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  */
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  */
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