systemd/src/boot/efi/bcd.c

/* SPDX-License-Identifier: LGPL-2.1-or-later */

#include <stdalign.h>

#include "bcd.h"
#include "efi-string.h"

enum {
        SIG_BASE_BLOCK  = 1718052210, /* regf */
        SIG_KEY         = 27502,      /* nk */
        SIG_SUBKEY_FAST = 26220,      /* lf */
        SIG_KEY_VALUE   = 27510,      /* vk */
};

enum {
        REG_SZ       = 1,
        REG_MULTI_SZ = 7,
};

/* These structs contain a lot more members than we care for. They have all
 * been squashed into _padN for our convenience. */

typedef struct {
        uint32_t sig;
        uint32_t primary_seqnum;
        uint32_t secondary_seqnum;
        uint64_t _pad1;
        uint32_t version_major;
        uint32_t version_minor;
        uint32_t type;
        uint32_t _pad2;
        uint32_t root_cell_offset;
        uint64_t _pad3[507];
} _packed_ BaseBlock;
assert_cc(sizeof(BaseBlock) == 4096);
assert_cc(offsetof(BaseBlock, sig) == 0);
assert_cc(offsetof(BaseBlock, primary_seqnum) == 4);
assert_cc(offsetof(BaseBlock, secondary_seqnum) == 8);
assert_cc(offsetof(BaseBlock, version_major) == 20);
assert_cc(offsetof(BaseBlock, version_minor) == 24);
assert_cc(offsetof(BaseBlock, type) == 28);
assert_cc(offsetof(BaseBlock, root_cell_offset) == 36);

/* All offsets are relative to the base block and technically point to a hive
 * cell struct. But for our use case we don't need to bother about that one,
 * so skip over the cell_size uint32_t. */
#define HIVE_CELL_OFFSET (sizeof(BaseBlock) + 4)

typedef struct {
        uint16_t sig;
        uint16_t _pad1[13];
        uint32_t subkeys_offset;
        uint32_t _pad2;
        uint32_t n_key_values;
        uint32_t key_values_offset;
        uint32_t _pad3[7];
        uint16_t key_name_len;
        uint16_t _pad4;
        char key_name[];
} _packed_ Key;
assert_cc(offsetof(Key, sig) == 0);
assert_cc(offsetof(Key, subkeys_offset) == 28);
assert_cc(offsetof(Key, n_key_values) == 36);
assert_cc(offsetof(Key, key_values_offset) == 40);
assert_cc(offsetof(Key, key_name_len) == 72);
assert_cc(offsetof(Key, key_name) == 76);

typedef struct {
        uint16_t sig;
        uint16_t n_entries;
        struct SubkeyFastEntry {
                uint32_t key_offset;
                char name_hint[4];
        } _packed_ entries[];
} _packed_ SubkeyFast;
assert_cc(offsetof(SubkeyFast, sig) == 0);
assert_cc(offsetof(SubkeyFast, n_entries) == 2);
assert_cc(offsetof(SubkeyFast, entries) == 4);

typedef struct {
        uint16_t sig;
        uint16_t name_len;
        uint32_t data_size;
        uint32_t data_offset;
        uint32_t data_type;
        uint32_t _pad;
        char name[];
} _packed_ KeyValue;
assert_cc(offsetof(KeyValue, sig) == 0);
assert_cc(offsetof(KeyValue, name_len) == 2);
assert_cc(offsetof(KeyValue, data_size) == 4);
assert_cc(offsetof(KeyValue, data_offset) == 8);
assert_cc(offsetof(KeyValue, data_type) == 12);
assert_cc(offsetof(KeyValue, name) == 20);

#define BAD_OFFSET(offset, len, max) \
        ((uint64_t) (offset) + (len) >= (max))

#define BAD_STRUCT(type, offset, max) \
        ((uint64_t) (offset) + sizeof(type) >= (max))

#define BAD_ARRAY(type, array, offset, array_len, max) \
        ((uint64_t) (offset) + offsetof(type, array) + \
         sizeof((type){}.array[0]) * (uint64_t) (array_len) >= (max))

static const Key *get_key(const uint8_t *bcd, uint32_t bcd_len, uint32_t offset, const char *name);

static const Key *get_subkey(const uint8_t *bcd, uint32_t bcd_len, uint32_t offset, const char *name) {
        assert(bcd);
        assert(name);

        if (BAD_STRUCT(SubkeyFast, offset, bcd_len))
                return NULL;

        const SubkeyFast *subkey = (const SubkeyFast *) (bcd + offset);
        if (subkey->sig != SIG_SUBKEY_FAST)
                return NULL;

        if (BAD_ARRAY(SubkeyFast, entries, offset, subkey->n_entries, bcd_len))
                return NULL;

        for (uint16_t i = 0; i < subkey->n_entries; i++) {
                if (!strncaseeq8(name, subkey->entries[i].name_hint, sizeof(subkey->entries[i].name_hint)))
                        continue;

                const Key *key = get_key(bcd, bcd_len, subkey->entries[i].key_offset, name);
                if (key)
                        return key;
        }

        return NULL;
}

/* We use NUL as registry path separators for convenience. To start from the root, begin
 * name with a NUL. Name must end with two NUL. The lookup depth is not restricted, so
 * name must be properly validated before calling get_key(). */
static const Key *get_key(const uint8_t *bcd, uint32_t bcd_len, uint32_t offset, const char *name) {
        assert(bcd);
        assert(name);

        if (BAD_STRUCT(Key, offset, bcd_len))
                return NULL;

        const Key *key = (const Key *) (bcd + offset);
        if (key->sig != SIG_KEY)
                return NULL;

        if (BAD_ARRAY(Key, key_name, offset, key->key_name_len, bcd_len))
                return NULL;

        if (*name) {
                if (strncaseeq8(name, key->key_name, key->key_name_len) && strlen8(name) == key->key_name_len)
                        name += key->key_name_len;
                else
                        return NULL;
        }

        name++;
        return *name ? get_subkey(bcd, bcd_len, key->subkeys_offset, name) : key;
}

static const KeyValue *get_key_value(const uint8_t *bcd, uint32_t bcd_len, const Key *key, const char *name) {
        assert(bcd);
        assert(key);
        assert(name);

        if (key->n_key_values == 0)
                return NULL;

        if (BAD_OFFSET(key->key_values_offset, sizeof(uint32_t) * (uint64_t) key->n_key_values, bcd_len) ||
            (uintptr_t) (bcd + key->key_values_offset) % alignof(uint32_t) != 0)
                return NULL;

        const uint32_t *key_value_list = (const uint32_t *) (bcd + key->key_values_offset);
        for (uint32_t i = 0; i < key->n_key_values; i++) {
                uint32_t offset = *(key_value_list + i);

                if (BAD_STRUCT(KeyValue, offset, bcd_len))
                        continue;

                const KeyValue *kv = (const KeyValue *) (bcd + offset);
                if (kv->sig != SIG_KEY_VALUE)
                        continue;

                if (BAD_ARRAY(KeyValue, name, offset, kv->name_len, bcd_len))
                        continue;

                /* If most significant bit is set, data is stored in data_offset itself, but
                 * we are only interested in UTF16 strings. The only strings that could fit
                 * would have just one char in it, so let's not bother with this. */
                if (FLAGS_SET(kv->data_size, UINT32_C(1) << 31))
                        continue;

                if (BAD_OFFSET(kv->data_offset, kv->data_size, bcd_len))
                        continue;

                if (strncaseeq8(name, kv->name, kv->name_len) && strlen8(name) == kv->name_len)
                        return kv;
        }

        return NULL;
}

/* The BCD store is really just a regular windows registry hive with a rather cryptic internal
 * key structure. On a running system it gets mounted to HKEY_LOCAL_MACHINE\BCD00000000.
 *
 * Of interest to us are these two keys:
 * - \Objects\{bootmgr}\Elements\24000001
 *   This key is the "displayorder" property and contains a value of type REG_MULTI_SZ
 *   with the name "Element" that holds a {GUID} list (UTF16, NUL-separated).
 * - \Objects\{GUID}\Elements\12000004
 *   This key is the "description" property and contains a value of type REG_SZ with the
 *   name "Element" that holds a NUL-terminated UTF16 string.
 *
 * The GUIDs and properties are as reported by "bcdedit.exe /v".
 *
 * To get a title for the BCD store we first look at the displayorder property of {bootmgr}
 * (it always has the GUID 9dea862c-5cdd-4e70-acc1-f32b344d4795). If it contains more than
 * one GUID, the BCD is multi-boot and we stop looking. Otherwise we take that GUID, look it
 * up, and return its description property. */
char16_t *get_bcd_title(uint8_t *bcd, size_t bcd_len) {
        assert(bcd);

        if (HIVE_CELL_OFFSET >= bcd_len)
                return NULL;

        BaseBlock *base_block = (BaseBlock *) bcd;
        if (base_block->sig != SIG_BASE_BLOCK ||
            base_block->version_major != 1 ||
            base_block->version_minor != 3 ||
            base_block->type != 0 ||
            base_block->primary_seqnum != base_block->secondary_seqnum)
                return NULL;

        bcd += HIVE_CELL_OFFSET;
        bcd_len -= HIVE_CELL_OFFSET;

        const Key *objects_key = get_key(bcd, bcd_len, base_block->root_cell_offset, "\0Objects\0");
        if (!objects_key)
                return NULL;

        const Key *displayorder_key = get_subkey(
                        bcd,
                        bcd_len,
                        objects_key->subkeys_offset,
                        "{9dea862c-5cdd-4e70-acc1-f32b344d4795}\0Elements\00024000001\0");
        if (!displayorder_key)
                return NULL;

        const KeyValue *displayorder_value = get_key_value(bcd, bcd_len, displayorder_key, "Element");
        if (!displayorder_value)
                return NULL;

        char order_guid[sizeof("{00000000-0000-0000-0000-000000000000}\0")];
        if (displayorder_value->data_type != REG_MULTI_SZ ||
            displayorder_value->data_size != sizeof(char16_t[sizeof(order_guid)]) ||
            (uintptr_t) (bcd + displayorder_value->data_offset) % alignof(char16_t) != 0)
                /* BCD is multi-boot. */
                return NULL;

        /* Keys are stored as ASCII in registry hives if the data fits (and GUIDS always should). */
        char16_t *order_guid_utf16 = (char16_t *) (bcd + displayorder_value->data_offset);
        for (size_t i = 0; i < sizeof(order_guid) - 2; i++) {
                char16_t c = order_guid_utf16[i];
                switch (c) {
                case '-':
                case '{':
                case '}':
                case '0' ... '9':
                case 'a' ... 'f':
                case 'A' ... 'F':
                        order_guid[i] = c;
                        break;
                default:
                        /* Not a valid GUID. */
                        return NULL;
                }
        }
        /* Our functions expect the lookup key to be double-derminated. */
        order_guid[sizeof(order_guid) - 2] = '\0';
        order_guid[sizeof(order_guid) - 1] = '\0';

        const Key *default_key = get_subkey(bcd, bcd_len, objects_key->subkeys_offset, order_guid);
        if (!default_key)
                return NULL;

        const Key *description_key = get_subkey(
                        bcd, bcd_len, default_key->subkeys_offset, "Elements\00012000004\0");
        if (!description_key)
                return NULL;

        const KeyValue *description_value = get_key_value(bcd, bcd_len, description_key, "Element");
        if (!description_value)
                return NULL;

        if (description_value->data_type != REG_SZ ||
            description_value->data_size < sizeof(char16_t) ||
            description_value->data_size % sizeof(char16_t) != 0 ||
            (uintptr_t) (bcd + description_value->data_offset) % alignof(char16_t))
                return NULL;

        /* The data should already be NUL-terminated. */
        char16_t *title = (char16_t *) (bcd + description_value->data_offset);
        title[description_value->data_size / sizeof(char16_t) - 1] = '\0';
        return title;
}