Fix build_principal memory bug [CVE-2015-2697]

In build_principal_va(), use k5memdup0() instead of strdup() to make a
copy of the realm, to ensure that we allocate the correct number of
bytes and do not read past the end of the input string.  This bug
affects krb5_build_principal(), krb5_build_principal_va(), and
krb5_build_principal_alloc_va().  krb5_build_principal_ext() is not
affected.

CVE-2015-2697:

In MIT krb5 1.7 and later, an authenticated attacker may be able to
cause a KDC to crash using a TGS request with a large realm field
beginning with a null byte.  If the KDC attempts to find a referral to
answer the request, it constructs a principal name for lookup using
krb5_build_principal() with the requested realm.  Due to a bug in this
function, the null byte causes only one byte be allocated for the
realm field of the constructed principal, far less than its length.
Subsequent operations on the lookup principal may cause a read beyond
the end of the mapped memory region, causing the KDC process to crash.

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

ticket: 8252 (new)
target_version: 1.14
tags: pullup
krb5_build_principal(krb5_context context,
                     krb5_principal * princ,
                     unsigned int rlen,
                     const char * realm, ...)
{
    krb5_error_code retval = 0;
    va_list ap;

    va_start(ap, realm);
    retval = krb5_build_principal_alloc_va(context, princ, rlen, realm, ap);
    va_end(ap);

    return retval;
}

krb5_build_principal(krb5_context context,
                     krb5_principal * princ,
                     unsigned int rlen,
                     const char * realm, ...)
{
    krb5_error_code retval = 0;
    va_list ap;

    va_start(ap, realm);
    retval = krb5_build_principal_alloc_va(context, princ, rlen, realm, ap);
    va_end(ap);

    return retval;
}
