Commit message: Fix krb5_read_message handling [CVE-2014-5355]

In recvauth_common, do not use strcmp against the data fields of
krb5_data objects populated by krb5_read_message(), as there is no
guarantee that they are C strings.  Instead, create an expected
krb5_data value and use data_eq().

In the sample user-to-user server application, check that the received
client principal name is null-terminated before using it with printf
and krb5_parse_name.

CVE-2014-5355:

In MIT krb5, when a server process uses the krb5_recvauth function, an
unauthenticated remote attacker can cause a NULL dereference by
sending a zero-byte version string, or a read beyond the end of
allocated storage by sending a non-null-terminated version string.
The example user-to-user server application (uuserver) is similarly
vulnerable to a zero-length or non-null-terminated principal name
string.

The krb5_recvauth function reads two version strings from the client
using krb5_read_message(), which produces a krb5_data structure
containing a length and a pointer to an octet sequence.  krb5_recvauth
assumes that the data pointer is a valid C string and passes it to
strcmp() to verify the versions.  If the client sends an empty octet
sequence, the data pointer will be NULL and strcmp() will dereference
a NULL pointer, causing the process to crash.  If the client sends a
non-null-terminated octet sequence, strcmp() will read beyond the end
of the allocated storage, possibly causing the process to crash.

uuserver similarly uses krb5_read_message() to read a client principal
name, and then passes it to printf() and krb5_parse_name() without
verifying that it is a valid C string.

The krb5_recvauth function is used by kpropd and the Kerberized
versions of the BSD rlogin and rsh daemons.  These daemons are usually
run out of inetd or in a mode which forks before processing incoming
connections, so a process crash will generally not result in a
complete denial of service.

Thanks to Tim Uglow for discovering this issue.

CVSSv2: AV:N/AC:L/Au:N/C:N/I:N/A:P/E:POC/RL:OF/RC:C

[tlyu@mit.edu: CVSS score]

ticket: 8050 (new)
target_version: 1.13.1
tags: pullup
CVE: CVE-2014-5355
CWE: CWE-703
Vulnerability description: MIT Kerberos 5 (aka krb5) through 1.13.1 incorrectly expects that a krb5_read_message data field is represented as a string ending with a '\0' character, which allows remote attackers to (1) cause a denial of service (NULL pointer dereference) via a zero-byte version string or (2) cause a denial of service (out-of-bounds read) by omitting the '\0' character, related to appl/user_user/server.c and lib/krb5/krb/recvauth.c.

Project: krb5
File: recvauth.c

recvauth_common(krb5_context context,
                krb5_auth_context * auth_context,
                /* IN */
                krb5_pointer fd,
                char *appl_version,
                krb5_principal server,
                krb5_int32 flags,
                krb5_keytab keytab,
                /* OUT */
                krb5_ticket ** ticket,
                krb5_data *version)
{
    krb5_auth_context     new_auth_context;
    krb5_flags            ap_option = 0;
    krb5_error_code       retval, problem;
    krb5_data             inbuf;
    krb5_data             outbuf;
     krb5_rcache           rcache = 0;
     krb5_octet            response;
     krb5_data             null_server;
     int                   need_error_free = 0;
     int                   local_rcache = 0, local_authcon = 0;
 
    /*
     * Zero out problem variable.  If problem is set at the end of
     * the intial version negotiation section, it means that we
     * need to send an error code back to the client application
     * and exit.
     */
    problem = 0;
    response = 0;

    if (!(flags & KRB5_RECVAUTH_SKIP_VERSION)) {
        /*
         * First read the sendauth version string and check it.
          */
         if ((retval = krb5_read_message(context, fd, &inbuf)))
             return(retval);
        if (strcmp(inbuf.data, sendauth_version)) {
             problem = KRB5_SENDAUTH_BADAUTHVERS;
             response = 1;
         }
        free(inbuf.data);
    }
    if (flags & KRB5_RECVAUTH_BADAUTHVERS) {
        problem = KRB5_SENDAUTH_BADAUTHVERS;
        response = 1;
    }

    /*
     * Do the same thing for the application version string.
      */
     if ((retval = krb5_read_message(context, fd, &inbuf)))
         return(retval);
    if (appl_version && strcmp(inbuf.data, appl_version)) {
        if (!problem) {
             problem = KRB5_SENDAUTH_BADAPPLVERS;
             response = 2;
         }
    }
    if (version && !problem)
        *version = inbuf;
    else
        free(inbuf.data);

    /*
     * Now we actually write the response.  If the response is non-zero,
     * exit with a return value of problem
     */
    if ((krb5_net_write(context, *((int *)fd), (char *)&response, 1)) < 0) {
        return(problem); /* We'll return the top-level problem */
    }
    if (problem)
        return(problem);

    /* We are clear of errors here */

    /*
     * Now, let's read the AP_REQ message and decode it
     */
    if ((retval = krb5_read_message(context, fd, &inbuf)))
        return retval;

    if (*auth_context == NULL) {
        problem = krb5_auth_con_init(context, &new_auth_context);
        *auth_context = new_auth_context;
        local_authcon = 1;
    }
    krb5_auth_con_getrcache(context, *auth_context, &rcache);
    if ((!problem) && rcache == NULL) {
        /*
         * Setup the replay cache.
         */
        if (server != NULL && server->length > 0) {
            problem = krb5_get_server_rcache(context, &server->data[0],
                                             &rcache);
        } else {
            null_server.length = 7;
            null_server.data = "default";
            problem = krb5_get_server_rcache(context, &null_server, &rcache);
        }
        if (!problem)
            problem = krb5_auth_con_setrcache(context, *auth_context, rcache);
        local_rcache = 1;
    }
    if (!problem) {
        problem = krb5_rd_req(context, auth_context, &inbuf, server,
                              keytab, &ap_option, ticket);
        free(inbuf.data);
    }

    /*
     * If there was a problem, send back a krb5_error message,
     * preceeded by the length of the krb5_error message.  If
     * everything's ok, send back 0 for the length.
     */
    if (problem) {
        krb5_error      error;
        const   char *message;

        memset(&error, 0, sizeof(error));
        krb5_us_timeofday(context, &error.stime, &error.susec);
        if(server)
            error.server = server;
        else {
            /* If this fails - ie. ENOMEM we are hosed
               we cannot even send the error if we wanted to... */
            (void) krb5_parse_name(context, "????", &error.server);
            need_error_free = 1;
        }

        error.error = problem - ERROR_TABLE_BASE_krb5;
        if (error.error > 127)
            error.error = KRB_ERR_GENERIC;
        message = error_message(problem);
        error.text.length  = strlen(message) + 1;
        error.text.data = strdup(message);
        if (!error.text.data) {
            retval = ENOMEM;
            goto cleanup;
        }
        if ((retval = krb5_mk_error(context, &error, &outbuf))) {
            free(error.text.data);
            goto cleanup;
        }
        free(error.text.data);
        if(need_error_free)
            krb5_free_principal(context, error.server);

    } else {
        outbuf.length = 0;
        outbuf.data = 0;
    }

    retval = krb5_write_message(context, fd, &outbuf);
    if (outbuf.data) {
        free(outbuf.data);
        /* We sent back an error, we need cleanup then return */
        retval = problem;
        goto cleanup;
    }
    if (retval)
        goto cleanup;

    /* Here lies the mutual authentication stuff... */
    if ((ap_option & AP_OPTS_MUTUAL_REQUIRED)) {
        if ((retval = krb5_mk_rep(context, *auth_context, &outbuf))) {
            return(retval);
        }
        retval = krb5_write_message(context, fd, &outbuf);
        free(outbuf.data);
    }

cleanup:;
    if (retval) {
        if (local_authcon) {
            krb5_auth_con_free(context, *auth_context);
        } else if (local_rcache && rcache != NULL) {
            krb5_rc_close(context, rcache);
            krb5_auth_con_setrcache(context, *auth_context, NULL);
        }
    }
    return retval;
}