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[efi] Use the EFI_RNG_PROTOCOL as an entropy source if available

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Entropy gathering via timer ticks is slow under UEFI (of the order of
20-30 seconds on some machines).  Use the EFI_RNG_PROTOCOL if
available, to speed up the process of entropy gathering.

Note that some implementations (including EDK2) will fail if we
request fewer than 32 random bytes at a time, and that the RNG
protocol provides no guarantees about the amount of entropy provided
by a call to GetRNG().  We take the (hopefully pessimistic) view that
a 32-byte block returned by GetRNG() will contain at least the 1.3
bits of entropy claimed by min_entropy_per_sample().

Signed-off-by: Michael Brown <mcb30@ipxe.org>
This commit is contained in:
Michael Brown
2015-04-14 17:53:13 +01:00
parent c6c7e78c42
commit 7ca801d637
2 changed files with 234 additions and 3 deletions
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79
src/arch/x86/interface/efi/efi_entropy.c
View File

@@ -25,7 +25,9 @@ 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
*
@@ -33,6 +35,23 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
*
*/
/** 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
@@ -56,6 +75,9 @@ static int efi_entropy_enable ( void ) {
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 ) {
@@ -80,7 +102,7 @@ static void efi_entropy_disable ( void ) {
}
/**
* Wait for an RTC tick
* Wait for a timer tick
*
* @ret low TSC low-order bits, or negative error
*/
@@ -114,12 +136,12 @@ static int efi_entropy_tick ( void ) {
}
/**
* Get noise sample
* Get noise sample from timer ticks
*
* @ret noise Noise sample
* @ret rc Return status code
*/
static int efi_get_noise ( noise_sample_t *noise ) {
static int efi_get_noise_ticks ( noise_sample_t *noise ) {
int before;
int after;
int rc;
@@ -144,6 +166,57 @@ static int efi_get_noise ( noise_sample_t *noise ) {
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 );