common/rollback: Add support for rollback protection

Implement actual rollback protection. First, we add a new field
in the version structure, which is an incrementing integer
(we'll start by shipping images with version 0, and gradually
increase the number as required). This allows us to release
new versions of the EC without necessarily bumping the rollback
protection.

For the rollback protection block itself, it contains 2 sub-blocks
of equal size (normally, 2k), that are individually erasable.
The rollback code looks at both, and takes the most restrictive one
to determine the desired rollback minimum version. The blocks
are also allowed to be erased (full of 1's), in which case the
rollback minimum version is assumed to be 0.

We also add an FMAP entry, in case we later decide to allow the
signer to increment the rollback version.

Also note that, like any version_data struct change, this change
breaks compatibility between old and new RO/RW.

Follow-up code will take care of auto-updating the rollback block
as required, and properly manage block protection.

BRANCH=none
BUG=b:35586219
TEST=Flash hammer
     rollbackinfo => 1 version 0 block, 1 empty block, RW verifies
           correctly.
     rollbackupdate 0; rollbackinfo => No change
     rollbackupdate 1; reboot => RO refuses to jump to RW
     rollbackupdate 2, 3, 4; rollbackinfo => Writes alternate
           between the 2 blocks.
     rollbackupdate 2 => Refuses to downgrade version

Change-Id: Ia969afb481a93deb912b9153bdd95ace01ad8fa7
Reviewed-on: https://chromium-review.googlesource.com/452815
Commit-Ready: Nicolas Boichat <drinkcat@chromium.org>
Tested-by: Nicolas Boichat <drinkcat@chromium.org>
Reviewed-by: Randall Spangler <rspangler@chromium.org>
Reviewed-by: Vincent Palatin <vpalatin@chromium.org>

1 files changed, 179 insertions, 0 deletions

diff --git a/common/rollback.c b/common/rollback.c
new file mode 100644
index 0000000000..dbeffaa051
--- /dev/null
+++ b/common/rollback.c
@@ -0,0 +1,179 @@
+/* Copyright 2017 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.
+ */
+
+/* Rollback protection logic. */
+
+#include "common.h"
+#include "console.h"
+#include "flash.h"
+#include "rollback.h"
+#include "system.h"
+#include "util.h"
+
+/* Number of rollback regions */
+#define ROLLBACK_REGIONS 2
+
+/*
+ * Note: Do not change this structure without also updating
+ * common/firmware_image.S .image.ROLLBACK section.
+ */
+struct rollback_data {
+	int32_t rollback_min_version;
+	uint32_t cookie;
+};
+
+/* We need at least 2 erasable blocks in the rollback region. */
+BUILD_ASSERT(CONFIG_ROLLBACK_SIZE >= ROLLBACK_REGIONS*CONFIG_FLASH_ERASE_SIZE);
+BUILD_ASSERT(sizeof(struct rollback_data) <= CONFIG_FLASH_ERASE_SIZE);
+
+static uintptr_t get_rollback_offset(int region)
+{
+	return CONFIG_ROLLBACK_OFF + region * CONFIG_FLASH_ERASE_SIZE;
+}
+
+static int read_rollback(int region, struct rollback_data *data)
+{
+	uintptr_t offset;
+
+	offset = get_rollback_offset(region);
+
+	if (flash_read(offset, sizeof(*data), (char *)data))
+		return EC_ERROR_UNKNOWN;
+
+	return EC_SUCCESS;
+}
+
+/*
+ * Get the most recent rollback information.
+ *
+ * @rollback_min_version: Minimum version to accept for rollback protection,
+ *                        or 0 if no rollback information is present.
+ *
+ * Return most recent region index on success (>= 0, or 0 if no rollback
+ * region is valid), negative value on error.
+ */
+static int get_latest_rollback(int32_t *rollback_min_version)
+{
+	int region;
+	int min_region = 0;
+
+	*rollback_min_version = 0;
+
+	for (region = 0; region < ROLLBACK_REGIONS; region++) {
+		struct rollback_data data;
+
+		if (read_rollback(region, &data))
+			return -1;
+
+		/* Check if not initialized or invalid cookie. */
+		if (data.cookie != CROS_EC_ROLLBACK_COOKIE)
+			continue;
+
+		if (data.rollback_min_version > *rollback_min_version) {
+			min_region = region;
+			*rollback_min_version = data.rollback_min_version;
+		}
+	}
+
+	return min_region;
+}
+
+int32_t rollback_get_minimum_version(void)
+{
+	int32_t rollback_min_version;
+
+	if (get_latest_rollback(&rollback_min_version) < 0)
+		return -1;
+
+	return rollback_min_version;
+}
+
+int rollback_update(int32_t next_min_version)
+{
+	struct rollback_data data;
+	uintptr_t offset;
+	int region;
+	int32_t current_min_version;
+	int ret;
+
+	region = get_latest_rollback(&current_min_version);
+
+	if (region < 0)
+		return EC_ERROR_UNKNOWN;
+
+	/* Do not accept to decrement the value. */
+	if (next_min_version < current_min_version)
+		return EC_ERROR_INVAL;
+
+	/* No need to update if version is already correct. */
+	if (next_min_version == current_min_version)
+		return EC_SUCCESS;
+
+	/* Use the other region. */
+	region = (region + 1) % ROLLBACK_REGIONS;
+
+	offset = get_rollback_offset(region);
+
+	data.rollback_min_version = next_min_version;
+	data.cookie = CROS_EC_ROLLBACK_COOKIE;
+
+	if (system_unsafe_to_overwrite(offset, CONFIG_FLASH_ERASE_SIZE))
+		return EC_ERROR_ACCESS_DENIED;
+
+	ret = flash_erase(offset, CONFIG_FLASH_ERASE_SIZE);
+	if (ret)
+		return ret;
+
+	return flash_write(offset, sizeof(data), (char *)&data);
+}
+
+static int command_rollback_info(int argc, char **argv)
+{
+	int region, ret, min_region;
+	int32_t rollback_min_version;
+
+	min_region = get_latest_rollback(&rollback_min_version);
+
+	if (min_region < 0)
+		return EC_ERROR_UNKNOWN;
+
+	ccprintf("rollback minimum version: %d\n", rollback_min_version);
+
+	for (region = 0; region < ROLLBACK_REGIONS; region++) {
+		struct rollback_data data;
+
+		ret = read_rollback(region, &data);
+		if (ret)
+			return ret;
+
+		ccprintf("rollback %d: %08x %08x%s\n",
+			region, data.rollback_min_version, data.cookie,
+			min_region == region ? " *" : "");
+	}
+
+	return EC_SUCCESS;
+}
+DECLARE_SAFE_CONSOLE_COMMAND(rollbackinfo, command_rollback_info,
+			     NULL,
+			     "Print rollback info");
+
+static int command_rollback_update(int argc, char **argv)
+{
+	int32_t min_version;
+	char *e;
+
+	if (argc < 2)
+		return EC_ERROR_PARAM_COUNT;
+
+	min_version = strtoi(argv[1], &e, 0);
+
+	if (*e || min_version < 0)
+		return EC_ERROR_PARAM1;
+
+	return rollback_update(min_version);
+}
+DECLARE_CONSOLE_COMMAND(rollbackupdate, command_rollback_update,
+			"min_version",
+			"Update rollback info");