1 // SPDX-License-Identifier: GPL-2.0 2 /* 3 * Copyright (C) 2022-2024 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved. 4 */ 5 6 #include <linux/cache.h> 7 #include <linux/kernel.h> 8 #include <linux/time64.h> 9 #include <vdso/datapage.h> 10 #include <vdso/getrandom.h> 11 #include <asm/vdso/getrandom.h> 12 #include <asm/vdso/vsyscall.h> 13 #include <asm/unaligned.h> 14 #include <uapi/linux/mman.h> 15 16 #define MEMCPY_AND_ZERO_SRC(type, dst, src, len) do { \ 17 while (len >= sizeof(type)) { \ 18 __put_unaligned_t(type, __get_unaligned_t(type, src), dst); \ 19 __put_unaligned_t(type, 0, src); \ 20 dst += sizeof(type); \ 21 src += sizeof(type); \ 22 len -= sizeof(type); \ 23 } \ 24 } while (0) 25 26 static void memcpy_and_zero_src(void *dst, void *src, size_t len) 27 { 28 if (IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)) { 29 if (IS_ENABLED(CONFIG_64BIT)) 30 MEMCPY_AND_ZERO_SRC(u64, dst, src, len); 31 MEMCPY_AND_ZERO_SRC(u32, dst, src, len); 32 MEMCPY_AND_ZERO_SRC(u16, dst, src, len); 33 } 34 MEMCPY_AND_ZERO_SRC(u8, dst, src, len); 35 } 36 37 /** 38 * __cvdso_getrandom_data - Generic vDSO implementation of getrandom() syscall. 39 * @rng_info: Describes state of kernel RNG, memory shared with kernel. 40 * @buffer: Destination buffer to fill with random bytes. 41 * @len: Size of @buffer in bytes. 42 * @flags: Zero or more GRND_* flags. 43 * @opaque_state: Pointer to an opaque state area. 44 * @opaque_len: Length of opaque state area. 45 * 46 * This implements a "fast key erasure" RNG using ChaCha20, in the same way that the kernel's 47 * getrandom() syscall does. It periodically reseeds its key from the kernel's RNG, at the same 48 * schedule that the kernel's RNG is reseeded. If the kernel's RNG is not ready, then this always 49 * calls into the syscall. 50 * 51 * If @buffer, @len, and @flags are 0, and @opaque_len is ~0UL, then @opaque_state is populated 52 * with a struct vgetrandom_opaque_params and the function returns 0; if it does not return 0, 53 * this function should not be used. 54 * 55 * @opaque_state *must* be allocated by calling mmap(2) using the mmap_prot and mmap_flags fields 56 * from the struct vgetrandom_opaque_params, and states must not straddle pages. Unless external 57 * locking is used, one state must be allocated per thread, as it is not safe to call this function 58 * concurrently with the same @opaque_state. However, it is safe to call this using the same 59 * @opaque_state that is shared between main code and signal handling code, within the same thread. 60 * 61 * Returns: The number of random bytes written to @buffer, or a negative value indicating an error. 62 */ 63 static __always_inline ssize_t 64 __cvdso_getrandom_data(const struct vdso_rng_data *rng_info, void *buffer, size_t len, 65 unsigned int flags, void *opaque_state, size_t opaque_len) 66 { 67 ssize_t ret = min_t(size_t, INT_MAX & PAGE_MASK /* = MAX_RW_COUNT */, len); 68 struct vgetrandom_state *state = opaque_state; 69 size_t batch_len, nblocks, orig_len = len; 70 bool in_use, have_retried = false; 71 unsigned long current_generation; 72 void *orig_buffer = buffer; 73 u32 counter[2] = { 0 }; 74 75 if (unlikely(opaque_len == ~0UL && !buffer && !len && !flags)) { 76 *(struct vgetrandom_opaque_params *)opaque_state = (struct vgetrandom_opaque_params) { 77 .size_of_opaque_state = sizeof(*state), 78 .mmap_prot = PROT_READ | PROT_WRITE, 79 .mmap_flags = MAP_DROPPABLE | MAP_ANONYMOUS 80 }; 81 return 0; 82 } 83 84 /* The state must not straddle a page, since pages can be zeroed at any time. */ 85 if (unlikely(((unsigned long)opaque_state & ~PAGE_MASK) + sizeof(*state) > PAGE_SIZE)) 86 return -EFAULT; 87 88 /* Handle unexpected flags by falling back to the kernel. */ 89 if (unlikely(flags & ~(GRND_NONBLOCK | GRND_RANDOM | GRND_INSECURE))) 90 goto fallback_syscall; 91 92 /* If the caller passes the wrong size, which might happen due to CRIU, fallback. */ 93 if (unlikely(opaque_len != sizeof(*state))) 94 goto fallback_syscall; 95 96 /* 97 * If the kernel's RNG is not yet ready, then it's not possible to provide random bytes from 98 * userspace, because A) the various @flags require this to block, or not, depending on 99 * various factors unavailable to userspace, and B) the kernel's behavior before the RNG is 100 * ready is to reseed from the entropy pool at every invocation. 101 */ 102 if (unlikely(!READ_ONCE(rng_info->is_ready))) 103 goto fallback_syscall; 104 105 /* 106 * This condition is checked after @rng_info->is_ready, because before the kernel's RNG is 107 * initialized, the @flags parameter may require this to block or return an error, even when 108 * len is zero. 109 */ 110 if (unlikely(!len)) 111 return 0; 112 113 /* 114 * @state->in_use is basic reentrancy protection against this running in a signal handler 115 * with the same @opaque_state, but obviously not atomic wrt multiple CPUs or more than one 116 * level of reentrancy. If a signal interrupts this after reading @state->in_use, but before 117 * writing @state->in_use, there is still no race, because the signal handler will run to 118 * its completion before returning execution. 119 */ 120 in_use = READ_ONCE(state->in_use); 121 if (unlikely(in_use)) 122 /* The syscall simply fills the buffer and does not touch @state, so fallback. */ 123 goto fallback_syscall; 124 WRITE_ONCE(state->in_use, true); 125 126 retry_generation: 127 /* 128 * @rng_info->generation must always be read here, as it serializes @state->key with the 129 * kernel's RNG reseeding schedule. 130 */ 131 current_generation = READ_ONCE(rng_info->generation); 132 133 /* 134 * If @state->generation doesn't match the kernel RNG's generation, then it means the 135 * kernel's RNG has reseeded, and so @state->key is reseeded as well. 136 */ 137 if (unlikely(state->generation != current_generation)) { 138 /* 139 * Write the generation before filling the key, in case of fork. If there is a fork 140 * just after this line, the parent and child will get different random bytes from 141 * the syscall, which is good. However, were this line to occur after the getrandom 142 * syscall, then both child and parent could have the same bytes and the same 143 * generation counter, so the fork would not be detected. Therefore, write 144 * @state->generation before the call to the getrandom syscall. 145 */ 146 WRITE_ONCE(state->generation, current_generation); 147 148 /* 149 * Prevent the syscall from being reordered wrt current_generation. Pairs with the 150 * smp_store_release(&_vdso_rng_data.generation) in random.c. 151 */ 152 smp_rmb(); 153 154 /* Reseed @state->key using fresh bytes from the kernel. */ 155 if (getrandom_syscall(state->key, sizeof(state->key), 0) != sizeof(state->key)) { 156 /* 157 * If the syscall failed to refresh the key, then @state->key is now 158 * invalid, so invalidate the generation so that it is not used again, and 159 * fallback to using the syscall entirely. 160 */ 161 WRITE_ONCE(state->generation, 0); 162 163 /* 164 * Set @state->in_use to false only after the last write to @state in the 165 * line above. 166 */ 167 WRITE_ONCE(state->in_use, false); 168 169 goto fallback_syscall; 170 } 171 172 /* 173 * Set @state->pos to beyond the end of the batch, so that the batch is refilled 174 * using the new key. 175 */ 176 state->pos = sizeof(state->batch); 177 } 178 179 /* Set len to the total amount of bytes that this function is allowed to read, ret. */ 180 len = ret; 181 more_batch: 182 /* 183 * First use bytes out of @state->batch, which may have been filled by the last call to this 184 * function. 185 */ 186 batch_len = min_t(size_t, sizeof(state->batch) - state->pos, len); 187 if (batch_len) { 188 /* Zeroing at the same time as memcpying helps preserve forward secrecy. */ 189 memcpy_and_zero_src(buffer, state->batch + state->pos, batch_len); 190 state->pos += batch_len; 191 buffer += batch_len; 192 len -= batch_len; 193 } 194 195 if (!len) { 196 /* Prevent the loop from being reordered wrt ->generation. */ 197 barrier(); 198 199 /* 200 * Since @rng_info->generation will never be 0, re-read @state->generation, rather 201 * than using the local current_generation variable, to learn whether a fork 202 * occurred or if @state was zeroed due to memory pressure. Primarily, though, this 203 * indicates whether the kernel's RNG has reseeded, in which case generate a new key 204 * and start over. 205 */ 206 if (unlikely(READ_ONCE(state->generation) != READ_ONCE(rng_info->generation))) { 207 /* 208 * Prevent this from looping forever in case of low memory or racing with a 209 * user force-reseeding the kernel's RNG using the ioctl. 210 */ 211 if (have_retried) { 212 WRITE_ONCE(state->in_use, false); 213 goto fallback_syscall; 214 } 215 216 have_retried = true; 217 buffer = orig_buffer; 218 goto retry_generation; 219 } 220 221 /* 222 * Set @state->in_use to false only when there will be no more reads or writes of 223 * @state. 224 */ 225 WRITE_ONCE(state->in_use, false); 226 return ret; 227 } 228 229 /* Generate blocks of RNG output directly into @buffer while there's enough room left. */ 230 nblocks = len / CHACHA_BLOCK_SIZE; 231 if (nblocks) { 232 __arch_chacha20_blocks_nostack(buffer, state->key, counter, nblocks); 233 buffer += nblocks * CHACHA_BLOCK_SIZE; 234 len -= nblocks * CHACHA_BLOCK_SIZE; 235 } 236 237 BUILD_BUG_ON(sizeof(state->batch_key) % CHACHA_BLOCK_SIZE != 0); 238 239 /* Refill the batch and overwrite the key, in order to preserve forward secrecy. */ 240 __arch_chacha20_blocks_nostack(state->batch_key, state->key, counter, 241 sizeof(state->batch_key) / CHACHA_BLOCK_SIZE); 242 243 /* Since the batch was just refilled, set the position back to 0 to indicate a full batch. */ 244 state->pos = 0; 245 goto more_batch; 246 247 fallback_syscall: 248 return getrandom_syscall(orig_buffer, orig_len, flags); 249 } 250 251 static __always_inline ssize_t 252 __cvdso_getrandom(void *buffer, size_t len, unsigned int flags, void *opaque_state, size_t opaque_len) 253 { 254 return __cvdso_getrandom_data(__arch_get_vdso_rng_data(), buffer, len, flags, opaque_state, opaque_len); 255 } 256