Merge pull request #25319 from zx2c4-forks/krngseed

boot: implement kernel EFI RNG seed protocol with proper hashing

src/core/efi-random.c

@@ -12,79 +12,23 @@
 #include "random-util.h"
 #include "strv.h"
 
-/* If a random seed was passed by the boot loader in the LoaderRandomSeed EFI variable, let's credit it to
- * the kernel's random pool, but only once per boot. If this is run very early during initialization we can
- * instantly boot up with a filled random pool.
- *
- * This makes no judgement on the entropy passed, it's the job of the boot loader to only pass us a seed that
- * is suitably validated. */
-
-static void lock_down_efi_variables(void) {
+void lock_down_efi_variables(void) {
+        _cleanup_close_ int fd = -1;
         int r;
 
+        fd = open(EFIVAR_PATH(EFI_LOADER_VARIABLE(LoaderSystemToken)), O_RDONLY|O_CLOEXEC);
+        if (fd < 0) {
+                if (errno != ENOENT)
+                        log_warning_errno(errno, "Unable to open LoaderSystemToken EFI variable, ignoring: %m");
+                return;
+        }
+
         /* Paranoia: let's restrict access modes of these a bit, so that unprivileged users can't use them to
          * identify the system or gain too much insight into what we might have credited to the entropy
          * pool. */
-        FOREACH_STRING(path,
-                       EFIVAR_PATH(EFI_LOADER_VARIABLE(LoaderRandomSeed)),
-                       EFIVAR_PATH(EFI_LOADER_VARIABLE(LoaderSystemToken))) {
-
-                r = chattr_path(path, 0, FS_IMMUTABLE_FL, NULL);
-                if (r == -ENOENT)
-                        continue;
-                if (r < 0)
-                        log_warning_errno(r, "Failed to drop FS_IMMUTABLE_FL from %s, ignoring: %m", path);
-
-                if (chmod(path, 0600) < 0)
-                        log_warning_errno(errno, "Failed to reduce access mode of %s, ignoring: %m", path);
-        }
-}
-
-int efi_take_random_seed(void) {
-        _cleanup_free_ void *value = NULL;
-        size_t size;
-        int r;
-
-        /* Paranoia comes first. */
-        lock_down_efi_variables();
-
-        if (access("/run/systemd/efi-random-seed-taken", F_OK) < 0) {
-                if (errno != ENOENT) {
-                        log_warning_errno(errno, "Failed to determine whether we already used the random seed token, not using it.");
-                        return 0;
-                }
-
-                /* ENOENT means we haven't used it yet. */
-        } else {
-                log_debug("EFI random seed already used, not using again.");
-                return 0;
-        }
-
-        r = efi_get_variable(EFI_LOADER_VARIABLE(LoaderRandomSeed), NULL, &value, &size);
-        if (r == -EOPNOTSUPP) {
-                log_debug_errno(r, "System lacks EFI support, not initializing random seed from EFI variable.");
-                return 0;
-        }
-        if (r == -ENOENT) {
-                log_debug_errno(r, "Boot loader did not pass LoaderRandomSeed EFI variable, not crediting any entropy.");
-                return 0;
-        }
+        r = chattr_fd(fd, 0, FS_IMMUTABLE_FL, NULL);
         if (r < 0)
-                return log_warning_errno(r, "Failed to read LoaderRandomSeed EFI variable, ignoring: %m");
-
-        if (size == 0)
-                return log_warning_errno(SYNTHETIC_ERRNO(EINVAL), "Random seed passed from boot loader has zero size? Ignoring.");
-
-        /* Before we use the seed, let's mark it as used, so that we never credit it twice. Also, it's a nice
-         * way to let users known that we successfully acquired entropy from the boot loader. */
-        r = touch("/run/systemd/efi-random-seed-taken");
-        if (r < 0)
-                return log_warning_errno(r, "Unable to mark EFI random seed as used, not using it: %m");
-
-        r = random_write_entropy(-1, value, size, true);
-        if (r < 0)
-                return log_warning_errno(errno, "Failed to credit entropy, ignoring: %m");
-
-        log_info("Successfully credited entropy passed from boot loader.");
-        return 1;
+                log_warning_errno(r, "Failed to drop FS_IMMUTABLE_FL from LoaderSystemToken EFI variable, ignoring: %m");
+        if (fchmod(fd, 0600) < 0)
+                log_warning_errno(errno, "Failed to reduce access mode of LoaderSystemToken EFI variable, ignoring: %m");
 }

src/core/efi-random.h

@@ -1,4 +1,4 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
-int efi_take_random_seed(void);
+void lock_down_efi_variables(void);

src/core/main.c

@@ -2831,8 +2831,8 @@ int main(int argc, char *argv[]) {
                         goto finish;
                 }
 
-                /* The efivarfs is now mounted, let's read the random seed off it */
-                (void) efi_take_random_seed();
+                /* The efivarfs is now mounted, let's lock down the system token. */
+                lock_down_efi_variables();
 
                 /* Cache command-line options passed from EFI variables */
                 if (!skip_setup)