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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/profile.h>
#include <ipxe/efi/efi.h>
/** @file
*
* EFI entropy source
*
*/
struct entropy_source efitick_entropy __entropy_source ( ENTROPY_FALLBACK );
/** 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;
/* Drop to external TPL to allow timer tick event to take place */
bs->RestoreTPL ( efi_external_tpl );
/* 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;
}
/* We use essentially the same mechanism as for the BIOS
* RTC-based entropy source, and so assume the same
* min-entropy per sample.
*/
entropy_init ( &efitick_entropy, MIN_ENTROPY ( 1.3 ) );
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 );
/* Return to internal TPL */
bs->RaiseTPL ( efi_internal_tpl );
}
/**
* Wait for a timer tick
*
* @ret low CPU profiling low-order bits, or negative error
*/
static int efi_entropy_tick ( void ) {
EFI_BOOT_SERVICES *bs = efi_systab->BootServices;
UINTN index;
uint16_t low;
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 CPU profiling timestamp low-order bits */
low = profile_timestamp();
return low;
}
/**
* Get noise sample from timer ticks
*
* @ret noise Noise sample
* @ret rc Return status code
*/
static int efi_get_noise ( 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;
}
/** EFI entropy source */
struct entropy_source efitick_entropy __entropy_source ( ENTROPY_FALLBACK ) = {
.name = "efitick",
.enable = efi_entropy_enable,
.disable = efi_entropy_disable,
.get_noise = efi_get_noise,
};
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