efi: random: refresh non-volatile random seed when RNG is initialized

EFI has a rather unique benefit that it has access to some limited
non-volatile storage, where the kernel can store a random seed. Register
a notification for when the RNG is initialized, and at that point, store
a new random seed.

Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
index a46df5d1d0942751635276f6f1dc2710305ffae1..16dae588f0e38bbfacaa745937c03df4050bfc64 100644 (file)
--- a/drivers/firmware/efi/efi.c
+++ b/drivers/firmware/efi/efi.c
@@ -337,6 +337,24 @@ static void __init efi_debugfs_init(void)
 static inline void efi_debugfs_init(void) {}
 #endif
 
+static void refresh_nv_rng_seed(struct work_struct *work)
+{
+       u8 seed[EFI_RANDOM_SEED_SIZE];
+
+       get_random_bytes(seed, sizeof(seed));
+       efi.set_variable(L"RandomSeed", &LINUX_EFI_RANDOM_SEED_TABLE_GUID,
+                        EFI_VARIABLE_NON_VOLATILE | EFI_VARIABLE_BOOTSERVICE_ACCESS |
+                        EFI_VARIABLE_RUNTIME_ACCESS, sizeof(seed), seed);
+       memzero_explicit(seed, sizeof(seed));
+}
+static int refresh_nv_rng_seed_notification(struct notifier_block *nb, unsigned long action, void *data)
+{
+       static DECLARE_WORK(work, refresh_nv_rng_seed);
+       schedule_work(&work);
+       return NOTIFY_DONE;
+}
+static struct notifier_block refresh_nv_rng_seed_nb = { .notifier_call = refresh_nv_rng_seed_notification };
+
 /*
  * We register the efi subsystem with the firmware subsystem and the
  * efivars subsystem with the efi subsystem, if the system was booted with
@@ -413,6 +431,7 @@ static int __init efisubsys_init(void)
                platform_device_register_simple("efi_secret", 0, NULL, 0);
 #endif
 
+       execute_with_initialized_rng(&refresh_nv_rng_seed_nb);
        return 0;
 
 err_remove_group: