linux-kernel - [PATCH v3 5/5] efi: random: refresh non-volatile random seed when RNG is initialized

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Message-Id: <20221122020404.3476063-6-Jason@zx2c4.com>
Date:   Tue, 22 Nov 2022 03:04:04 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     linux-efi@...r.kernel.org, linux-crypto@...r.kernel.org,
        patches@...ts.linux.dev, linux-kernel@...r.kernel.org,
        ardb@...nel.org
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>
Subject: [PATCH v3 5/5] 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.

Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
---
 drivers/firmware/efi/efi.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
index f12cc29bd4b8..b23ec97d68ea 100644
--- 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:
-- 
2.38.1