EFI entropy source. More...

#include <errno.h>
#include <ipxe/entropy.h>
#include <ipxe/profile.h>
#include <ipxe/efi/efi.h>

Macros
#define	EFI_ENTROPY_TRIGGER_TIME 10
	Time (in 100ns units) to delay waiting for timer tick. More...

Functions
	FILE_LICENCE (GPL2_OR_LATER_OR_UBDL)

struct entropy_source efitick_entropy	__entropy_source (ENTROPY_FALLBACK)
	EFI entropy source. More...

static int	efi_entropy_enable (void)
	Enable entropy gathering. More...

static void	efi_entropy_disable (void)
	Disable entropy gathering. More...

static int	efi_entropy_tick (void)
	Wait for a timer tick. More...

static int	efi_get_noise (noise_sample_t *noise)
	Get noise sample from timer ticks. More...

Variables
static EFI_EVENT	tick
	Event used to wait for timer tick. More...

Detailed Description

EFI entropy source.

Definition in file efi_entropy.c.

Macro Definition Documentation

◆ EFI_ENTROPY_TRIGGER_TIME

#define EFI_ENTROPY_TRIGGER_TIME 10

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.

Definition at line 47 of file efi_entropy.c.

Function Documentation

◆ FILE_LICENCE()

FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL )

◆ __entropy_source()

struct entropy_source efitick_entropy __entropy_source ( ENTROPY_FALLBACK )

Initial value:

= {
        .name = "efitick",
        .enable = efi_entropy_enable,
        .disable = efi_entropy_disable,
        .get_noise = efi_get_noise,
}

EFI entropy source.

◆ efi_entropy_enable()

static int efi_entropy_enable ( void )

static

Enable entropy gathering.

Return values

rc	Return status code

Definition at line 57 of file efi_entropy.c.

                                        {
         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;
 }

References EFI_SYSTEM_TABLE::BootServices, EFI_BOOT_SERVICES::CreateEvent, DBGC, EEFI, efi_external_tpl, efi_systab, entropy_init(), EVT_TIMER, MIN_ENTROPY, NULL, rc, EFI_BOOT_SERVICES::RestoreTPL, strerror(), tick, and TPL_NOTIFY.

◆ efi_entropy_disable()

static void efi_entropy_disable ( void )

static

Disable entropy gathering.

Definition at line 87 of file efi_entropy.c.

                                          {
         EFI_BOOT_SERVICES *bs = efi_systab->BootServices;
 
         /* Close timer tick event */
         bs->CloseEvent ( tick );
 
         /* Return to internal TPL */
         bs->RaiseTPL ( efi_internal_tpl );
 }

References EFI_SYSTEM_TABLE::BootServices, EFI_BOOT_SERVICES::CloseEvent, efi_internal_tpl, efi_systab, EFI_BOOT_SERVICES::RaiseTPL, and tick.

◆ efi_entropy_tick()

static int efi_entropy_tick ( void )

static

Wait for a timer tick.

Return values

low	CPU profiling low-order bits, or negative error

Definition at line 102 of file efi_entropy.c.

                                      {
         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;
 }

References EFI_SYSTEM_TABLE::BootServices, DBGC, EEFI, EFI_ENTROPY_TRIGGER_TIME, efi_systab, index, low, rc, EFI_BOOT_SERVICES::SetTimer, strerror(), tick, TimerRelative, and EFI_BOOT_SERVICES::WaitForEvent.

Referenced by efi_get_noise().

◆ efi_get_noise()

static int efi_get_noise ( noise_sample_t * noise )

static

Get noise sample from timer ticks.

Return values

noise	Noise sample
rc	Return status code

Definition at line 136 of file efi_entropy.c.

                                                    {
         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;
 }