src/arch/x86/interface/efi/efi_entropy.c

   1 /*
   2  * Copyright (C) 2015 Michael Brown <mbrown@fensystems.co.uk>.
   3  *
   4  * This program is free software; you can redistribute it and/or
   5  * modify it under the terms of the GNU General Public License as
   6  * published by the Free Software Foundation; either version 2 of the
   7  * License, or (at your option) any later version.
   8  *
   9  * This program is distributed in the hope that it will be useful, but
  10  * WITHOUT ANY WARRANTY; without even the implied warranty of
  11  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12  * General Public License for more details.
  13  *
  14  * You should have received a copy of the GNU General Public License
  15  * along with this program; if not, write to the Free Software
  16  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
  17  * 02110-1301, USA.
  18  *
  19  * You can also choose to distribute this program under the terms of
  20  * the Unmodified Binary Distribution Licence (as given in the file
  21  * COPYING.UBDL), provided that you have satisfied its requirements.
  22  */
  23
  24 FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
  25
  26 #include <errno.h>
  27 #include <ipxe/entropy.h>
  28 #include <ipxe/efi/efi.h>
  29
  30 /** @file
  31  *
  32  * EFI entropy source
  33  *
  34  */
  35
  36 /** Time (in 100ns units) to delay waiting for timer tick
  37  *
  38  * In theory, UEFI allows us to specify a trigger time of zero to
  39  * simply wait for the next timer tick.  In practice, specifying zero
  40  * seems to often return immediately, which produces almost no
  41  * entropy.  Specify a delay of 1000ns to try to force an existent
  42  * delay.
  43  */
  44 #define EFI_ENTROPY_TRIGGER_TIME 10
  45
  46 /** Event used to wait for timer tick */
  47 static EFI_EVENT tick;
  48
  49 /**
  50  * Enable entropy gathering
  51  *
  52  * @ret rc              Return status code
  53  */
  54 static int efi_entropy_enable ( void ) {
  55         EFI_BOOT_SERVICES *bs = efi_systab->BootServices;
  56         EFI_STATUS efirc;
  57         int rc;
  58
  59         /* Create timer tick event */
  60         if ( ( efirc = bs->CreateEvent ( EVT_TIMER, TPL_NOTIFY, NULL, NULL,
  61                                          &tick ) ) != 0 ) {
  62                 rc = -EEFI ( efirc );
  63                 DBGC ( &tick, "ENTROPY could not create event: %s\n",
  64                        strerror ( rc ) );
  65                 return rc;
  66         }
  67
  68         return 0;
  69 }
  70
  71 /**
  72  * Disable entropy gathering
  73  *
  74  */
  75 static void efi_entropy_disable ( void ) {
  76         EFI_BOOT_SERVICES *bs = efi_systab->BootServices;
  77
  78         /* Close timer tick event */
  79         bs->CloseEvent ( tick );
  80 }
  81
  82 /**
  83  * Wait for an RTC tick
  84  *
  85  * @ret low             TSC low-order bits, or negative error
  86  */
  87 static int efi_entropy_tick ( void ) {
  88         EFI_BOOT_SERVICES *bs = efi_systab->BootServices;
  89         UINTN index;
  90         uint16_t low;
  91         uint32_t discard_d;
  92         EFI_STATUS efirc;
  93         int rc;
  94
  95         /* Wait for next timer tick */
  96         if ( ( efirc = bs->SetTimer ( tick, TimerRelative,
  97                                       EFI_ENTROPY_TRIGGER_TIME ) ) != 0 ) {
  98                 rc = -EEFI ( efirc );
  99                 DBGC ( &tick, "ENTROPY could not set timer: %s\n",
 100                        strerror ( rc ) );
 101                 return rc;
 102         }
 103         if ( ( efirc = bs->WaitForEvent ( 1, &tick, &index ) ) != 0 ) {
 104                 rc = -EEFI ( efirc );
 105                 DBGC ( &tick, "ENTROPY could not wait for timer tick: %s\n",
 106                        strerror ( rc ) );
 107                 return rc;
 108         }
 109
 110         /* Get current TSC low-order bits */
 111         __asm__ __volatile__ ( "rdtsc" : "=a" ( low ), "=d" ( discard_d ) );
 112
 113         return low;
 114 }
 115
 116 /**
 117  * Get noise sample
 118  *
 119  * @ret noise           Noise sample
 120  * @ret rc              Return status code
 121  */
 122 static int efi_get_noise ( noise_sample_t *noise ) {
 123         int before;
 124         int after;
 125         int rc;
 126
 127         /* Wait for a timer tick */
 128         before = efi_entropy_tick();
 129         if ( before < 0 ) {
 130                 rc = before;
 131                 return rc;
 132         }
 133
 134         /* Wait for another timer tick */
 135         after = efi_entropy_tick();
 136         if ( after < 0 ) {
 137                 rc = after;
 138                 return rc;
 139         }
 140
 141         /* Use TSC delta as noise sample */
 142         *noise = ( after - before );
 143
 144         return 0;
 145 }
 146
 147 PROVIDE_ENTROPY_INLINE ( efi, min_entropy_per_sample );
 148 PROVIDE_ENTROPY ( efi, entropy_enable, efi_entropy_enable );
 149 PROVIDE_ENTROPY ( efi, entropy_disable, efi_entropy_disable );
 150 PROVIDE_ENTROPY ( efi, get_noise, efi_get_noise );