[efi] Split out EFI_RNG_PROTOCOL as a separate entropy source

Commit 7ca801d ("[efi] Use the EFI_RNG_PROTOCOL as an entropy source
if available") added EFI_RNG_PROTOCOL as an alternative entropy source
via an ad-hoc mechanism specific to efi_entropy.c.

Split out EFI_RNG_PROTOCOL to a separate entropy source, and allow the
entropy core to handle the selection of RDRAND, EFI_RNG_PROTOCOL, or
timer ticks as the active source.

The fault detection logic added in commit a87537d ("[efi] Detect and
disable seriously broken EFI_RNG_PROTOCOL implementations") may be
removed completely, since the failure will already be detected by the
generic ANS X9.82-mandated repetition count test and will now be
handled gracefully by the entropy core.

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

src/interface/efi/efi_rng.c

@@ -0,0 +1,118 @@
+/*
+ * 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/efi/efi.h>
+#include <ipxe/efi/Protocol/Rng.h>
+
+/** @file
+ *
+ * EFI random number generator protocol entropy source
+ *
+ */
+
+struct entropy_source efirng_entropy __entropy_source ( ENTROPY_NORMAL );
+
+/** 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 EFIRNG_LEN 32
+
+/**
+ * Enable entropy gathering
+ *
+ * @ret rc		Return status code
+ */
+static int efirng_enable ( void ) {
+
+	/* Check for RNG protocol support */
+	if ( ! efirng ) {
+		DBGC ( &efirng, "EFIRNG has no RNG protocol\n" );
+		return -ENOTSUP;
+	}
+
+	/* Nothing in the EFI specification provides any clue as to
+	 * how much entropy will be returned by GetRNG().  Make a
+	 * totally uninformed (and conservative guess) that each
+	 * sample will contain at least one bit of entropy.
+	 */
+	entropy_init ( &efirng_entropy, MIN_ENTROPY ( 1.0 ) );
+
+	return 0;
+}
+
+/**
+ * Get noise sample from RNG protocol
+ *
+ * @ret noise		Noise sample
+ * @ret rc		Return status code
+ */
+static int efirng_get_noise ( noise_sample_t *noise ) {
+	uint8_t buf[EFIRNG_LEN];
+	EFI_STATUS efirc;
+	int rc;
+
+	/* Sanity check */
+	assert ( efirng != NULL );
+
+	/* Get the minimum allowed number of random bytes */
+	if ( ( efirc = efirng->GetRNG ( efirng, NULL, sizeof ( buf ),
+					buf ) ) != 0 ) {
+		rc = -EEFI ( efirc );
+		DBGC ( &efirng, "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;
+}
+
+/** EFI random number generator protocol entropy source */
+struct entropy_source efirng_entropy __entropy_source ( ENTROPY_NORMAL ) = {
+	.name = "efirng",
+	.enable = efirng_enable,
+	.get_noise = efirng_get_noise,
+};