diff options
Diffstat (limited to 'src/boot/efi/random-seed.c')
| -rw-r--r-- | src/boot/efi/random-seed.c | 325 |
1 files changed, 325 insertions, 0 deletions
diff --git a/src/boot/efi/random-seed.c b/src/boot/efi/random-seed.c new file mode 100644 index 0000000..8147e54 --- /dev/null +++ b/src/boot/efi/random-seed.c @@ -0,0 +1,325 @@ +/* SPDX-License-Identifier: LGPL-2.1-or-later */ + +#include "memory-util-fundamental.h" +#include "proto/rng.h" +#include "random-seed.h" +#include "secure-boot.h" +#include "sha256.h" +#include "util.h" + +#define RANDOM_MAX_SIZE_MIN (32U) +#define RANDOM_MAX_SIZE_MAX (32U*1024U) + +struct linux_efi_random_seed { + uint32_t size; + uint8_t seed[]; +}; + +#define LINUX_EFI_RANDOM_SEED_TABLE_GUID \ + { 0x1ce1e5bc, 0x7ceb, 0x42f2, { 0x81, 0xe5, 0x8a, 0xad, 0xf1, 0x80, 0xf5, 0x7b } } + +/* SHA256 gives us 256/8=32 bytes */ +#define HASH_VALUE_SIZE 32 + +/* Linux's RNG is 256 bits, so let's provide this much */ +#define DESIRED_SEED_SIZE 32 + +/* Some basic domain separation in case somebody uses this data elsewhere */ +#define HASH_LABEL "systemd-boot random seed label v1" + +static EFI_STATUS acquire_rng(void *ret, size_t size) { + EFI_RNG_PROTOCOL *rng; + EFI_STATUS err; + + assert(ret); + + /* Try to acquire the specified number of bytes from the UEFI RNG */ + + err = BS->LocateProtocol(MAKE_GUID_PTR(EFI_RNG_PROTOCOL), NULL, (void **) &rng); + if (err != EFI_SUCCESS) + return err; + if (!rng) + return EFI_UNSUPPORTED; + + err = rng->GetRNG(rng, NULL, size, ret); + if (err != EFI_SUCCESS) + return log_error_status(err, "Failed to acquire RNG data: %m"); + return EFI_SUCCESS; +} + +static EFI_STATUS acquire_system_token(void **ret, size_t *ret_size) { + _cleanup_free_ char *data = NULL; + EFI_STATUS err; + size_t size; + + assert(ret); + assert(ret_size); + + err = efivar_get_raw(MAKE_GUID_PTR(LOADER), u"LoaderSystemToken", &data, &size); + if (err != EFI_SUCCESS) { + if (err != EFI_NOT_FOUND) + log_error_status(err, "Failed to read LoaderSystemToken EFI variable: %m"); + return err; + } + + if (size <= 0) + return log_error_status(EFI_NOT_FOUND, "System token too short, ignoring."); + + *ret = TAKE_PTR(data); + *ret_size = size; + + return EFI_SUCCESS; +} + +static void validate_sha256(void) { + +#ifdef EFI_DEBUG + /* Let's validate our SHA256 implementation. We stole it from glibc, and converted it to UEFI + * style. We better check whether it does the right stuff. We use the simpler test vectors from the + * SHA spec. Note that we strip this out in optimization builds. */ + + static const struct { + const char *string; + uint8_t hash[HASH_VALUE_SIZE]; + } array[] = { + { "abc", + { 0xba, 0x78, 0x16, 0xbf, 0x8f, 0x01, 0xcf, 0xea, + 0x41, 0x41, 0x40, 0xde, 0x5d, 0xae, 0x22, 0x23, + 0xb0, 0x03, 0x61, 0xa3, 0x96, 0x17, 0x7a, 0x9c, + 0xb4, 0x10, 0xff, 0x61, 0xf2, 0x00, 0x15, 0xad }}, + + { "", + { 0xe3, 0xb0, 0xc4, 0x42, 0x98, 0xfc, 0x1c, 0x14, + 0x9a, 0xfb, 0xf4, 0xc8, 0x99, 0x6f, 0xb9, 0x24, + 0x27, 0xae, 0x41, 0xe4, 0x64, 0x9b, 0x93, 0x4c, + 0xa4, 0x95, 0x99, 0x1b, 0x78, 0x52, 0xb8, 0x55 }}, + + { "abcdbcdecdefdefgefghfghighijhijkijkljklmklmnlmnomnopnopq", + { 0x24, 0x8d, 0x6a, 0x61, 0xd2, 0x06, 0x38, 0xb8, + 0xe5, 0xc0, 0x26, 0x93, 0x0c, 0x3e, 0x60, 0x39, + 0xa3, 0x3c, 0xe4, 0x59, 0x64, 0xff, 0x21, 0x67, + 0xf6, 0xec, 0xed, 0xd4, 0x19, 0xdb, 0x06, 0xc1 }}, + + { "abcdefghbcdefghicdefghijdefghijkefghijklfghijklmghijklmnhijklmnoijklmnopjklmnopqklmnopqrlmnopqrsmnopqrstnopqrstu", + { 0xcf, 0x5b, 0x16, 0xa7, 0x78, 0xaf, 0x83, 0x80, + 0x03, 0x6c, 0xe5, 0x9e, 0x7b, 0x04, 0x92, 0x37, + 0x0b, 0x24, 0x9b, 0x11, 0xe8, 0xf0, 0x7a, 0x51, + 0xaf, 0xac, 0x45, 0x03, 0x7a, 0xfe, 0xe9, 0xd1 }}, + }; + + for (size_t i = 0; i < ELEMENTSOF(array); i++) + assert(memcmp(SHA256_DIRECT(array[i].string, strlen8(array[i].string)), array[i].hash, HASH_VALUE_SIZE) == 0); +#endif +} + +EFI_STATUS process_random_seed(EFI_FILE *root_dir) { + uint8_t random_bytes[DESIRED_SEED_SIZE], hash_key[HASH_VALUE_SIZE]; + _cleanup_free_ struct linux_efi_random_seed *new_seed_table = NULL; + struct linux_efi_random_seed *previous_seed_table = NULL; + _cleanup_free_ void *seed = NULL, *system_token = NULL; + _cleanup_(file_closep) EFI_FILE *handle = NULL; + _cleanup_free_ EFI_FILE_INFO *info = NULL; + struct sha256_ctx hash; + uint64_t uefi_monotonic_counter = 0; + size_t size, rsize, wsize; + bool seeded_by_efi = false; + EFI_STATUS err; + EFI_TIME now; + + CLEANUP_ERASE(random_bytes); + CLEANUP_ERASE(hash_key); + CLEANUP_ERASE(hash); + + assert(root_dir); + assert_cc(DESIRED_SEED_SIZE == HASH_VALUE_SIZE); + + validate_sha256(); + + /* hash = LABEL || sizeof(input1) || input1 || ... || sizeof(inputN) || inputN */ + sha256_init_ctx(&hash); + + /* Some basic domain separation in case somebody uses this data elsewhere */ + sha256_process_bytes(HASH_LABEL, sizeof(HASH_LABEL) - 1, &hash); + + previous_seed_table = find_configuration_table(MAKE_GUID_PTR(LINUX_EFI_RANDOM_SEED_TABLE)); + if (!previous_seed_table) { + size = 0; + sha256_process_bytes(&size, sizeof(size), &hash); + } else { + size = previous_seed_table->size; + seeded_by_efi = size >= DESIRED_SEED_SIZE; + sha256_process_bytes(&size, sizeof(size), &hash); + sha256_process_bytes(previous_seed_table->seed, size, &hash); + + /* Zero and free the previous seed table only at the end after we've managed to install a new + * one, so that in case this function fails or aborts, Linux still receives whatever the + * previous bootloader chain set. So, the next line of this block is not an explicit_bzero() + * call. */ + } + + /* Request some random data from the UEFI RNG. We don't need this to work safely, but it's a good + * idea to use it because it helps us for cases where users mistakenly include a random seed in + * golden master images that are replicated many times. */ + err = acquire_rng(random_bytes, sizeof(random_bytes)); + if (err != EFI_SUCCESS) { + size = 0; + /* If we can't get any randomness from EFI itself, then we'll only be relying on what's in + * ESP. But ESP is mutable, so if secure boot is enabled, we probably shouldn't trust that + * alone, in which case we bail out early. */ + if (!seeded_by_efi && secure_boot_enabled()) + return EFI_NOT_FOUND; + } else { + seeded_by_efi = true; + size = sizeof(random_bytes); + } + sha256_process_bytes(&size, sizeof(size), &hash); + sha256_process_bytes(random_bytes, size, &hash); + + /* Get some system specific seed that the installer might have placed in an EFI variable. We include + * it in our hash. This is protection against golden master image sloppiness, and it remains on the + * system, even when disk images are duplicated or swapped out. */ + size = 0; + err = acquire_system_token(&system_token, &size); + if ((err != EFI_SUCCESS || size < DESIRED_SEED_SIZE) && !seeded_by_efi) + return err; + sha256_process_bytes(&size, sizeof(size), &hash); + if (system_token) { + sha256_process_bytes(system_token, size, &hash); + explicit_bzero_safe(system_token, size); + } + + err = root_dir->Open( + root_dir, + &handle, + (char16_t *) u"\\loader\\random-seed", + EFI_FILE_MODE_READ | EFI_FILE_MODE_WRITE, + 0); + if (err != EFI_SUCCESS) { + if (err != EFI_NOT_FOUND && err != EFI_WRITE_PROTECTED) + log_error_status(err, "Failed to open random seed file: %m"); + return err; + } + + err = get_file_info(handle, &info, NULL); + if (err != EFI_SUCCESS) + return log_error_status(err, "Failed to get file info for random seed: %m"); + + size = info->FileSize; + if (size < RANDOM_MAX_SIZE_MIN) + return log_error("Random seed file is too short."); + + if (size > RANDOM_MAX_SIZE_MAX) + return log_error("Random seed file is too large."); + + seed = xmalloc(size); + rsize = size; + err = handle->Read(handle, &rsize, seed); + if (err != EFI_SUCCESS) + return log_error_status(err, "Failed to read random seed file: %m"); + if (rsize != size) { + explicit_bzero_safe(seed, rsize); + return log_error_status(EFI_PROTOCOL_ERROR, "Short read on random seed file."); + } + + sha256_process_bytes(&size, sizeof(size), &hash); + sha256_process_bytes(seed, size, &hash); + explicit_bzero_safe(seed, size); + + err = handle->SetPosition(handle, 0); + if (err != EFI_SUCCESS) + return log_error_status(err, "Failed to seek to beginning of random seed file: %m"); + + /* Let's also include the UEFI monotonic counter (which is supposedly increasing on every single + * boot) in the hash, so that even if the changes to the ESP for some reason should not be + * persistent, the random seed we generate will still be different on every single boot. */ + err = BS->GetNextMonotonicCount(&uefi_monotonic_counter); + if (err != EFI_SUCCESS && !seeded_by_efi) + return log_error_status(err, "Failed to acquire UEFI monotonic counter: %m"); + size = sizeof(uefi_monotonic_counter); + sha256_process_bytes(&size, sizeof(size), &hash); + sha256_process_bytes(&uefi_monotonic_counter, size, &hash); + + err = RT->GetTime(&now, NULL); + size = err == EFI_SUCCESS ? sizeof(now) : 0; /* Known to be flaky, so don't bark on error. */ + sha256_process_bytes(&size, sizeof(size), &hash); + sha256_process_bytes(&now, size, &hash); + + /* hash_key = HASH(hash) */ + sha256_finish_ctx(&hash, hash_key); + + /* hash = hash_key || 0 */ + sha256_init_ctx(&hash); + sha256_process_bytes(hash_key, sizeof(hash_key), &hash); + sha256_process_bytes(&(const uint8_t){ 0 }, sizeof(uint8_t), &hash); + /* random_bytes = HASH(hash) */ + sha256_finish_ctx(&hash, random_bytes); + + size = sizeof(random_bytes); + /* If the file size is too large, zero out the remaining bytes on disk. */ + if (size < info->FileSize) { + err = handle->SetPosition(handle, size); + if (err != EFI_SUCCESS) + return log_error_status(err, "Failed to seek to offset of random seed file: %m"); + wsize = info->FileSize - size; + err = handle->Write(handle, &wsize, seed /* All zeros now */); + if (err != EFI_SUCCESS) + return log_error_status(err, "Failed to write random seed file: %m"); + if (wsize != info->FileSize - size) + return log_error_status(EFI_PROTOCOL_ERROR, "Short write on random seed file."); + err = handle->Flush(handle); + if (err != EFI_SUCCESS) + return log_error_status(err, "Failed to flush random seed file: %m"); + err = handle->SetPosition(handle, 0); + if (err != EFI_SUCCESS) + return log_error_status(err, "Failed to seek to beginning of random seed file: %m"); + + /* We could truncate the file here with something like: + * + * info->FileSize = size; + * err = handle->SetInfo(handle, &GenericFileInfo, info->Size, info); + * if (err != EFI_SUCCESS) + * return log_error_status(err, "Failed to truncate random seed file: %u"); + * + * But this is considered slightly risky, because EFI filesystem drivers are a little bit + * flimsy. So instead we rely on userspace eventually truncating this when it writes a new + * seed. For now the best we do is zero it. */ + } + /* Update the random seed on disk before we use it */ + wsize = size; + err = handle->Write(handle, &wsize, random_bytes); + if (err != EFI_SUCCESS) + return log_error_status(err, "Failed to write random seed file: %m"); + if (wsize != size) + return log_error_status(EFI_PROTOCOL_ERROR, "Short write on random seed file."); + err = handle->Flush(handle); + if (err != EFI_SUCCESS) + return log_error_status(err, "Failed to flush random seed file: %m"); + + err = BS->AllocatePool(EfiACPIReclaimMemory, + offsetof(struct linux_efi_random_seed, seed) + DESIRED_SEED_SIZE, + (void **) &new_seed_table); + if (err != EFI_SUCCESS) + return log_error_status(err, "Failed to allocate EFI table for random seed: %m"); + new_seed_table->size = DESIRED_SEED_SIZE; + + /* hash = hash_key || 1 */ + sha256_init_ctx(&hash); + sha256_process_bytes(hash_key, sizeof(hash_key), &hash); + sha256_process_bytes(&(const uint8_t){ 1 }, sizeof(uint8_t), &hash); + /* new_seed_table->seed = HASH(hash) */ + sha256_finish_ctx(&hash, new_seed_table->seed); + + err = BS->InstallConfigurationTable(MAKE_GUID_PTR(LINUX_EFI_RANDOM_SEED_TABLE), new_seed_table); + if (err != EFI_SUCCESS) + return log_error_status(err, "Failed to install EFI table for random seed: %m"); + TAKE_PTR(new_seed_table); + + if (previous_seed_table) { + /* Now that we've succeeded in installing the new table, we can safely nuke the old one. */ + explicit_bzero_safe(previous_seed_table->seed, previous_seed_table->size); + explicit_bzero_safe(previous_seed_table, sizeof(*previous_seed_table)); + free(previous_seed_table); + } + + return EFI_SUCCESS; +} |