[PATCH RFC v1 4/6] efi: stub: use random seed from EFI variable

["Jason A. Donenfeld" <Jason@zx2c4.com>]

Since we have some storage, we can manage non-volatile seeds directly
from EFISTUB. This commit passes the contents of an EFI variable to the
kernel via the existing configuration table.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 drivers/firmware/efi/libstub/random.c | 59 +++++++++++++++++++++------
 include/linux/efi.h                   |  1 +
 2 files changed, 48 insertions(+), 12 deletions(-)

diff --git a/drivers/firmware/efi/libstub/random.c b/drivers/firmware/efi/libstub/random.c
index f85d2c066877..1e72013a6457 100644
--- a/drivers/firmware/efi/libstub/random.c
+++ b/drivers/firmware/efi/libstub/random.c
@@ -67,14 +67,25 @@ efi_status_t efi_random_get_seed(void)
 	efi_guid_t rng_proto = EFI_RNG_PROTOCOL_GUID;
 	efi_guid_t rng_algo_raw = EFI_RNG_ALGORITHM_RAW;
 	efi_guid_t rng_table_guid = LINUX_EFI_RANDOM_SEED_TABLE_GUID;
+	efi_char16_t rng_nv_seed_var[] = LINUX_EFI_RANDOM_NV_SEED_VAR;
 	struct linux_efi_random_seed *prev_seed, *seed = NULL;
-	int prev_seed_size = 0, seed_size = EFI_RANDOM_SEED_SIZE;
+	u8 nv_seed[EFI_RANDOM_SEED_SIZE];
+	unsigned long prev_seed_size = 0, nv_seed_size = sizeof(nv_seed), seed_size = 0, offset = 0;
 	efi_rng_protocol_t *rng = NULL;
 	efi_status_t status;
 
 	status = efi_bs_call(locate_protocol, &rng_proto, NULL, (void **)&rng);
-	if (status != EFI_SUCCESS)
-		return status;
+	if (status == EFI_SUCCESS)
+		seed_size += EFI_RANDOM_SEED_SIZE;
+
+	status = get_efi_var(rng_nv_seed_var, &rng_table_guid, NULL, &nv_seed_size, nv_seed);
+	if (status == EFI_SUCCESS)
+		seed_size += nv_seed_size;
+	else
+		nv_seed_size = 0;
+
+	if (!seed_size)
+		return EFI_NOT_FOUND;
 
 	/*
 	 * Check whether a seed was provided by a prior boot stage. In that
@@ -83,7 +94,7 @@ efi_status_t efi_random_get_seed(void)
 	 * Note that we should read the seed size with caution, in case the
 	 * table got corrupted in memory somehow.
 	 */
-	prev_seed = get_efi_config_table(LINUX_EFI_RANDOM_SEED_TABLE_GUID);
+	prev_seed = get_efi_config_table(rng_table_guid);
 	if (prev_seed && prev_seed->size <= 512U) {
 		prev_seed_size = prev_seed->size;
 		seed_size += prev_seed_size;
@@ -103,7 +114,7 @@ efi_status_t efi_random_get_seed(void)
 	}
 
 	status = efi_call_proto(rng, get_rng, &rng_algo_raw,
-				EFI_RANDOM_SEED_SIZE, seed->bits);
+				EFI_RANDOM_SEED_SIZE, seed->bits + offset);
 
 	if (status == EFI_UNSUPPORTED)
 		/*
@@ -111,16 +122,40 @@ efi_status_t efi_random_get_seed(void)
 		 * is not implemented.
 		 */
 		status = efi_call_proto(rng, get_rng, NULL,
-					EFI_RANDOM_SEED_SIZE, seed->bits);
+					EFI_RANDOM_SEED_SIZE, seed->bits + offset);
 
-	if (status != EFI_SUCCESS)
+	if (status == EFI_SUCCESS)
+		offset += EFI_RANDOM_SEED_SIZE;
+
+	if (nv_seed_size) {
+		memcpy(seed->bits + offset, nv_seed, nv_seed_size);
+		memzero_explicit(nv_seed, nv_seed_size);
+		/*
+		 * Zero it out before committing to using it. TODO: in the
+		 * future, maybe we can hash it forward instead, which is
+		 * better and also reduces the amount of writes here.
+		 */
+		status = set_efi_var(rng_nv_seed_var, &rng_table_guid,
+				     EFI_VARIABLE_NON_VOLATILE |
+				     EFI_VARIABLE_BOOTSERVICE_ACCESS |
+				     EFI_VARIABLE_RUNTIME_ACCESS, nv_seed_size, nv_seed);
+		if (status == EFI_SUCCESS)
+			status = set_efi_var(rng_nv_seed_var, &rng_table_guid, 0, 0, NULL);
+		if (status == EFI_SUCCESS)
+			offset += nv_seed_size;
+		else
+			memzero_explicit(seed->bits + offset, nv_seed_size);
+	}
+
+	if (!offset)
 		goto err_freepool;
 
-	seed->size = seed_size;
-	if (prev_seed_size)
-		memcpy(seed->bits + EFI_RANDOM_SEED_SIZE, prev_seed->bits,
-		       prev_seed_size);
+	if (prev_seed_size) {
+		memcpy(seed->bits + offset, prev_seed->bits, prev_seed_size);
+		offset += prev_seed_size;
+	}
 
+	seed->size = offset;
 	status = efi_bs_call(install_configuration_table, &rng_table_guid, seed);
 	if (status != EFI_SUCCESS)
 		goto err_freepool;
@@ -135,7 +170,7 @@ efi_status_t efi_random_get_seed(void)
 err_freepool:
 	memzero_explicit(seed, struct_size(seed, bits, seed_size));
 	efi_bs_call(free_pool, seed);
-	efi_warn("Failed to obtain seed from EFI_RNG_PROTOCOL\n");
+	efi_warn("Failed to obtain seed from EFI_RNG_PROTOCOL and EFI variable\n");
 err_warn:
 	if (prev_seed)
 		efi_warn("Retaining bootloader-supplied seed only");
diff --git a/include/linux/efi.h b/include/linux/efi.h
index 4aa1dbc7b064..0e93510b23b5 100644
--- a/include/linux/efi.h
+++ b/include/linux/efi.h
@@ -408,6 +408,7 @@ void efi_native_runtime_setup(void);
 #define LINUX_EFI_ARM_CPU_STATE_TABLE_GUID	EFI_GUID(0xef79e4aa, 0x3c3d, 0x4989,  0xb9, 0x02, 0x07, 0xa9, 0x43, 0xe5, 0x50, 0xd2)
 #define LINUX_EFI_LOADER_ENTRY_GUID		EFI_GUID(0x4a67b082, 0x0a4c, 0x41cf,  0xb6, 0xc7, 0x44, 0x0b, 0x29, 0xbb, 0x8c, 0x4f)
 #define LINUX_EFI_RANDOM_SEED_TABLE_GUID	EFI_GUID(0x1ce1e5bc, 0x7ceb, 0x42f2,  0x81, 0xe5, 0x8a, 0xad, 0xf1, 0x80, 0xf5, 0x7b)
+#define LINUX_EFI_RANDOM_NV_SEED_VAR		L"RandomNVSeed"
 #define LINUX_EFI_TPM_EVENT_LOG_GUID		EFI_GUID(0xb7799cb0, 0xeca2, 0x4943,  0x96, 0x67, 0x1f, 0xae, 0x07, 0xb7, 0x47, 0xfa)
 #define LINUX_EFI_TPM_FINAL_LOG_GUID		EFI_GUID(0x1e2ed096, 0x30e2, 0x4254,  0xbd, 0x89, 0x86, 0x3b, 0xbe, 0xf8, 0x23, 0x25)
 #define LINUX_EFI_MEMRESERVE_TABLE_GUID		EFI_GUID(0x888eb0c6, 0x8ede, 0x4ff5,  0xa8, 0xf0, 0x9a, 0xee, 0x5c, 0xb9, 0x77, 0xc2)
-- 
2.38.1