciphersuites */
    /*
     * If key is GOST and n is exactly 64, it is bare signature without
     * length field
     */
    if (n == 64 && (pkey->type == NID_id_GostR3410_94 ||
                    pkey->type == NID_id_GostR3410_2001)) {
        i = 64;
    } else {
        if (SSL_USE_SIGALGS(s)) {
            int rv = tls12_check_peer_sigalg(&md, s, p, pkey);
            if (rv == -1) {
                al = SSL_AD_INTERNAL_ERROR;
                goto f_err;
            } else if (rv == 0) {
                al = SSL_AD_DECODE_ERROR;
                goto f_err;
            }
#ifdef SSL_DEBUG
            fprintf(stderr, "USING TLSv1.2 HASH %s\n", EVP_MD_name(md));
#endif
            p += 2;
            n -= 2;
        }
        n2s(p, i);
        n -= 2;
        if (i > n) {
            SSLerr(SSL_F_SSL3_GET_CERT_VERIFY, SSL_R_LENGTH_MISMATCH);
            al = SSL_AD_DECODE_ERROR;
            goto f_err;
        }
    }
    j = EVP_PKEY_size(pkey);
    if ((i > j) || (n > j) || (n <= 0)) {
        SSLerr(SSL_F_SSL3_GET_CERT_VERIFY, SSL_R_WRONG_SIGNATURE_SIZE);
        al = SSL_AD_DECODE_ERROR;
        goto f_err;
    }

    if (SSL_USE_SIGALGS(s)) {
        long hdatalen = 0;
        void *hdata;
        hdatalen = BIO_get_mem_data(s->s3->handshake_buffer, &hdata);
        if (hdatalen <= 0) {
            SSLerr(SSL_F_SSL3_GET_CERT_VERIFY, ERR_R_INTERNAL_ERROR);
            al = SSL_AD_INTERNAL_ERROR;
            goto f_err;
        }
#ifdef SSL_DEBUG
        fprintf(stderr, "Using TLS 1.2 with client verify alg %s\n",
                EVP_MD_name(md));
#endif
        if (!EVP_VerifyInit_ex(&mctx, md, NULL)
            || !EVP_VerifyUpdate(&mctx, hdata, hdatalen)) {
            SSLerr(SSL_F_SSL3_GET_CERT_VERIFY, ERR_R_EVP_LIB);
            al = SSL_AD_INTERNAL_ERROR;
            goto f_err;
        }

        if (EVP_VerifyFinal(&mctx, p, i, pkey) <= 0) {
            al = SSL_AD_DECRYPT_ERROR;
            SSLerr(SSL_F_SSL3_GET_CERT_VERIFY, SSL_R_BAD_SIGNATURE);
            goto f_err;
        }
    } else
#ifndef OPENSSL_NO_RSA
    if (pkey->type == EVP_PKEY_RSA) {
        i = RSA_verify(NID_md5_sha1, s->s3->tmp.cert_verify_md,
                       MD5_DIGEST_LENGTH + SHA_DIGEST_LENGTH, p, i,
                       pkey->pkey.rsa);
        if (i < 0) {
            al = SSL_AD_DECRYPT_ERROR;
            SSLerr(SSL_F_SSL3_GET_CERT_VERIFY, SSL_R_BAD_RSA_DECRYPT);
            goto f_err;
        }
        if (i == 0) {
            al = SSL_AD_DECRYPT_ERROR;
            SSLerr(SSL_F_SSL3_GET_CERT_VERIFY, SSL_R_BAD_RSA_SIGNATURE);
            goto f_err;
        }
    } else
#endif
#ifndef OPENSSL_NO_DSA
    if (pkey->type == EVP_PKEY_DSA) {
        j = DSA_verify(pkey->save_type,
                       &(s->s3->tmp.cert_verify_md[MD5_DIGEST_LENGTH]),
                       SHA_DIGEST_LENGTH, p, i, pkey->pkey.dsa);
        if (j <= 0) {
            /* bad signature */
            al = SSL_AD_DECRYPT_ERROR;
            SSLerr(SSL_F_SSL3_GET_CERT_VERIFY, SSL_R_BAD_DSA_SIGNATURE);
            goto f_err;
        }
    } else
#endif
#ifndef OPENSSL_NO_ECDSA
    if (pkey->type == EVP_PKEY_EC) {
        j = ECDSA_verify(pkey->save_type,
                         &(s->s3->tmp.cert_verify_md[MD5_DIGEST_LENGTH]),
                         SHA_DIGEST_LENGTH, p, i, pkey->pkey.ec);
        if (j <= 0) {
            /* bad signature */
            al = SSL_AD_DECRYPT_ERROR;
            SSLerr(SSL_F_SSL3_GET_CERT_VERIFY, SSL_R_BAD_ECDSA_SIGNATURE);
            goto f_err;
        }
    } else
#endif
    if (pkey->type == NID_id_GostR3410_94
            || pkey->type == NID_id_GostR3410_2001) {
        unsigned char signature[64];
        int idx;
        EVP_PKEY_CTX *pctx = EVP_PKEY_CTX_new(pkey, NULL);
        EVP_PKEY_verify_init(pctx);
        if (i != 64) {
            fprintf(stderr, "GOST signature length is %d", i);
        }
        for (idx = 0; idx < 64; idx++) {
            signature[63 - idx] = p[idx];
        }
        j = EVP_PKEY_verify(pctx, signature, 64, s->s3->tmp.cert_verify_md,
                            32);
        EVP_PKEY_CTX_free(pctx);
        if (j <= 0) {
            al = SSL_AD_DECRYPT_ERROR;
            SSLerr(SSL_F_SSL3_GET_CERT_VERIFY, SSL_R_BAD_ECDSA_SIGNATURE);
            goto f_err;
        }
    } else {
        SSLerr(SSL_F_SSL3_GET_CERT_VERIFY, ERR_R_INTERNAL_ERROR);
        al = SSL_AD_UNSUPPORTED_CERTIFICATE;
        goto f_err;
    }

    ret = 1;
    if (0) {
 f_err:
        ssl3_send_alert(s, SSL3_AL_FATAL, al);
        s->state = SSL_ST_ERR;
    }
 end:
    if (s->s3->handshake_buffer) {
        BIO_free(s->s3->handshake_buffer);
        s->s3->handshake_buffer = NULL;
        s->s3->flags &= ~TLS1_FLAGS_KEEP_HANDSHAKE;
    }
    EVP_MD_CTX_cleanup(&mctx);
    EVP_PKEY_free(pkey);
    return (ret);
}

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.