rdrand.c

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/*
 * Copyright (C) 2023 Michael Brown <mbrown@fensystems.co.uk>.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License as
 * published by the Free Software Foundation; either version 2 of the
 * License, or any later version.
 *
 * This program is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
 * 02110-1301, USA.
 *
 * You can also choose to distribute this program under the terms of
 * the Unmodified Binary Distribution Licence (as given in the file
 * COPYING.UBDL), provided that you have satisfied its requirements.
 */

FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
FILE_SECBOOT ( PERMITTED );

/** @file
 *
 * Hardware random number generator
 *
 */

#include <errno.h>
#include <ipxe/cpuid.h>
#include <ipxe/entropy.h>
#include <ipxe/drbg.h>

struct entropy_source rdrand_entropy __entropy_source ( ENTROPY_PREFERRED );

/** Number of times to retry RDRAND instruction */
#define RDRAND_RETRY_COUNT 16

/** Colour for debug messages */
#define colour &rdrand_entropy

/**
 * Enable entropy gathering
 *
 * @ret rc              Return status code
 */
static int rdrand_entropy_enable ( void ) {
        struct x86_features features;

        /* Check that RDRAND is supported */
        x86_features ( &features );
        if ( ! ( features.intel.ecx & CPUID_FEATURES_INTEL_ECX_RDRAND ) ) {
                DBGC ( colour, "RDRAND not supported\n" );
                return -ENOTSUP;
        }

        /* Data returned by RDRAND is theoretically full entropy, up
         * to a security strength of 128 bits, so assume that each
         * sample contains exactly 8 bits of entropy.
         */
        if ( DRBG_SECURITY_STRENGTH > 128 )
                return -ENOTSUP;
        entropy_init ( &rdrand_entropy, MIN_ENTROPY ( 8.0 ) );

        return 0;
}

/**
 * Get noise sample
 *
 * @ret noise           Noise sample
 * @ret rc              Return status code
 */
static int rdrand_get_noise ( noise_sample_t *noise ) {
        unsigned int result;
        unsigned int discard_c;
        unsigned int ok;

        /* Issue RDRAND, retrying until CF is set */
        __asm__ ( "\n1:\n\t"
                  "rdrand %0\n\t"
                  "sbb %1, %1\n\t"
                  "loopz 1b\n\t"
                  : "=r" ( result ), "=r" ( ok ), "=c" ( discard_c )
                  : "2" ( RDRAND_RETRY_COUNT ) );
        if ( ! ok ) {
                DBGC ( colour, "RDRAND failed to become ready\n" );
                return -EBUSY;
        }

        *noise = result;
        return 0;
}

/** Hardware random number generator entropy source */
struct entropy_source rdrand_entropy __entropy_source ( ENTROPY_PREFERRED ) = {
        .name = "rdrand",
        .enable = rdrand_entropy_enable,
        .get_noise = rdrand_get_noise,
};