[efi] Generalise EFI entropy generation to non-x86 CPUs

Signed-off-by: Michael Brown <mcb30@ipxe.org>

1 files changed, 223 insertions, 0 deletions

diff --git a/src/interface/efi/efi_entropy.c b/src/interface/efi/efi_entropy.c
new file mode 100644
index 00000000..881c4c9a
--- /dev/null
+++ b/src/interface/efi/efi_entropy.c
@@ -0,0 +1,223 @@
+/*
+ * 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/profile.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		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_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 );