sd-boot: read random seed from ESP and pass it to OS

4 files changed, 397 insertions, 2 deletions

diff --git a/src/boot/efi/boot.c b/src/boot/efi/boot.c
index 2a8035f0b2..fd0c972951 100644
--- a/src/boot/efi/boot.c
+++ b/src/boot/efi/boot.c
@@ -12,6 +12,7 @@
 #include "loader-features.h"
 #include "measure.h"
 #include "pe.h"
+#include "random-seed.h"
 #include "shim.h"
 #include "util.h"
 
@@ -68,6 +69,7 @@ typedef struct {
         BOOLEAN force_menu;
         UINTN console_mode;
         enum console_mode_change_type console_mode_change;
+        RandomSeedMode random_seed_mode;
 } Config;
 
 static VOID cursor_left(UINTN *cursor, UINTN *first) {
@@ -396,6 +398,21 @@ static VOID print_status(Config *config, CHAR16 *loaded_image_path) {
         Print(L"editor:                 %s\n", yes_no(config->editor));
         Print(L"auto-entries:           %s\n", yes_no(config->auto_entries));
         Print(L"auto-firmware:          %s\n", yes_no(config->auto_firmware));
+
+        switch (config->random_seed_mode) {
+        case RANDOM_SEED_OFF:
+                Print(L"random-seed-mode:       off\n");
+                break;
+        case RANDOM_SEED_WITH_SYSTEM_TOKEN:
+                Print(L"random-seed-node:       with-system-token\n");
+                break;
+        case RANDOM_SEED_ALWAYS:
+                Print(L"random-seed-node:       always\n");
+                break;
+        default:
+                ;
+        }
+
         Print(L"\n");
 
         Print(L"config entry count:     %d\n", config->entry_count);
@@ -1080,6 +1097,23 @@ static VOID config_defaults_load_from_file(Config *config, CHAR8 *content) {
 
                         continue;
                 }
+
+                if (strcmpa((CHAR8*) "random-seed-mode", key) == 0) {
+                        if (strcmpa((CHAR8*) "off", value) == 0)
+                                config->random_seed_mode = RANDOM_SEED_OFF;
+                        else if (strcmpa((CHAR8*) "with-system-token", value) == 0)
+                                config->random_seed_mode = RANDOM_SEED_WITH_SYSTEM_TOKEN;
+                        else if (strcmpa((CHAR8*) "always", value) == 0)
+                                config->random_seed_mode = RANDOM_SEED_ALWAYS;
+                        else {
+                                BOOLEAN on;
+
+                                if (EFI_ERROR(parse_boolean(value, &on)))
+                                        continue;
+
+                                config->random_seed_mode = on ? RANDOM_SEED_ALWAYS : RANDOM_SEED_OFF;
+                        }
+                }
         }
 }
 
@@ -1410,6 +1444,7 @@ static VOID config_load_defaults(Config *config, EFI_FILE *root_dir) {
                 .editor = TRUE,
                 .auto_entries = TRUE,
                 .auto_firmware = TRUE,
+                .random_seed_mode = RANDOM_SEED_WITH_SYSTEM_TOKEN,
         };
 
         err = file_read(root_dir, L"\\loader\\loader.conf", 0, 0, &content, NULL);
@@ -2290,6 +2325,7 @@ EFI_STATUS efi_main(EFI_HANDLE image, EFI_SYSTEM_TABLE *sys_table) {
                 EFI_LOADER_FEATURE_ENTRY_ONESHOT |
                 EFI_LOADER_FEATURE_BOOT_COUNTING |
                 EFI_LOADER_FEATURE_XBOOTLDR |
+                EFI_LOADER_FEATURE_RANDOM_SEED |
                 0;
 
         _cleanup_freepool_ CHAR16 *infostr = NULL, *typestr = NULL;
@@ -2441,8 +2477,11 @@ EFI_STATUS efi_main(EFI_HANDLE image, EFI_SYSTEM_TABLE *sys_table) {
 
                 config_entry_bump_counters(entry, root_dir);
 
-                /* export the selected boot entry to the system */
-                efivar_set(L"LoaderEntrySelected", entry->id, FALSE);
+                /* Export the selected boot entry to the system */
+                (VOID) efivar_set(L"LoaderEntrySelected", entry->id, FALSE);
+
+                /* Optionally, read a random seed off the ESP and pass it to the OS */
+                (VOID) process_random_seed(root_dir, config.random_seed_mode);
 
                 uefi_call_wrapper(BS->SetWatchdogTimer, 4, 5 * 60, 0x10000, 0, NULL);
                 err = image_start(image, &config, entry);
diff --git a/src/boot/efi/meson.build b/src/boot/efi/meson.build
index d74660b27f..277432c7e0 100644
--- a/src/boot/efi/meson.build
+++ b/src/boot/efi/meson.build
@@ -8,6 +8,7 @@ efi_headers = files('''
         linux.h
         measure.h
         pe.h
+        random-seed.h
         sha256.h
         shim.h
         splash.h
@@ -26,6 +27,7 @@ systemd_boot_sources = '''
         boot.c
         console.c
         crc32.c
+        random-seed.c
         sha256.c
         shim.c
 '''.split()
diff --git a/src/boot/efi/random-seed.c b/src/boot/efi/random-seed.c
new file mode 100644
index 0000000000..2185c7a7c2
--- /dev/null
+++ b/src/boot/efi/random-seed.c
@@ -0,0 +1,340 @@
+#include <efi.h>
+#include <efilib.h>
+
+#include "random-seed.h"
+#include "sha256.h"
+#include "util.h"
+#include "shim.h"
+
+#define RANDOM_MAX_SIZE_MIN (32U)
+#define RANDOM_MAX_SIZE_MAX (32U*1024U)
+
+static const EFI_GUID rng_protocol_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_freepool_ VOID *data = NULL;
+        EFI_RNG_PROTOCOL *rng;
+        EFI_STATUS err;
+
+        /* Try to acquire the specified number of bytes from the UEFI RNG */
+
+        err = LibLocateProtocol((EFI_GUID*) &rng_protocol_guid, (VOID**) &rng);
+        if (EFI_ERROR(err)) {
+                Print(L"Failed to acquire RNG protocol: %r\n", err);
+                return err;
+        }
+        if (!rng) {
+                /* Print(L"RNG protocol not available.\n"); */
+                return EFI_UNSUPPORTED;
+        }
+
+        data = AllocatePool(size);
+        if (!data)
+                return log_oom();
+
+        err = uefi_call_wrapper(rng->GetRNG, 3, rng, NULL, size, data);
+        if (EFI_ERROR(err)) {
+                Print(L"Failed to acquire RNG data: %r\n", err);
+                return 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,
+                UINTN counter,
+                UINT8 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. A counter value
+         *
+         * And writes the result to the specified buffer.
+         */
+
+        struct sha256_ctx hash;
+
+        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(&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,
+                UINTN counter_start,
+                UINTN n,
+                VOID **ret) {
+
+        _cleanup_freepool_ VOID *output = NULL;
+        UINTN i;
+
+        /* Hashes the specified parameters in counter mode, generating n hash values, with the counter in the
+         * range counter_start…counter_start+n-1. */
+
+        output = AllocatePool(n * HASH_VALUE_SIZE);
+        if (!output)
+                return log_oom();
+
+        for (i = 0; i < n; i++)
+                hash_once(old_seed, rng, size,
+                          system_token, system_token_size,
+                          counter_start + i,
+                          (UINT8*) 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,
+                VOID **ret_new_seed,
+                VOID **ret_for_kernel) {
+
+        _cleanup_freepool_ VOID *new_seed = NULL, *for_kernel = NULL;
+        EFI_STATUS err;
+        UINTN n;
+
+        /* 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, 0, n, &new_seed);
+        if (EFI_ERROR(err))
+                return err;
+
+        /* Continue counting at 'n' for the seed for the kernel */
+        err = hash_many(old_seed, rng, size, system_token, system_token_size, n, n, &for_kernel);
+        if (EFI_ERROR(err))
+                return err;
+
+        *ret_new_seed = TAKE_PTR(new_seed);
+        *ret_for_kernel = TAKE_PTR(for_kernel);
+
+        return EFI_SUCCESS;
+}
+
+EFI_STATUS acquire_system_token(VOID **ret, UINTN *ret_size) {
+        _cleanup_freepool_ CHAR8 *data = NULL;
+        EFI_STATUS err;
+        UINTN size;
+
+        err = efivar_get_raw(&loader_guid, L"LoaderSystemToken", &data, &size);
+        if (EFI_ERROR(err)) {
+                if (err != EFI_NOT_FOUND)
+                        Print(L"Failed to read LoaderSystemToken EFI variable: %r", err);
+                return err;
+        }
+
+        if (size <= 0) {
+                Print(L"System token too short, ignoring.");
+                return EFI_NOT_FOUND;
+        }
+
+        *ret = TAKE_PTR(data);
+        *ret_size = size;
+
+        return EFI_SUCCESS;
+}
+
+static VOID validate_sha256(void) {
+
+#ifndef __OPTIMIZE__
+        /* 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 }},
+        };
+
+        UINTN i;
+
+        for (i = 0; i < ELEMENTSOF(array); i++) {
+                struct sha256_ctx hash;
+                uint8_t result[HASH_VALUE_SIZE];
+
+                sha256_init_ctx(&hash);
+                sha256_process_bytes(array[i].string, strlena((const CHAR8*) array[i].string), &hash);
+                sha256_finish_ctx(&hash, result);
+
+                if (CompareMem(result, array[i].hash, HASH_VALUE_SIZE) != 0) {
+                        Print(L"SHA256 failed validation.\n");
+                        uefi_call_wrapper(BS->Stall, 1, 120 * 1000 * 1000);
+                        return;
+                }
+        }
+
+        Print(L"SHA256 validated\n");
+#endif
+}
+
+EFI_STATUS process_random_seed(EFI_FILE *root_dir, RandomSeedMode mode) {
+        _cleanup_freepool_ VOID *seed = NULL, *new_seed = NULL, *rng = NULL, *for_kernel = NULL, *system_token = NULL;
+        _cleanup_(FileHandleClosep) EFI_FILE_HANDLE handle = NULL;
+        UINTN size, rsize, wsize, system_token_size = 0;
+        _cleanup_freepool_ EFI_FILE_INFO *info = NULL;
+        EFI_STATUS err;
+
+        validate_sha256();
+
+        if (mode == RANDOM_SEED_OFF) {
+                /* Print(L"Random seed handling turned off.\n"); */
+                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()) {
+                /* Print(L"Not loading random seed, because we are in SecureBoot mode.\n"); */
+                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) {
+                /* if (err == EFI_NOT_FOUND) */
+                /*         Print(L"Not loading random seed, because no system token is set.\n"); */
+                if (EFI_ERROR(err))
+                        return err; /* in all other error cases we already logged */
+        }
+
+        err = uefi_call_wrapper(root_dir->Open, 5, root_dir, &handle, L"\\loader\\random-seed", EFI_FILE_MODE_READ|EFI_FILE_MODE_WRITE, 0ULL);
+        if (EFI_ERROR(err)) {
+                if (err != EFI_NOT_FOUND)
+                        Print(L"Failed to open random seed file: %r\n", err);
+                /* else */
+                /*         Print(L"Not loading random seed, because there is none.\n"); */
+
+                return err;
+        }
+
+        info = LibFileInfo(handle);
+        if (!info)
+                return log_oom();
+
+        size = info->FileSize;
+        if (size < RANDOM_MAX_SIZE_MIN) {
+                Print(L"Random seed file is too short?\n");
+                return EFI_INVALID_PARAMETER;
+        }
+
+        if (size > RANDOM_MAX_SIZE_MAX) {
+                Print(L"Random seed file is too large?\n");
+                return EFI_INVALID_PARAMETER;
+        }
+
+        seed = AllocatePool(size);
+        if (!seed)
+                return log_oom();
+
+        rsize = size;
+        err = uefi_call_wrapper(handle->Read, 3, handle, &rsize, seed);
+        if (EFI_ERROR(err)) {
+                Print(L"Failed to read random seed file: %r\n", err);
+                return err;
+        }
+        if (rsize != size) {
+                Print(L"Short read on random seed file\n");
+                return EFI_PROTOCOL_ERROR;
+        }
+
+        err = uefi_call_wrapper(handle->SetPosition, 2, handle, 0);
+        if (EFI_ERROR(err)) {
+                Print(L"Failed to seek to beginning of random seed file: %r\n", err);
+                return 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 */
+
+        /* 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, &new_seed, &for_kernel);
+        if (EFI_ERROR(err))
+                return err;
+
+        /* Update the random seed on disk before we use it */
+        wsize = size;
+        err = uefi_call_wrapper(handle->Write, 3, handle, &wsize, new_seed);
+        if (EFI_ERROR(err)) {
+                Print(L"Failed to write random seed file: %r\n", err);
+                return err;
+        }
+        if (wsize != size) {
+                Print(L"Short write on random seed file\n");
+                return EFI_PROTOCOL_ERROR;
+        }
+
+        err = uefi_call_wrapper(handle->Flush, 1, handle);
+        if (EFI_ERROR(err)) {
+                Print(L"Failed to flush random seed file: %r\n");
+                return err;
+        }
+
+        /* We are good to go */
+        err = efivar_set_raw(&loader_guid, L"LoaderRandomSeed", for_kernel, size, FALSE);
+        if (EFI_ERROR(err)) {
+                Print(L"Failed to write random seed to EFI variable: %r\n", err);
+                return err;
+        }
+
+        return EFI_SUCCESS;
+}
diff --git a/src/boot/efi/random-seed.h b/src/boot/efi/random-seed.h
new file mode 100644
index 0000000000..0f750716fd
--- /dev/null
+++ b/src/boot/efi/random-seed.h
@@ -0,0 +1,14 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+#pragma once
+
+#include <efi.h>
+
+typedef enum RandomSeedMode {
+        RANDOM_SEED_OFF,
+        RANDOM_SEED_WITH_SYSTEM_TOKEN,
+        RANDOM_SEED_ALWAYS,
+        _RANDOM_SEED_MODE_MAX,
+        _RANDOM_SEED_MODE_INVALID = -1,
+} RandomSeedMode;
+
+EFI_STATUS process_random_seed(EFI_FILE *root_dir, RandomSeedMode mode);