path: root/firmware/lib/vboot_kernel.c

/* Copyright (c) 2013 The Chromium OS Authors. All rights reserved.
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 *
 * Functions for loading a kernel from disk.
 * (Firmware portion)
 */

#include "2common.h"
#include "2misc.h"
#include "2nvstorage.h"
#include "2packed_key.h"
#include "2secdata.h"
#include "2sysincludes.h"
#include "cgptlib.h"
#include "cgptlib_internal.h"
#include "gpt_misc.h"
#include "load_kernel_fw.h"
#include "vboot_api.h"

enum vb2_load_partition_flags {
	VB2_LOAD_PARTITION_FLAG_VBLOCK_ONLY = (1 << 0),
	VB2_LOAD_PARTITION_FLAG_MINIOS = (1 << 1),
};

#define KBUF_SIZE 65536  /* Bytes to read at start of kernel partition */

/* Minimum context work buffer size needed for vb2_load_partition() */
#define VB2_LOAD_PARTITION_WORKBUF_BYTES	\
	(VB2_VERIFY_KERNEL_PREAMBLE_WORKBUF_BYTES + KBUF_SIZE)

#define LOWEST_TPM_VERSION 0xffffffff

enum vb2_boot_mode {
	/* Normal boot: kernel must be verified. */
	VB2_BOOT_MODE_NORMAL = 0,

	/* Recovery boot, regardless of dev mode state. */
	VB2_BOOT_MODE_RECOVERY = 1,

	/* Developer boot: self-signed kernel okay. */
	VB2_BOOT_MODE_DEVELOPER = 2,
};

/**
 * Return the current boot mode (normal, recovery, or dev).
 *
 * @param ctx          Vboot context
 * @return Current boot mode (see vb2_boot_mode enum).
 */
static enum vb2_boot_mode get_boot_mode(struct vb2_context *ctx)
{
	if (ctx->flags & VB2_CONTEXT_RECOVERY_MODE)
		return VB2_BOOT_MODE_RECOVERY;

	if (ctx->flags & VB2_CONTEXT_DEVELOPER_MODE)
		return VB2_BOOT_MODE_DEVELOPER;

	return VB2_BOOT_MODE_NORMAL;
}

/**
 * Check if a valid keyblock is required.
 *
 * @param ctx		Vboot context
 * @return 1 if valid keyblock required (officially signed kernel);
 *         0 if valid hash is enough (self-signed kernel).
 */
static int need_valid_keyblock(struct vb2_context *ctx)
{
	/* Normal and recovery modes always require official OS */
	if (get_boot_mode(ctx) != VB2_BOOT_MODE_DEVELOPER)
		return 1;

	/* FWMP can require developer mode to use signed kernels */
	if (vb2_secdata_fwmp_get_flag(
		ctx, VB2_SECDATA_FWMP_DEV_ENABLE_OFFICIAL_ONLY))
		return 1;

	/* Developers may require signed kernels */
	if (vb2_nv_get(ctx, VB2_NV_DEV_BOOT_SIGNED_ONLY))
		return 1;

	return 0;
}

/**
 * Return a pointer to the keyblock inside a vblock.
 *
 * Must only be called during or after vb2_verify_kernel_vblock().
 *
 * @param kbuf		Buffer containing vblock
 * @return The keyblock pointer.
 */
static struct vb2_keyblock *get_keyblock(uint8_t *kbuf)
{
	return (struct vb2_keyblock *)kbuf;
}

/**
 * Return a pointer to the kernel preamble inside a vblock.
 *
 * Must only be called during or after vb2_verify_kernel_vblock().
 *
 * @param kbuf		Buffer containing vblock
 * @return The kernel preamble pointer.
 */
static struct vb2_kernel_preamble *get_preamble(uint8_t *kbuf)
{
	return (struct vb2_kernel_preamble *)
			(kbuf + get_keyblock(kbuf)->keyblock_size);
}

/**
 * Return the offset of the kernel body from the start of the vblock.
 *
 * Must only be called during or after vb2_verify_kernel_vblock().
 *
 * @param kbuf		Buffer containing vblock
 * @return The offset of the kernel body from the vblock start, in bytes.
 */
static uint32_t get_body_offset(uint8_t *kbuf)
{
	return (get_keyblock(kbuf)->keyblock_size +
		get_preamble(kbuf)->preamble_size);
}

/**
 * Verify developer mode key hash.
 *
 * @param ctx		Vboot context
 * @param keyblock	Keyblock to verify
 * @return VB2_SUCCESS, or non-zero error code.
 */
static vb2_error_t vb2_verify_kernel_dev_key_hash(
	struct vb2_context *ctx, struct vb2_keyblock *keyblock)
{
	struct vb2_packed_key *key = &keyblock->data_key;
	uint8_t *buf = ((uint8_t *)key) + key->key_offset;
	uint32_t buflen = key->key_size;
	uint8_t digest[VB2_SHA256_DIGEST_SIZE];

	VB2_DEBUG("Checking developer key hash.\n");
	VB2_TRY(vb2_digest_buffer(buf, buflen, VB2_HASH_SHA256, digest,
				  sizeof(digest)));

	uint8_t *fwmp_dev_key_hash =
		vb2_secdata_fwmp_get_dev_key_hash(ctx);
	if (fwmp_dev_key_hash == NULL) {
		VB2_DEBUG("Couldn't retrieve developer key hash.\n");
		return VB2_ERROR_KERNEL_KEYBLOCK_DEV_KEY_HASH;
	}

	if (vb2_safe_memcmp(digest, fwmp_dev_key_hash,
			    VB2_SHA256_DIGEST_SIZE)) {
		int i;

		VB2_DEBUG("Wrong developer key hash.\n");
		VB2_DEBUG("Want: ");
		for (i = 0; i < VB2_SHA256_DIGEST_SIZE; i++)
			VB2_DEBUG_RAW("%02x ", fwmp_dev_key_hash[i]);
		VB2_DEBUG_RAW("\n");
		VB2_DEBUG("Got:  ");
		for (i = 0; i < VB2_SHA256_DIGEST_SIZE; i++)
			VB2_DEBUG_RAW("%02x ", digest[i]);
		VB2_DEBUG_RAW("\n");

		return VB2_ERROR_KERNEL_KEYBLOCK_DEV_KEY_HASH;
	}

	return VB2_SUCCESS;
}

/**
 * Verify a kernel vblock.
 *
 * @param ctx		Vboot context
 * @param kbuf		Buffer containing the vblock
 * @param kbuf_size	Size of the buffer in bytes
 * @param lpflags	Flags (one or more of vb2_load_partition_flags)
 * @param wb		Work buffer.  Must be at least
 *			VB2_VERIFY_KERNEL_PREAMBLE_WORKBUF_BYTES bytes.
 * @return VB2_SUCCESS, or non-zero error code.
 */
static vb2_error_t vb2_verify_kernel_vblock(
	struct vb2_context *ctx, uint8_t *kbuf, uint32_t kbuf_size,
	uint32_t lpflags, struct vb2_workbuf *wb)
{
	struct vb2_shared_data *sd = vb2_get_sd(ctx);

	uint8_t *key_data;
	uint32_t key_size;
	struct vb2_public_key kernel_key;

	int need_keyblock_valid = need_valid_keyblock(ctx);
	int keyblock_valid = 1;  /* Assume valid */

	vb2_error_t rv;

	/* Locate key to verify kernel.  This will either be a recovery key, or
	   a kernel subkey passed from firmware verification. */
	key_data = vb2_member_of(sd, sd->kernel_key_offset);
	key_size = sd->kernel_key_size;
	VB2_TRY(vb2_unpack_key_buffer(&kernel_key, key_data, key_size));

	if (vb2_hwcrypto_allowed(ctx))
		kernel_key.allow_hwcrypto = 1;

	/*
	 * Clear any previous keyblock-valid flag (for example, from a previous
	 * kernel where the keyblock was signed but the preamble failed
	 * verification).
	 */
	sd->flags &= ~VB2_SD_FLAG_KERNEL_SIGNED;

	/* Verify the keyblock. */
	struct vb2_keyblock *keyblock = get_keyblock(kbuf);
	rv = vb2_verify_keyblock(keyblock, kbuf_size, &kernel_key, wb);
	if (rv) {
		VB2_DEBUG("Verifying keyblock signature failed.\n");
		keyblock_valid = 0;

		/* Check if we must have an officially signed kernel */
		if (need_keyblock_valid) {
			VB2_DEBUG("Self-signed kernels not enabled.\n");
			return rv;
		}

		/* Otherwise, allow the kernel if the keyblock hash is valid */
		rv = vb2_verify_keyblock_hash(keyblock, kbuf_size, wb);
		if (rv) {
			VB2_DEBUG("Verifying keyblock hash failed.\n");
			return rv;
		}
	}

	/* Check the keyblock flags against boot flags. */
	if (!(keyblock->keyblock_flags &
	      ((ctx->flags & VB2_CONTEXT_DEVELOPER_MODE) ?
	       VB2_KEYBLOCK_FLAG_DEVELOPER_1 :
	       VB2_KEYBLOCK_FLAG_DEVELOPER_0))) {
		VB2_DEBUG("Keyblock developer flag mismatch.\n");
		keyblock_valid = 0;
		if (need_keyblock_valid)
			return VB2_ERROR_KERNEL_KEYBLOCK_DEV_FLAG;
	}
	if (!(keyblock->keyblock_flags &
	      ((ctx->flags & VB2_CONTEXT_RECOVERY_MODE) ?
	       VB2_KEYBLOCK_FLAG_RECOVERY_1 :
	       VB2_KEYBLOCK_FLAG_RECOVERY_0))) {
		VB2_DEBUG("Keyblock recovery flag mismatch.\n");
		keyblock_valid = 0;
		if (need_keyblock_valid)
			return VB2_ERROR_KERNEL_KEYBLOCK_REC_FLAG;
	}
	if (!(keyblock->keyblock_flags &
	      ((lpflags & VB2_LOAD_PARTITION_FLAG_MINIOS) ?
	       VB2_KEYBLOCK_FLAG_MINIOS_1 :
	       VB2_KEYBLOCK_FLAG_MINIOS_0))) {
		VB2_DEBUG("Keyblock miniOS flag mismatch.\n");
		keyblock_valid = 0;
		if (need_keyblock_valid)
			return VB2_ERROR_KERNEL_KEYBLOCK_MINIOS_FLAG;
	}

	/* Check for rollback of key version except in recovery mode. */
	enum vb2_boot_mode boot_mode = get_boot_mode(ctx);
	uint32_t key_version = keyblock->data_key.key_version;
	if (boot_mode != VB2_BOOT_MODE_RECOVERY) {
		if (key_version < (sd->kernel_version_secdata >> 16)) {
			keyblock_valid = 0;
			if (need_keyblock_valid) {
				VB2_DEBUG("Key version too old.\n");
				return VB2_ERROR_KERNEL_KEYBLOCK_VERSION_ROLLBACK;
			}
		}
		if (key_version > VB2_MAX_KEY_VERSION) {
			/*
			 * Key version is stored in 16 bits in the TPM, so key
			 * versions greater than 0xFFFF can't be stored
			 * properly.
			 */
			VB2_DEBUG("Key version > 0xFFFF.\n");
			keyblock_valid = 0;
			if (need_keyblock_valid)
				return VB2_ERROR_KERNEL_KEYBLOCK_VERSION_RANGE;
		}
	}

	/* If in developer mode and using key hash, check it. */
	if (boot_mode == VB2_BOOT_MODE_DEVELOPER &&
	    vb2_secdata_fwmp_get_flag(ctx, VB2_SECDATA_FWMP_DEV_USE_KEY_HASH)) {
		VB2_TRY(vb2_verify_kernel_dev_key_hash(ctx, keyblock));
	}

	/*
	 * At this point, we've checked everything.  The kernel keyblock is at
	 * least self-consistent, and has either a valid signature or a valid
	 * hash.  Track if it had a valid signature (that is, would we have
	 * been willing to boot it even if developer mode was off).
	 */
	if (keyblock_valid)
		sd->flags |= VB2_SD_FLAG_KERNEL_SIGNED;

	/* Get key for preamble verification from the keyblock. */
	struct vb2_public_key data_key;
	rv = vb2_unpack_key(&data_key, &keyblock->data_key);
	if (rv) {
		VB2_DEBUG("Unable to unpack kernel data key\n");
		return rv;
	}

	/* Verify the preamble, which follows the keyblock */
	struct vb2_kernel_preamble *preamble = get_preamble(kbuf);
	rv = vb2_verify_kernel_preamble(preamble,
					kbuf_size - keyblock->keyblock_size,
					&data_key,
					wb);
	if (rv) {
		VB2_DEBUG("Preamble verification failed.\n");
		return rv;
	}

	/* Rollback check for miniOS */
	if (need_keyblock_valid && (lpflags & VB2_LOAD_PARTITION_FLAG_MINIOS)) {
		if (preamble->kernel_version <
		    (sd->kernel_version_secdata >> 24)) {
			keyblock_valid = 0;
			if (need_keyblock_valid) {
				VB2_DEBUG("miniOS kernel version too old.\n");
				return VB2_ERROR_KERNEL_PREAMBLE_VERSION_ROLLBACK;
			}
		}
		if (preamble->kernel_version > 0xff) {
			/*
			 * Key version is stored in the top 8 bits of 16 bits
			 * in the TPM, so key versions greater than 0xFF can't
			 * be stored properly.
			 */
			VB2_DEBUG("Key version > 0xFF.\n");
			keyblock_valid = 0;
			if (need_keyblock_valid)
				return VB2_ERROR_KERNEL_PREAMBLE_VERSION_RANGE;
		}
	}

	/*
	 * Kernel preamble version is the lower 16 bits of the composite
	 * kernel version.
	 */
	if (preamble->kernel_version > VB2_MAX_PREAMBLE_VERSION)
		return VB2_ERROR_KERNEL_PREAMBLE_VERSION_RANGE;

	/* Combine with the key version. */
	sd->kernel_version = key_version << 16 | preamble->kernel_version;

	/* If not in recovery mode, check for rollback of the kernel version. */
	if (need_keyblock_valid &&
	    boot_mode != VB2_BOOT_MODE_RECOVERY &&
	    sd->kernel_version < sd->kernel_version_secdata) {
		VB2_DEBUG("Kernel version too low.\n");
		return VB2_ERROR_KERNEL_PREAMBLE_VERSION_ROLLBACK;
	}

	VB2_DEBUG("Kernel preamble is good.\n");
	return VB2_SUCCESS;
}

/**
 * Load and verify a partition from the stream.
 *
 * @param ctx		Vboot context
 * @param params	Load-kernel parameters
 * @param stream	Stream to load kernel from
 * @param lpflags	Flags (one or more of vb2_load_partition_flags)
 * @return VB2_SUCCESS, or non-zero error code.
 */
static vb2_error_t vb2_load_partition(
	struct vb2_context *ctx, VbSelectAndLoadKernelParams *params,
	VbExStream_t stream, uint32_t lpflags)
{
	uint32_t read_ms = 0, start_ts;
	struct vb2_workbuf wb;

	vb2_workbuf_from_ctx(ctx, &wb);

	/* Allocate kernel header buffer in workbuf */
	uint8_t *kbuf = vb2_workbuf_alloc(&wb, KBUF_SIZE);
	if (!kbuf)
		return VB2_ERROR_LOAD_PARTITION_WORKBUF;

	start_ts = vb2ex_mtime();
	if (VbExStreamRead(stream, KBUF_SIZE, kbuf)) {
		VB2_DEBUG("Unable to read start of partition.\n");
		return VB2_ERROR_LOAD_PARTITION_READ_VBLOCK;
	}
	read_ms += vb2ex_mtime() - start_ts;

	if (vb2_verify_kernel_vblock(ctx, kbuf, KBUF_SIZE, lpflags, &wb))
		return VB2_ERROR_LOAD_PARTITION_VERIFY_VBLOCK;

	if (lpflags & VB2_LOAD_PARTITION_FLAG_VBLOCK_ONLY)
		return VB2_SUCCESS;

	struct vb2_keyblock *keyblock = get_keyblock(kbuf);
	struct vb2_kernel_preamble *preamble = get_preamble(kbuf);

	/*
	 * Make sure the kernel starts at or before what we already read into
	 * kbuf.
	 *
	 * We could deal with a larger offset by reading and discarding the
	 * data in between the vblock and the kernel data.
	 */
	uint32_t body_offset = get_body_offset(kbuf);
	if (body_offset > KBUF_SIZE) {
		VB2_DEBUG("Kernel body offset is %u > 64KB.\n", body_offset);
		return VB2_ERROR_LOAD_PARTITION_BODY_OFFSET;
	}

	uint8_t *kernbuf = params->kernel_buffer;
	uint32_t kernbuf_size = params->kernel_buffer_size;
	if (!kernbuf) {
		/* Get kernel load address and size from the header. */
		kernbuf = (uint8_t *)((long)preamble->body_load_address);
		kernbuf_size = preamble->body_signature.data_size;
	} else if (preamble->body_signature.data_size > kernbuf_size) {
		VB2_DEBUG("Kernel body doesn't fit in memory.\n");
		return 	VB2_ERROR_LOAD_PARTITION_BODY_SIZE;
	}

	uint32_t body_toread = preamble->body_signature.data_size;
	uint8_t *body_readptr = kernbuf;

	/*
	 * If we've already read part of the kernel, copy that to the beginning
	 * of the kernel buffer.
	 */
	uint32_t body_copied = KBUF_SIZE - body_offset;
	if (body_copied > body_toread)
		body_copied = body_toread;  /* Don't over-copy tiny kernel */
	memcpy(body_readptr, kbuf + body_offset, body_copied);
	body_toread -= body_copied;
	body_readptr += body_copied;

	/* Read the kernel data */
	start_ts = vb2ex_mtime();
	if (body_toread && VbExStreamRead(stream, body_toread, body_readptr)) {
		VB2_DEBUG("Unable to read kernel data.\n");
		return VB2_ERROR_LOAD_PARTITION_READ_BODY;
	}
	read_ms += vb2ex_mtime() - start_ts;
	if (read_ms == 0)  /* Avoid division by 0 in speed calculation */
		read_ms = 1;
	VB2_DEBUG("read %u KB in %u ms at %u KB/s.\n",
		  (body_toread + KBUF_SIZE) / 1024, read_ms,
		  (uint32_t)(((body_toread + KBUF_SIZE) * VB2_MSEC_PER_SEC) /
			     (read_ms * 1024)));

	/* Get key for preamble/data verification from the keyblock. */
	struct vb2_public_key data_key;
	if (vb2_unpack_key(&data_key, &keyblock->data_key)) {
		VB2_DEBUG("Unable to unpack kernel data key\n");
		return VB2_ERROR_LOAD_PARTITION_DATA_KEY;
	}

	if (vb2_hwcrypto_allowed(ctx))
		data_key.allow_hwcrypto = 1;

	/* Verify kernel data */
	if (vb2_verify_data(kernbuf, kernbuf_size, &preamble->body_signature,
			    &data_key, &wb)) {
		VB2_DEBUG("Kernel data verification failed.\n");
		return VB2_ERROR_LOAD_PARTITION_VERIFY_BODY;
	}

	/* If we're still here, the kernel is valid */
	VB2_DEBUG("Partition is good.\n");

	/* Save kernel data back to parameters */
	params->bootloader_address = preamble->bootloader_address;
	params->bootloader_size = preamble->bootloader_size;
	params->flags = vb2_kernel_get_flags(preamble);
	if (!params->kernel_buffer) {
		params->kernel_buffer = kernbuf;
		params->kernel_buffer_size = kernbuf_size;
	}

	return VB2_SUCCESS;
}

static vb2_error_t try_minios_kernel(struct vb2_context *ctx,
				     VbSelectAndLoadKernelParams *params,
				     VbDiskInfo *disk_info,
				     uint64_t sector) {
	VbExStream_t stream;
	uint64_t sectors_left = disk_info->lba_count - sector;
	const uint32_t lpflags = VB2_LOAD_PARTITION_FLAG_MINIOS;
	vb2_error_t rv = VB2_ERROR_LK_NO_KERNEL_FOUND;

	/* Re-open stream at correct offset to pass to vb2_load_partition. */
	if (VbExStreamOpen(params->disk_handle, sector, sectors_left,
			   &stream)) {
		VB2_DEBUG("Unable to open disk handle.\n");
		return rv;
	}

	rv = vb2_load_partition(ctx, params, stream, lpflags);
	VB2_DEBUG("vb2_load_partition returned: %d\n", rv);

	VbExStreamClose(stream);

	if (rv)
		return VB2_ERROR_LK_NO_KERNEL_FOUND;
	return rv;
}

static vb2_error_t try_minios_sectors(struct vb2_context *ctx,
				      VbSelectAndLoadKernelParams *params,
				      VbDiskInfo *disk_info,
				      uint64_t start, uint64_t count)
{
	const uint32_t buf_size = count * disk_info->bytes_per_lba;
	char *buf;
	VbExStream_t stream;
	uint64_t isector;
	vb2_error_t rv = VB2_ERROR_LK_NO_KERNEL_FOUND;

	buf = malloc(buf_size);
	if (buf == NULL) {
		VB2_DEBUG("Unable to allocate disk read buffer.\n");
		return rv;
	}

	if (VbExStreamOpen(params->disk_handle, start, count, &stream)) {
		VB2_DEBUG("Unable to open disk handle.\n");
		free(buf);
		return rv;
	}
	if (VbExStreamRead(stream, buf_size, buf)) {
		VB2_DEBUG("Unable to read disk.\n");
		free(buf);
		VbExStreamClose(stream);
		return rv;
	}
	VbExStreamClose(stream);

	for (isector = 0; isector < count; isector++) {
		if (memcmp(buf + isector * disk_info->bytes_per_lba,
			   VB2_KEYBLOCK_MAGIC, VB2_KEYBLOCK_MAGIC_SIZE))
			continue;
		VB2_DEBUG("Match on sector %" PRIu64 " / %" PRIu64 "\n",
			  start + isector,
			  disk_info->lba_count - 1);
		rv = try_minios_kernel(ctx, params, disk_info, start + isector);
		if (rv == VB2_SUCCESS)
			break;
	}

	free(buf);
	return rv;
}

static vb2_error_t try_minios_sector_region(struct vb2_context *ctx,
					    VbSelectAndLoadKernelParams *params,
					    VbDiskInfo *disk_info,
					    int end_region)
{
	const uint64_t disk_count_half = (disk_info->lba_count + 1) / 2;
	const uint64_t check_count_256 = 256 * 1024
		* 1024 / disk_info->bytes_per_lba;  // 256 MB
	const uint64_t batch_count_1 = 1024
		* 1024 / disk_info->bytes_per_lba;  // 1 MB
	const uint64_t check_count = VB2_MIN(disk_count_half, check_count_256);
	const uint64_t batch_count = VB2_MIN(disk_count_half, batch_count_1);
	uint64_t sector;
	uint64_t start;
	uint64_t end;
	const char *region_name;
	vb2_error_t rv = VB2_ERROR_LK_NO_KERNEL_FOUND;

	if (!end_region) {
		start = 0;
		end = check_count;
		region_name = "start";
	} else {
		start = disk_info->lba_count - check_count;
		end = disk_info->lba_count;
		region_name = "end";
	}

	VB2_DEBUG("Checking %s of disk for kernels...\n", region_name);
	for (sector = start; sector < end; sector += batch_count) {
		rv = try_minios_sectors(ctx, params, disk_info, sector,
					batch_count);
		if (rv == VB2_SUCCESS)
			return rv;
	}

	return rv;
}

/*
 * Search for kernels by sector, rather than by partition.  Only sectors near
 * the start and end of disks are considered, and the kernel must start exactly
 * at the first byte of the sector.
 */
vb2_error_t LoadMiniOsKernel(struct vb2_context *ctx,
			     VbSelectAndLoadKernelParams *params,
			     VbDiskInfo *disk_info, uint32_t minios_flags)
{
	vb2_error_t rv;
	int end_region_first = vb2_nv_get(ctx, VB2_NV_MINIOS_PRIORITY);

	if (minios_flags & VB_MINIOS_FLAG_NON_ACTIVE)
		rv = VB2_ERROR_UNKNOWN;  /* Ignore active partition */
	else
		rv = try_minios_sector_region(ctx, params, disk_info,
					      end_region_first);

	if (rv)
		rv = try_minios_sector_region(ctx, params, disk_info,
					      !end_region_first);
	if (rv)
		return rv;

	rv = vb2ex_tpm_set_mode(VB2_TPM_MODE_DISABLED);
	if (rv)
		VB2_DEBUG("Failed to disable TPM\n");

	return rv;
}

vb2_error_t LoadKernel(struct vb2_context *ctx,
		       VbSelectAndLoadKernelParams *params,
		       VbDiskInfo *disk_info)
{
	struct vb2_shared_data *sd = vb2_get_sd(ctx);
	int found_partitions = 0;
	uint32_t lowest_version = LOWEST_TPM_VERSION;
	vb2_error_t rv;

	/* Clear output params */
	params->partition_number = 0;

	/* Read GPT data */
	GptData gpt;
	gpt.sector_bytes = (uint32_t)disk_info->bytes_per_lba;
	gpt.streaming_drive_sectors = disk_info->streaming_lba_count
		?: disk_info->lba_count;
	gpt.gpt_drive_sectors = disk_info->lba_count;
	gpt.flags = disk_info->flags & VB_DISK_FLAG_EXTERNAL_GPT
			? GPT_FLAG_EXTERNAL : 0;
	if (AllocAndReadGptData(disk_info->handle, &gpt)) {
		VB2_DEBUG("Unable to read GPT data\n");
		goto gpt_done;
	}

	/* Initialize GPT library */
	if (GptInit(&gpt)) {
		VB2_DEBUG("Error parsing GPT\n");
		goto gpt_done;
	}

	/* Loop over candidate kernel partitions */
	uint64_t part_start, part_size;
	while (GptNextKernelEntry(&gpt, &part_start, &part_size) ==
	       GPT_SUCCESS) {

		VB2_DEBUG("Found kernel entry at %"
			  PRIu64 " size %" PRIu64 "\n",
			  part_start, part_size);

		/* Found at least one kernel partition. */
		found_partitions++;

		/* Set up the stream */
		VbExStream_t stream = NULL;
		if (VbExStreamOpen(disk_info->handle,
				   part_start, part_size, &stream)) {
			VB2_DEBUG("Partition error getting stream.\n");
			VB2_DEBUG("Marking kernel as invalid.\n");
			GptUpdateKernelEntry(&gpt, GPT_UPDATE_ENTRY_BAD);
			continue;
		}

		uint32_t lpflags = 0;
		if (params->partition_number > 0) {
			/*
			 * If we already have a good kernel, we only needed to
			 * look at the vblock versions to check for rollback.
			 */
			lpflags |= VB2_LOAD_PARTITION_FLAG_VBLOCK_ONLY;
		}

		rv = vb2_load_partition(ctx, params, stream, lpflags);
		VbExStreamClose(stream);

		if (rv) {
			VB2_DEBUG("Marking kernel as invalid (err=%x).\n", rv);
			GptUpdateKernelEntry(&gpt, GPT_UPDATE_ENTRY_BAD);
			continue;
		}

		int keyblock_valid = sd->flags & VB2_SD_FLAG_KERNEL_SIGNED;
		/* Track lowest version from a valid header. */
		if (keyblock_valid && lowest_version > sd->kernel_version) {
			lowest_version = sd->kernel_version;
		}
		VB2_DEBUG("Keyblock valid: %d\n", keyblock_valid);
		VB2_DEBUG("Combined version: %u\n", sd->kernel_version);

		/*
		 * If we're only looking at headers, we're done with this
		 * partition.
		 */
		if (lpflags & VB2_LOAD_PARTITION_FLAG_VBLOCK_ONLY)
			continue;

		/*
		 * Otherwise, we found a partition we like.
		 *
		 * TODO: GPT partitions start at 1, but cgptlib starts them at
		 * 0.  Adjust here, until cgptlib is fixed.
		 */
		params->partition_number = gpt.current_kernel + 1;

		/*
		 * TODO: GetCurrentKernelUniqueGuid() should take a destination
		 * size, or the dest should be a struct, so we know it's big
		 * enough.
		 */
		GetCurrentKernelUniqueGuid(&gpt, &params->partition_guid);

		/* Update GPT to note this is the kernel we're trying.
		 * But not when we assume that the boot process may
		 * not complete for valid reasons (eg. early shutdown).
		 */
		if (!(ctx->flags & VB2_CONTEXT_NOFAIL_BOOT))
			GptUpdateKernelEntry(&gpt, GPT_UPDATE_ENTRY_TRY);

		/*
		 * If we're in recovery mode or we're about to boot a
		 * non-officially-signed kernel, there's no rollback
		 * protection, so we can stop at the first valid kernel.
		 */
		if (get_boot_mode(ctx) == VB2_BOOT_MODE_RECOVERY ||
		    !keyblock_valid) {
			VB2_DEBUG("In recovery mode or dev-signed kernel\n");
			break;
		}

		/*
		 * Otherwise, we do care about the key index in the TPM.  If
		 * the good partition's key version is the same as the tpm,
		 * then the TPM doesn't need updating; we can stop now.
		 * Otherwise, we'll check all the other headers to see if they
		 * contain a newer key.
		 */
		if (sd->kernel_version == sd->kernel_version_secdata) {
			VB2_DEBUG("Same kernel version\n");
			break;
		}
	} /* while(GptNextKernelEntry) */

 gpt_done:
	/* Write and free GPT data */
	WriteAndFreeGptData(disk_info->handle, &gpt);

	/* Handle finding a good partition */
	if (params->partition_number > 0) {
		VB2_DEBUG("Good partition %d\n", params->partition_number);
		/*
		 * Validity check - only store a new TPM version if we found
		 * one. If lowest_version is still at its initial value, we
		 * didn't find one; for example, we're in developer mode and
		 * just didn't look.
		 */
		if (lowest_version != LOWEST_TPM_VERSION &&
		    lowest_version > sd->kernel_version_secdata)
			sd->kernel_version = lowest_version;

		/* Success! */
		rv = VB2_SUCCESS;
	} else if (found_partitions > 0) {
		rv = VB2_ERROR_LK_INVALID_KERNEL_FOUND;
	} else {
		rv = VB2_ERROR_LK_NO_KERNEL_FOUND;
	}

	return rv;
}