efi_rng.c

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/*
 * Copyright (C) 2015 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 (at your option) 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 );

#include <errno.h>
#include <ipxe/entropy.h>
#include <ipxe/crc32.h>
#include <ipxe/efi/efi.h>
#include <ipxe/efi/Protocol/Rng.h>

/** @file
 *
 * EFI random number generator protocol entropy source
 *
 */

struct entropy_source efirng_entropy __entropy_source ( ENTROPY_NORMAL );

/** Random number generator protocol */
static EFI_RNG_PROTOCOL *efirng;
EFI_REQUEST_PROTOCOL ( EFI_RNG_PROTOCOL, &efirng );

/** Minimum number of bytes to request from RNG
 *
 * The UEFI spec states (for no apparently good reason) that "When a
 * Deterministic Random Bit Generator (DRBG) is used on the output of
 * a (raw) entropy source, its security level must be at least 256
 * bits."  The EDK2 codebase (mis)interprets this to mean that the
 * call to GetRNG() should fail if given a buffer less than 32 bytes.
 *
 * Incidentally, nothing in the EFI RNG protocol provides any way to
 * report the actual amount of entropy returned by GetRNG().
 */
#define EFIRNG_LEN 32

/** Maximum number of times to attempting requesting data from RNG
 *
 * The UEFI spec allows GetRNG() to return EFI_NOT_READY, which is not
 * a particularly helpful error status since there is nothing that can
 * sensibly be done except to retry immediately.  We retry failed
 * calls to GetRNG() (for any reason) up to this number of times.
 */
#define EFIRNG_MAX_RETRY 16

/**
 * Enable entropy gathering
 *
 * @ret rc              Return status code
 */
static int efirng_enable ( void ) {

        /* Check for RNG protocol support */
        if ( ! efirng ) {
                DBGC ( &efirng, "EFIRNG has no RNG protocol\n" );
                return -ENOTSUP;
        }

        /* Nothing in the EFI specification provides any clue as to
         * how much entropy will be returned by GetRNG().  Make a
         * totally uninformed (and conservative guess) that each
         * sample will contain at least one bit of entropy.
         */
        entropy_init ( &efirng_entropy, MIN_ENTROPY ( 1.0 ) );

        return 0;
}

/**
 * Get noise sample from RNG protocol
 *
 * @ret noise           Noise sample
 * @ret rc              Return status code
 */
static int efirng_get_noise ( noise_sample_t *noise ) {
        uint8_t buf[EFIRNG_LEN];
        unsigned int i;
        EFI_STATUS efirc;
        int rc;

        /* Sanity check */
        assert ( efirng != NULL );

        /* Get random bytes, retrying if needed */
        for ( i = 0 ; i < EFIRNG_MAX_RETRY ; i++ ) {

                /* Get the minimum allowed number of random bytes */
                if ( ( efirc = efirng->GetRNG ( efirng, NULL, sizeof ( buf ),
                                                buf ) ) != 0 ) {
                        rc = -EEFI ( efirc );
                        continue;
                }

                /* Reduce random bytes to a single noise sample.  This
                 * seems like overkill, but we have no way of knowing
                 * how much entropy is actually present in the bytes
                 * returned by the RNG protocol.
                 */
                *noise = crc32_le ( 0, buf, sizeof ( buf ) );
                return 0;
        }

        DBGC ( &efirng, "ENTROPY could not read from RNG: %s\n",
               strerror ( rc ) );
        return rc;
}

/** EFI random number generator protocol entropy source */
struct entropy_source efirng_entropy __entropy_source ( ENTROPY_NORMAL ) = {
        .name = "efirng",
        .enable = efirng_enable,
        .get_noise = efirng_get_noise,
};