Commit message: Don't allow too many consecutive warning alerts

Certain warning alerts are ignored if they are received. This can mean that
no progress will be made if one peer continually sends those warning alerts.
Implement a count so that we abort the connection if we receive too many.

Issue reported by Shi Lei.

Reviewed-by: Rich Salz <rsalz@openssl.org>
CVE: CVE-2016-8610
CWE: CWE-400
Vulnerability description: A denial of service flaw was found in OpenSSL 0.9.8, 1.0.1, 1.0.2 through 1.0.2h, and 1.1.0 in the way the TLS/SSL protocol defined processing of ALERT packets during a connection handshake. A remote attacker could use this flaw to make a TLS/SSL server consume an excessive amount of CPU and fail to accept connections from other clients.

Project: openssl

int dtls1_read_bytes(SSL *s, int type, int *recvd_type, unsigned char *buf,
                     int len, int peek)
{
    int al, i, j, ret;
    unsigned int n;
    SSL3_RECORD *rr;
    void (*cb) (const SSL *ssl, int type2, int val) = NULL;

    if (!SSL3_BUFFER_is_initialised(&s->rlayer.rbuf)) {
        /* Not initialized yet */
        if (!ssl3_setup_buffers(s))
            return (-1);
    }

    if ((type && (type != SSL3_RT_APPLICATION_DATA) &&
         (type != SSL3_RT_HANDSHAKE)) ||
        (peek && (type != SSL3_RT_APPLICATION_DATA))) {
        SSLerr(SSL_F_DTLS1_READ_BYTES, ERR_R_INTERNAL_ERROR);
        return -1;
    }

    /*
     * check whether there's a handshake message (client hello?) waiting
     */
    if ((ret = have_handshake_fragment(s, type, buf, len)))
        return ret;

    /*
     * Now s->rlayer.d->handshake_fragment_len == 0 if
     * type == SSL3_RT_HANDSHAKE.
     */

#ifndef OPENSSL_NO_SCTP
    /*
     * Continue handshake if it had to be interrupted to read app data with
     * SCTP.
     */
    if ((!ossl_statem_get_in_handshake(s) && SSL_in_init(s)) ||
        (BIO_dgram_is_sctp(SSL_get_rbio(s))
         && ossl_statem_in_sctp_read_sock(s)
         && s->s3->in_read_app_data != 2))
#else
    if (!ossl_statem_get_in_handshake(s) && SSL_in_init(s))
#endif
    {
        /* type == SSL3_RT_APPLICATION_DATA */
        i = s->handshake_func(s);
        if (i < 0)
            return (i);
        if (i == 0) {
            SSLerr(SSL_F_DTLS1_READ_BYTES, SSL_R_SSL_HANDSHAKE_FAILURE);
            return (-1);
        }
    }

 start:
    s->rwstate = SSL_NOTHING;

    /*-
     * s->s3->rrec.type         - is the type of record
     * s->s3->rrec.data,    - data
     * s->s3->rrec.off,     - offset into 'data' for next read
     * s->s3->rrec.length,  - number of bytes.
     */
    rr = s->rlayer.rrec;

    /*
     * We are not handshaking and have no data yet, so process data buffered
     * during the last handshake in advance, if any.
     */
    if (SSL_is_init_finished(s) && SSL3_RECORD_get_length(rr) == 0) {
        pitem *item;
        item = pqueue_pop(s->rlayer.d->buffered_app_data.q);
        if (item) {
#ifndef OPENSSL_NO_SCTP
            /* Restore bio_dgram_sctp_rcvinfo struct */
            if (BIO_dgram_is_sctp(SSL_get_rbio(s))) {
                DTLS1_RECORD_DATA *rdata = (DTLS1_RECORD_DATA *)item->data;
                BIO_ctrl(SSL_get_rbio(s), BIO_CTRL_DGRAM_SCTP_SET_RCVINFO,
                         sizeof(rdata->recordinfo), &rdata->recordinfo);
            }
#endif

            dtls1_copy_record(s, item);

            OPENSSL_free(item->data);
            pitem_free(item);
        }
    }

    /* Check for timeout */
    if (dtls1_handle_timeout(s) > 0)
        goto start;

    /* get new packet if necessary */
    if ((SSL3_RECORD_get_length(rr) == 0)
        || (s->rlayer.rstate == SSL_ST_READ_BODY)) {
        ret = dtls1_get_record(s);
        if (ret <= 0) {
            ret = dtls1_read_failed(s, ret);
            /* anything other than a timeout is an error */
            if (ret <= 0)
                return (ret);
            else
                goto start;
         }
     }
 
    /*
     * Reset the count of consecutive warning alerts if we've got a non-empty
     * record that isn't an alert.
     */
    if (SSL3_RECORD_get_type(rr) != SSL3_RT_ALERT
            && SSL3_RECORD_get_length(rr) != 0)
        s->rlayer.alert_count = 0;

     /* we now have a packet which can be read and processed */
 
     if (s->s3->change_cipher_spec /* set when we receive ChangeCipherSpec,
                                SSL3_RECORD_get_seq_num(rr)) < 0) {
            SSLerr(SSL_F_DTLS1_READ_BYTES, ERR_R_INTERNAL_ERROR);
            return -1;
        }
        SSL3_RECORD_set_length(rr, 0);
        goto start;
    }