1 /*
2 * Copyright (c) 2021, ARM Limited. All rights reserved.
3 *
4 * SPDX-License-Identifier: BSD-3-Clause
5 */
6
7 #include <assert.h>
8 #include <stdbool.h>
9 #include <stdint.h>
10 #include <lib/spinlock.h>
11 #include <plat/common/plat_trng.h>
12
13 /*
14 * # Entropy pool
15 * Note that the TRNG Firmware interface can request up to 192 bits of entropy
16 * in a single call or three 64bit words per call. We have 4 words in the pool
17 * so that when we have 1-63 bits in the pool, and we have a request for
18 * 192 bits of entropy, we don't have to throw out the leftover 1-63 bits of
19 * entropy.
20 */
21 #define WORDS_IN_POOL (4)
22 static uint64_t entropy[WORDS_IN_POOL];
23 /* index in bits of the first bit of usable entropy */
24 static uint32_t entropy_bit_index;
25 /* then number of valid bits in the entropy pool */
26 static uint32_t entropy_bit_size;
27
28 static spinlock_t trng_pool_lock;
29
30 #define BITS_PER_WORD (sizeof(entropy[0]) * 8)
31 #define BITS_IN_POOL (WORDS_IN_POOL * BITS_PER_WORD)
32 #define ENTROPY_MIN_WORD (entropy_bit_index / BITS_PER_WORD)
33 #define ENTROPY_FREE_BIT (entropy_bit_size + entropy_bit_index)
34 #define _ENTROPY_FREE_WORD (ENTROPY_FREE_BIT / BITS_PER_WORD)
35 #define ENTROPY_FREE_INDEX (_ENTROPY_FREE_WORD % WORDS_IN_POOL)
36 /* ENTROPY_WORD_INDEX(0) includes leftover bits in the lower bits */
37 #define ENTROPY_WORD_INDEX(i) ((ENTROPY_MIN_WORD + i) % WORDS_IN_POOL)
38
39 /*
40 * Fill the entropy pool until we have at least as many bits as requested.
41 * Returns true after filling the pool, and false if the entropy source is out
42 * of entropy and the pool could not be filled.
43 * Assumes locks are taken.
44 */
trng_fill_entropy(uint32_t nbits)45 static bool trng_fill_entropy(uint32_t nbits)
46 {
47 while (nbits > entropy_bit_size) {
48 bool valid = plat_get_entropy(&entropy[ENTROPY_FREE_INDEX]);
49
50 if (valid) {
51 entropy_bit_size += BITS_PER_WORD;
52 assert(entropy_bit_size <= BITS_IN_POOL);
53 } else {
54 return false;
55 }
56 }
57 return true;
58 }
59
60 /*
61 * Pack entropy into the out buffer, filling and taking locks as needed.
62 * Returns true on success, false on failure.
63 *
64 * Note: out must have enough space for nbits of entropy
65 */
trng_pack_entropy(uint32_t nbits,uint64_t * out)66 bool trng_pack_entropy(uint32_t nbits, uint64_t *out)
67 {
68 bool success = true;
69
70 spin_lock(&trng_pool_lock);
71
72 if (!trng_fill_entropy(nbits)) {
73 success = false;
74 goto out;
75 }
76
77 const unsigned int rshift = entropy_bit_index % BITS_PER_WORD;
78 const unsigned int lshift = BITS_PER_WORD - rshift;
79 const int to_fill = ((nbits + BITS_PER_WORD - 1) / BITS_PER_WORD);
80 int word_i;
81
82 for (word_i = 0; word_i < to_fill; word_i++) {
83 /*
84 * Repack the entropy from the pool into the passed in out
85 * buffer. This takes the lower bits from the valid upper bits
86 * of word_i and the upper bits from the lower bits of
87 * (word_i + 1).
88 *
89 * I found the following diagram useful. note: `e` represents
90 * valid entropy, ` ` represents invalid bits (not entropy) and
91 * `x` represents valid entropy that must not end up in the
92 * packed word.
93 *
94 * |---------entropy pool----------|
95 * C var |--(word_i + 1)-|----word_i-----|
96 * bit idx |7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|
97 * [x,x,e,e,e,e,e,e|e,e, , , , , , ]
98 * | [e,e,e,e,e,e,e,e] |
99 * | |--out[word_i]--| |
100 * lshift|---| |--rshift---|
101 *
102 * ==== Which is implemented as ====
103 *
104 * |---------entropy pool----------|
105 * C var |--(word_i + 1)-|----word_i-----|
106 * bit idx |7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|
107 * [x,x,e,e,e,e,e,e|e,e, , , , , , ]
108 * C expr << lshift >> rshift
109 * bit idx 5 4 3 2 1 0 7 6
110 * [e,e,e,e,e,e,0,0|0,0,0,0,0,0,e,e]
111 * ==== bit-wise or ====
112 * 5 4 3 2 1 0 7 6
113 * [e,e,e,e,e,e,e,e]
114 */
115 out[word_i] = 0;
116 out[word_i] |= entropy[ENTROPY_WORD_INDEX(word_i)] >> rshift;
117
118 /*
119 * Note that a shift of 64 bits is treated as a shift of 0 bits.
120 * When the shift amount is the same as the BITS_PER_WORD, we
121 * don't want to include the next word of entropy, so we skip
122 * the `|=` operation.
123 */
124 if (lshift != BITS_PER_WORD) {
125 out[word_i] |= entropy[ENTROPY_WORD_INDEX(word_i + 1)]
126 << lshift;
127 }
128 }
129 const uint64_t mask = ~0ULL >> (BITS_PER_WORD - (nbits % BITS_PER_WORD));
130
131 out[to_fill - 1] &= mask;
132
133 entropy_bit_index = (entropy_bit_index + nbits) % BITS_IN_POOL;
134 entropy_bit_size -= nbits;
135
136 out:
137 spin_unlock(&trng_pool_lock);
138
139 return success;
140 }
141
trng_entropy_pool_setup(void)142 void trng_entropy_pool_setup(void)
143 {
144 int i;
145
146 for (i = 0; i < WORDS_IN_POOL; i++) {
147 entropy[i] = 0;
148 }
149 entropy_bit_index = 0;
150 entropy_bit_size = 0;
151 }
152