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-rw-r--r--src/boot/efi/bcd.c306
1 files changed, 306 insertions, 0 deletions
diff --git a/src/boot/efi/bcd.c b/src/boot/efi/bcd.c
new file mode 100644
index 0000000..7200012
--- /dev/null
+++ b/src/boot/efi/bcd.c
@@ -0,0 +1,306 @@
+/* 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 usecase 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 the 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;
+}
generated by cgit v1.2.3 (git 2.39.1) at 2024-12-23 16:33:20 +0000