diff options
Diffstat (limited to 'src/boot/efi/random-seed.c')
| -rw-r--r-- | src/boot/efi/random-seed.c | 321 |
1 files changed, 321 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..aea4f7e --- /dev/null +++ b/src/boot/efi/random-seed.c @@ -0,0 +1,321 @@ +/* SPDX-License-Identifier: LGPL-2.1-or-later */ + +#include <efi.h> +#include <efilib.h> + +#include "missing_efi.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) + +#define EFI_RNG_GUID &(const EFI_GUID) EFI_RNG_PROTOCOL_GUID + +/* SHA256 gives us 256/8=32 bytes */ +#define HASH_VALUE_SIZE 32 + +static EFI_STATUS acquire_rng(UINTN size, void **ret) { + _cleanup_free_ void *data = NULL; + EFI_RNG_PROTOCOL *rng; + EFI_STATUS err; + + assert(ret); + + /* Try to acquire the specified number of bytes from the UEFI RNG */ + + err = BS->LocateProtocol((EFI_GUID *) EFI_RNG_GUID, NULL, (void **) &rng); + if (err != EFI_SUCCESS) + return err; + if (!rng) + return EFI_UNSUPPORTED; + + data = xmalloc(size); + + err = rng->GetRNG(rng, NULL, size, data); + if (err != EFI_SUCCESS) + return log_error_status_stall(err, L"Failed to acquire RNG data: %r", err); + + *ret = TAKE_PTR(data); + return EFI_SUCCESS; +} + +static void hash_once( + const void *old_seed, + const void *rng, + UINTN size, + const void *system_token, + UINTN system_token_size, + uint64_t uefi_monotonic_counter, + UINTN counter, + uint8_t ret[static HASH_VALUE_SIZE]) { + + /* This hashes together: + * + * 1. The contents of the old seed file + * 2. Some random data acquired from the UEFI RNG (optional) + * 3. Some 'system token' the installer installed as EFI variable (optional) + * 4. The UEFI "monotonic counter" that increases with each boot + * 5. A supplied counter value + * + * And writes the result to the specified buffer. + */ + + struct sha256_ctx hash; + + assert(old_seed); + assert(system_token_size == 0 || system_token); + + sha256_init_ctx(&hash); + sha256_process_bytes(old_seed, size, &hash); + if (rng) + sha256_process_bytes(rng, size, &hash); + if (system_token_size > 0) + sha256_process_bytes(system_token, system_token_size, &hash); + sha256_process_bytes(&uefi_monotonic_counter, sizeof(uefi_monotonic_counter), &hash); + sha256_process_bytes(&counter, sizeof(counter), &hash); + sha256_finish_ctx(&hash, ret); +} + +static EFI_STATUS hash_many( + const void *old_seed, + const void *rng, + UINTN size, + const void *system_token, + UINTN system_token_size, + uint64_t uefi_monotonic_counter, + UINTN counter_start, + UINTN n, + void **ret) { + + _cleanup_free_ void *output = NULL; + + assert(old_seed); + assert(system_token_size == 0 || system_token); + assert(ret); + + /* Hashes the specified parameters in counter mode, generating n hash values, with the counter in the + * range counter_start…counter_start+n-1. */ + + output = xmalloc_multiply(HASH_VALUE_SIZE, n); + + for (UINTN i = 0; i < n; i++) + hash_once(old_seed, rng, size, + system_token, system_token_size, + uefi_monotonic_counter, + counter_start + i, + (uint8_t*) output + (i * HASH_VALUE_SIZE)); + + *ret = TAKE_PTR(output); + return EFI_SUCCESS; +} + +static EFI_STATUS mangle_random_seed( + const void *old_seed, + const void *rng, + UINTN size, + const void *system_token, + UINTN system_token_size, + uint64_t uefi_monotonic_counter, + void **ret_new_seed, + void **ret_for_kernel) { + + _cleanup_free_ void *new_seed = NULL, *for_kernel = NULL; + EFI_STATUS err; + UINTN n; + + assert(old_seed); + assert(system_token_size == 0 || system_token); + assert(ret_new_seed); + assert(ret_for_kernel); + + /* This takes the old seed file contents, an (optional) random number acquired from the UEFI RNG, an + * (optional) system 'token' installed once by the OS installer in an EFI variable, and hashes them + * together in counter mode, generating a new seed (to replace the file on disk) and the seed for the + * kernel. To keep things simple, the new seed and kernel data have the same size as the old seed and + * RNG data. */ + + n = (size + HASH_VALUE_SIZE - 1) / HASH_VALUE_SIZE; + + /* Begin hashing in counter mode at counter 0 for the new seed for the disk */ + err = hash_many(old_seed, rng, size, system_token, system_token_size, uefi_monotonic_counter, 0, n, &new_seed); + if (err != EFI_SUCCESS) + return err; + + /* Continue counting at 'n' for the seed for the kernel */ + err = hash_many(old_seed, rng, size, system_token, system_token_size, uefi_monotonic_counter, n, n, &for_kernel); + if (err != EFI_SUCCESS) + return err; + + *ret_new_seed = TAKE_PTR(new_seed); + *ret_for_kernel = TAKE_PTR(for_kernel); + + return EFI_SUCCESS; +} + +static EFI_STATUS acquire_system_token(void **ret, UINTN *ret_size) { + _cleanup_free_ char *data = NULL; + EFI_STATUS err; + UINTN size; + + assert(ret); + assert(ret_size); + + err = efivar_get_raw(LOADER_GUID, L"LoaderSystemToken", &data, &size); + if (err != EFI_SUCCESS) { + if (err != EFI_NOT_FOUND) + log_error_stall(L"Failed to read LoaderSystemToken EFI variable: %r", err); + return err; + } + + if (size <= 0) + return log_error_status_stall(EFI_NOT_FOUND, L"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 (UINTN 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, RandomSeedMode mode) { + _cleanup_free_ void *seed = NULL, *new_seed = NULL, *rng = NULL, *for_kernel = NULL, *system_token = NULL; + _cleanup_(file_closep) EFI_FILE *handle = NULL; + UINTN size, rsize, wsize, system_token_size = 0; + _cleanup_free_ EFI_FILE_INFO *info = NULL; + uint64_t uefi_monotonic_counter = 0; + EFI_STATUS err; + + assert(root_dir); + + validate_sha256(); + + if (mode == RANDOM_SEED_OFF) + return EFI_NOT_FOUND; + + /* Let's better be safe than sorry, and for now disable this logic in SecureBoot mode, so that we + * don't credit a random seed that is not authenticated. */ + if (secure_boot_enabled()) + return EFI_NOT_FOUND; + + /* 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. */ + err = acquire_system_token(&system_token, &system_token_size); + if (mode != RANDOM_SEED_ALWAYS && err != EFI_SUCCESS) + return err; + + err = root_dir->Open( + root_dir, + &handle, + (char16_t *) L"\\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_stall(L"Failed to open random seed file: %r", err); + return err; + } + + err = get_file_info_harder(handle, &info, NULL); + if (err != EFI_SUCCESS) + return log_error_status_stall(err, L"Failed to get file info for random seed: %r"); + + size = info->FileSize; + if (size < RANDOM_MAX_SIZE_MIN) + return log_error_status_stall(EFI_INVALID_PARAMETER, L"Random seed file is too short."); + + if (size > RANDOM_MAX_SIZE_MAX) + return log_error_status_stall(EFI_INVALID_PARAMETER, L"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_stall(err, L"Failed to read random seed file: %r", err); + if (rsize != size) + return log_error_status_stall(EFI_PROTOCOL_ERROR, L"Short read on random seed file."); + + err = handle->SetPosition(handle, 0); + if (err != EFI_SUCCESS) + return log_error_status_stall(err, L"Failed to seek to beginning of random seed file: %r", err); + + /* 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. */ + (void) acquire_rng(size, &rng); /* It's fine if this fails */ + + /* 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) + return log_error_status_stall(err, L"Failed to acquire UEFI monotonic counter: %r", err); + + /* Calculate new random seed for the disk and what to pass to the kernel */ + err = mangle_random_seed(seed, rng, size, system_token, system_token_size, uefi_monotonic_counter, &new_seed, &for_kernel); + if (err != EFI_SUCCESS) + return err; + + /* Update the random seed on disk before we use it */ + wsize = size; + err = handle->Write(handle, &wsize, new_seed); + if (err != EFI_SUCCESS) + return log_error_status_stall(err, L"Failed to write random seed file: %r", err); + if (wsize != size) + return log_error_status_stall(EFI_PROTOCOL_ERROR, L"Short write on random seed file."); + + err = handle->Flush(handle); + if (err != EFI_SUCCESS) + return log_error_status_stall(err, L"Failed to flush random seed file: %r", err); + + /* We are good to go */ + err = efivar_set_raw(LOADER_GUID, L"LoaderRandomSeed", for_kernel, size, 0); + if (err != EFI_SUCCESS) + return log_error_status_stall(err, L"Failed to write random seed to EFI variable: %r", err); + + return EFI_SUCCESS; +} |