1// SPDX-License-Identifier: GPL-2.0
2/*
3 * Copyright (C) 2016 Linaro Ltd;  <ard.biesheuvel@linaro.org>
4 */
5
6#include <linux/efi.h>
7#include <asm/efi.h>
8
9#include "efistub.h"
10
11typedef union efi_rng_protocol efi_rng_protocol_t;
12
13union efi_rng_protocol {
14	struct {
15		efi_status_t (__efiapi *get_info)(efi_rng_protocol_t *,
16						  unsigned long *,
17						  efi_guid_t *);
18		efi_status_t (__efiapi *get_rng)(efi_rng_protocol_t *,
19						 efi_guid_t *, unsigned long,
20						 u8 *out);
21	};
22	struct {
23		u32 get_info;
24		u32 get_rng;
25	} mixed_mode;
26};
27
28/**
29 * efi_get_random_bytes() - fill a buffer with random bytes
30 * @size:	size of the buffer
31 * @out:	caller allocated buffer to receive the random bytes
32 *
33 * The call will fail if either the firmware does not implement the
34 * EFI_RNG_PROTOCOL or there are not enough random bytes available to fill
35 * the buffer.
36 *
37 * Return:	status code
38 */
39efi_status_t efi_get_random_bytes(unsigned long size, u8 *out)
40{
41	efi_guid_t rng_proto = EFI_RNG_PROTOCOL_GUID;
42	efi_status_t status;
43	efi_rng_protocol_t *rng = NULL;
44
45	status = efi_bs_call(locate_protocol, &rng_proto, NULL, (void **)&rng);
46	if (status != EFI_SUCCESS)
47		return status;
48
49	return efi_call_proto(rng, get_rng, NULL, size, out);
50}
51
52/**
53 * efi_random_get_seed() - provide random seed as configuration table
54 *
55 * The EFI_RNG_PROTOCOL is used to read random bytes. These random bytes are
56 * saved as a configuration table which can be used as entropy by the kernel
57 * for the initialization of its pseudo random number generator.
58 *
59 * If the EFI_RNG_PROTOCOL is not available or there are not enough random bytes
60 * available, the configuration table will not be installed and an error code
61 * will be returned.
62 *
63 * Return:	status code
64 */
65efi_status_t efi_random_get_seed(void)
66{
67	efi_guid_t rng_proto = EFI_RNG_PROTOCOL_GUID;
68	efi_guid_t rng_algo_raw = EFI_RNG_ALGORITHM_RAW;
69	efi_guid_t rng_table_guid = LINUX_EFI_RANDOM_SEED_TABLE_GUID;
70	struct linux_efi_random_seed *prev_seed, *seed = NULL;
71	int prev_seed_size = 0, seed_size = EFI_RANDOM_SEED_SIZE;
72	unsigned long nv_seed_size = 0, offset = 0;
73	efi_rng_protocol_t *rng = NULL;
74	efi_status_t status;
75
76	status = efi_bs_call(locate_protocol, &rng_proto, NULL, (void **)&rng);
77	if (status != EFI_SUCCESS)
78		seed_size = 0;
79
80	// Call GetVariable() with a zero length buffer to obtain the size
81	get_efi_var(L"RandomSeed", &rng_table_guid, NULL, &nv_seed_size, NULL);
82	if (!seed_size && !nv_seed_size)
83		return status;
84
85	seed_size += nv_seed_size;
86
87	/*
88	 * Check whether a seed was provided by a prior boot stage. In that
89	 * case, instead of overwriting it, let's create a new buffer that can
90	 * hold both, and concatenate the existing and the new seeds.
91	 * Note that we should read the seed size with caution, in case the
92	 * table got corrupted in memory somehow.
93	 */
94	prev_seed = get_efi_config_table(rng_table_guid);
95	if (prev_seed && prev_seed->size <= 512U) {
96		prev_seed_size = prev_seed->size;
97		seed_size += prev_seed_size;
98	}
99
100	/*
101	 * Use EFI_ACPI_RECLAIM_MEMORY here so that it is guaranteed that the
102	 * allocation will survive a kexec reboot (although we refresh the seed
103	 * beforehand)
104	 */
105	status = efi_bs_call(allocate_pool, EFI_ACPI_RECLAIM_MEMORY,
106			     struct_size(seed, bits, seed_size),
107			     (void **)&seed);
108	if (status != EFI_SUCCESS) {
109		efi_warn("Failed to allocate memory for RNG seed.\n");
110		goto err_warn;
111	}
112
113	if (rng) {
114		status = efi_call_proto(rng, get_rng, &rng_algo_raw,
115					EFI_RANDOM_SEED_SIZE, seed->bits);
116
117		if (status == EFI_UNSUPPORTED)
118			/*
119			 * Use whatever algorithm we have available if the raw algorithm
120			 * is not implemented.
121			 */
122			status = efi_call_proto(rng, get_rng, NULL,
123						EFI_RANDOM_SEED_SIZE, seed->bits);
124
125		if (status == EFI_SUCCESS)
126			offset = EFI_RANDOM_SEED_SIZE;
127	}
128
129	if (nv_seed_size) {
130		status = get_efi_var(L"RandomSeed", &rng_table_guid, NULL,
131				     &nv_seed_size, seed->bits + offset);
132
133		if (status == EFI_SUCCESS)
134			/*
135			 * We delete the seed here, and /hope/ that this causes
136			 * EFI to also zero out its representation on disk.
137			 * This is somewhat idealistic, but overwriting the
138			 * variable with zeros is likely just as fraught too.
139			 * TODO: in the future, maybe we can hash it forward
140			 * instead, and write a new seed.
141			 */
142			status = set_efi_var(L"RandomSeed", &rng_table_guid, 0,
143					     0, NULL);
144
145		if (status == EFI_SUCCESS)
146			offset += nv_seed_size;
147		else
148			memzero_explicit(seed->bits + offset, nv_seed_size);
149	}
150
151	if (!offset)
152		goto err_freepool;
153
154	if (prev_seed_size) {
155		memcpy(seed->bits + offset, prev_seed->bits, prev_seed_size);
156		offset += prev_seed_size;
157	}
158
159	seed->size = offset;
160	status = efi_bs_call(install_configuration_table, &rng_table_guid, seed);
161	if (status != EFI_SUCCESS)
162		goto err_freepool;
163
164	if (prev_seed_size) {
165		/* wipe and free the old seed if we managed to install the new one */
166		memzero_explicit(prev_seed->bits, prev_seed_size);
167		efi_bs_call(free_pool, prev_seed);
168	}
169	return EFI_SUCCESS;
170
171err_freepool:
172	memzero_explicit(seed, struct_size(seed, bits, seed_size));
173	efi_bs_call(free_pool, seed);
174	efi_warn("Failed to obtain seed from EFI_RNG_PROTOCOL or EFI variable\n");
175err_warn:
176	if (prev_seed)
177		efi_warn("Retaining bootloader-supplied seed only");
178	return status;
179}
180