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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 );
#include <errno.h>
#include <ipxe/entropy.h>
#include <ipxe/crc32.h>
#include <ipxe/efi/efi.h>
#include <ipxe/efi/Protocol/Rng.h>
/** @file
*
* EFI entropy source
*
*/
/** 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 EFI_ENTROPY_RNG_LEN 32
/** Time (in 100ns units) to delay waiting for timer tick
*
* In theory, UEFI allows us to specify a trigger time of zero to
* simply wait for the next timer tick. In practice, specifying zero
* seems to often return immediately, which produces almost no
* entropy. Specify a delay of 1000ns to try to force an existent
* delay.
*/
#define EFI_ENTROPY_TRIGGER_TIME 10
/** Event used to wait for timer tick */
static EFI_EVENT tick;
/**
* Enable entropy gathering
*
* @ret rc Return status code
*/
static int efi_entropy_enable ( void ) {
EFI_BOOT_SERVICES *bs = efi_systab->BootServices;
EFI_STATUS efirc;
int rc;
DBGC ( &tick, "ENTROPY %s RNG protocol\n",
( efirng ? "has" : "has no" ) );
/* Create timer tick event */
if ( ( efirc = bs->CreateEvent ( EVT_TIMER, TPL_NOTIFY, NULL, NULL,
&tick ) ) != 0 ) {
rc = -EEFI ( efirc );
DBGC ( &tick, "ENTROPY could not create event: %s\n",
strerror ( rc ) );
return rc;
}
return 0;
}
/**
* Disable entropy gathering
*
*/
static void efi_entropy_disable ( void ) {
EFI_BOOT_SERVICES *bs = efi_systab->BootServices;
/* Close timer tick event */
bs->CloseEvent ( tick );
}
/**
* Wait for a timer tick
*
* @ret low TSC low-order bits, or negative error
*/
static int efi_entropy_tick ( void ) {
EFI_BOOT_SERVICES *bs = efi_systab->BootServices;
UINTN index;
uint16_t low;
uint32_t discard_d;
EFI_STATUS efirc;
int rc;
/* Wait for next timer tick */
if ( ( efirc = bs->SetTimer ( tick, TimerRelative,
EFI_ENTROPY_TRIGGER_TIME ) ) != 0 ) {
rc = -EEFI ( efirc );
DBGC ( &tick, "ENTROPY could not set timer: %s\n",
strerror ( rc ) );
return rc;
}
if ( ( efirc = bs->WaitForEvent ( 1, &tick, &index ) ) != 0 ) {
rc = -EEFI ( efirc );
DBGC ( &tick, "ENTROPY could not wait for timer tick: %s\n",
strerror ( rc ) );
return rc;
}
/* Get current TSC low-order bits */
__asm__ __volatile__ ( "rdtsc" : "=a" ( low ), "=d" ( discard_d ) );
return low;
}
/**
* Get noise sample from timer ticks
*
* @ret noise Noise sample
* @ret rc Return status code
*/
static int efi_get_noise_ticks ( noise_sample_t *noise ) {
int before;
int after;
int rc;
/* Wait for a timer tick */
before = efi_entropy_tick();
if ( before < 0 ) {
rc = before;
return rc;
}
/* Wait for another timer tick */
after = efi_entropy_tick();
if ( after < 0 ) {
rc = after;
return rc;
}
/* Use TSC delta as noise sample */
*noise = ( after - before );
return 0;
}
/**
* Get noise sample from RNG protocol
*
* @ret noise Noise sample
* @ret rc Return status code
*/
static int efi_get_noise_rng ( noise_sample_t *noise ) {
uint8_t buf[EFI_ENTROPY_RNG_LEN];
EFI_STATUS efirc;
int rc;
/* Fail if we have no EFI RNG protocol */
if ( ! efirng )
return -ENOTSUP;
/* Get the minimum allowed number of random bytes */
if ( ( efirc = efirng->GetRNG ( efirng, NULL, EFI_ENTROPY_RNG_LEN,
buf ) ) != 0 ) {
rc = -EEFI ( efirc );
DBGC ( &tick, "ENTROPY could not read from RNG: %s\n",
strerror ( rc ) );
return rc;
}
/* 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;
}
/**
* Get noise sample
*
* @ret noise Noise sample
* @ret rc Return status code
*/
static int efi_get_noise ( noise_sample_t *noise ) {
int rc;
/* Try RNG first, falling back to timer ticks */
if ( ( ( rc = efi_get_noise_rng ( noise ) ) != 0 ) &&
( ( rc = efi_get_noise_ticks ( noise ) ) != 0 ) )
return rc;
return 0;
}
PROVIDE_ENTROPY_INLINE ( efi, min_entropy_per_sample );
PROVIDE_ENTROPY ( efi, entropy_enable, efi_entropy_enable );
PROVIDE_ENTROPY ( efi, entropy_disable, efi_entropy_disable );
PROVIDE_ENTROPY ( efi, get_noise, efi_get_noise );
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