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 */ 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 */ 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 } 141 142 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