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
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