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
11 typedef union efi_rng_protocol efi_rng_protocol_t;
12
13 union 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 */
efi_get_random_bytes(unsigned long size,u8 * out)39 efi_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 */
efi_random_get_seed(void)65 efi_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
171 err_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");
175 err_warn:
176 if (prev_seed)
177 efi_warn("Retaining bootloader-supplied seed only");
178 return status;
179 }
180