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/array_size.h>
7 #include <linux/minmax.h>
8 #include <vdso/datapage.h>
9 #include <vdso/getrandom.h>
10 #include <vdso/unaligned.h>
11 #include <asm/vdso/getrandom.h>
12 #include <uapi/linux/mman.h>
13 #include <uapi/linux/random.h>
14
15 #undef PAGE_SIZE
16 #undef PAGE_MASK
17 #define PAGE_SIZE (1UL << CONFIG_PAGE_SHIFT)
18 #define PAGE_MASK (~(PAGE_SIZE - 1))
19
20 #define MEMCPY_AND_ZERO_SRC(type, dst, src, len) do { \
21 while (len >= sizeof(type)) { \
22 __put_unaligned_t(type, __get_unaligned_t(type, src), dst); \
23 __put_unaligned_t(type, 0, src); \
24 dst += sizeof(type); \
25 src += sizeof(type); \
26 len -= sizeof(type); \
27 } \
28 } while (0)
29
memcpy_and_zero_src(void * dst,void * src,size_t len)30 static void memcpy_and_zero_src(void *dst, void *src, size_t len)
31 {
32 if (IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)) {
33 if (IS_ENABLED(CONFIG_64BIT))
34 MEMCPY_AND_ZERO_SRC(u64, dst, src, len);
35 MEMCPY_AND_ZERO_SRC(u32, dst, src, len);
36 MEMCPY_AND_ZERO_SRC(u16, dst, src, len);
37 }
38 MEMCPY_AND_ZERO_SRC(u8, dst, src, len);
39 }
40
41 /**
42 * __cvdso_getrandom_data - Generic vDSO implementation of getrandom() syscall.
43 * @rng_info: Describes state of kernel RNG, memory shared with kernel.
44 * @buffer: Destination buffer to fill with random bytes.
45 * @len: Size of @buffer in bytes.
46 * @flags: Zero or more GRND_* flags.
47 * @opaque_state: Pointer to an opaque state area.
48 * @opaque_len: Length of opaque state area.
49 *
50 * This implements a "fast key erasure" RNG using ChaCha20, in the same way that the kernel's
51 * getrandom() syscall does. It periodically reseeds its key from the kernel's RNG, at the same
52 * schedule that the kernel's RNG is reseeded. If the kernel's RNG is not ready, then this always
53 * calls into the syscall.
54 *
55 * If @buffer, @len, and @flags are 0, and @opaque_len is ~0UL, then @opaque_state is populated
56 * with a struct vgetrandom_opaque_params and the function returns 0; if it does not return 0,
57 * this function should not be used.
58 *
59 * @opaque_state *must* be allocated by calling mmap(2) using the mmap_prot and mmap_flags fields
60 * from the struct vgetrandom_opaque_params, and states must not straddle pages. Unless external
61 * locking is used, one state must be allocated per thread, as it is not safe to call this function
62 * concurrently with the same @opaque_state. However, it is safe to call this using the same
63 * @opaque_state that is shared between main code and signal handling code, within the same thread.
64 *
65 * Returns: The number of random bytes written to @buffer, or a negative value indicating an error.
66 */
67 static __always_inline ssize_t
__cvdso_getrandom_data(const struct vdso_rng_data * rng_info,void * buffer,size_t len,unsigned int flags,void * opaque_state,size_t opaque_len)68 __cvdso_getrandom_data(const struct vdso_rng_data *rng_info, void *buffer, size_t len,
69 unsigned int flags, void *opaque_state, size_t opaque_len)
70 {
71 ssize_t ret = min_t(size_t, INT_MAX & PAGE_MASK /* = MAX_RW_COUNT */, len);
72 struct vgetrandom_state *state = opaque_state;
73 size_t batch_len, nblocks, orig_len = len;
74 bool in_use, have_retried = false;
75 void *orig_buffer = buffer;
76 u64 current_generation;
77 u32 counter[2] = { 0 };
78
79 if (unlikely(opaque_len == ~0UL && !buffer && !len && !flags)) {
80 struct vgetrandom_opaque_params *params = opaque_state;
81 params->size_of_opaque_state = sizeof(*state);
82 params->mmap_prot = PROT_READ | PROT_WRITE;
83 params->mmap_flags = MAP_DROPPABLE | MAP_ANONYMOUS;
84 for (size_t i = 0; i < ARRAY_SIZE(params->reserved); ++i)
85 params->reserved[i] = 0;
86 return 0;
87 }
88
89 /* The state must not straddle a page, since pages can be zeroed at any time. */
90 if (unlikely(((unsigned long)opaque_state & ~PAGE_MASK) + sizeof(*state) > PAGE_SIZE))
91 return -EFAULT;
92
93 /* Handle unexpected flags by falling back to the kernel. */
94 if (unlikely(flags & ~(GRND_NONBLOCK | GRND_RANDOM | GRND_INSECURE)))
95 goto fallback_syscall;
96
97 /* If the caller passes the wrong size, which might happen due to CRIU, fallback. */
98 if (unlikely(opaque_len != sizeof(*state)))
99 goto fallback_syscall;
100
101 /*
102 * If the kernel's RNG is not yet ready, then it's not possible to provide random bytes from
103 * userspace, because A) the various @flags require this to block, or not, depending on
104 * various factors unavailable to userspace, and B) the kernel's behavior before the RNG is
105 * ready is to reseed from the entropy pool at every invocation.
106 */
107 if (unlikely(!READ_ONCE(rng_info->is_ready)))
108 goto fallback_syscall;
109
110 /*
111 * This condition is checked after @rng_info->is_ready, because before the kernel's RNG is
112 * initialized, the @flags parameter may require this to block or return an error, even when
113 * len is zero.
114 */
115 if (unlikely(!len))
116 return 0;
117
118 /*
119 * @state->in_use is basic reentrancy protection against this running in a signal handler
120 * with the same @opaque_state, but obviously not atomic wrt multiple CPUs or more than one
121 * level of reentrancy. If a signal interrupts this after reading @state->in_use, but before
122 * writing @state->in_use, there is still no race, because the signal handler will run to
123 * its completion before returning execution.
124 */
125 in_use = READ_ONCE(state->in_use);
126 if (unlikely(in_use))
127 /* The syscall simply fills the buffer and does not touch @state, so fallback. */
128 goto fallback_syscall;
129 WRITE_ONCE(state->in_use, true);
130
131 retry_generation:
132 /*
133 * @rng_info->generation must always be read here, as it serializes @state->key with the
134 * kernel's RNG reseeding schedule.
135 */
136 current_generation = READ_ONCE(rng_info->generation);
137
138 /*
139 * If @state->generation doesn't match the kernel RNG's generation, then it means the
140 * kernel's RNG has reseeded, and so @state->key is reseeded as well.
141 */
142 if (unlikely(state->generation != current_generation)) {
143 /*
144 * Write the generation before filling the key, in case of fork. If there is a fork
145 * just after this line, the parent and child will get different random bytes from
146 * the syscall, which is good. However, were this line to occur after the getrandom
147 * syscall, then both child and parent could have the same bytes and the same
148 * generation counter, so the fork would not be detected. Therefore, write
149 * @state->generation before the call to the getrandom syscall.
150 */
151 WRITE_ONCE(state->generation, current_generation);
152
153 /*
154 * Prevent the syscall from being reordered wrt current_generation. Pairs with the
155 * smp_store_release(&_vdso_rng_data.generation) in random.c.
156 */
157 smp_rmb();
158
159 /* Reseed @state->key using fresh bytes from the kernel. */
160 if (getrandom_syscall(state->key, sizeof(state->key), 0) != sizeof(state->key)) {
161 /*
162 * If the syscall failed to refresh the key, then @state->key is now
163 * invalid, so invalidate the generation so that it is not used again, and
164 * fallback to using the syscall entirely.
165 */
166 WRITE_ONCE(state->generation, 0);
167
168 /*
169 * Set @state->in_use to false only after the last write to @state in the
170 * line above.
171 */
172 WRITE_ONCE(state->in_use, false);
173
174 goto fallback_syscall;
175 }
176
177 /*
178 * Set @state->pos to beyond the end of the batch, so that the batch is refilled
179 * using the new key.
180 */
181 state->pos = sizeof(state->batch);
182 }
183
184 /* Set len to the total amount of bytes that this function is allowed to read, ret. */
185 len = ret;
186 more_batch:
187 /*
188 * First use bytes out of @state->batch, which may have been filled by the last call to this
189 * function.
190 */
191 batch_len = min_t(size_t, sizeof(state->batch) - state->pos, len);
192 if (batch_len) {
193 /* Zeroing at the same time as memcpying helps preserve forward secrecy. */
194 memcpy_and_zero_src(buffer, state->batch + state->pos, batch_len);
195 state->pos += batch_len;
196 buffer += batch_len;
197 len -= batch_len;
198 }
199
200 if (!len) {
201 /* Prevent the loop from being reordered wrt ->generation. */
202 barrier();
203
204 /*
205 * Since @rng_info->generation will never be 0, re-read @state->generation, rather
206 * than using the local current_generation variable, to learn whether a fork
207 * occurred or if @state was zeroed due to memory pressure. Primarily, though, this
208 * indicates whether the kernel's RNG has reseeded, in which case generate a new key
209 * and start over.
210 */
211 if (unlikely(READ_ONCE(state->generation) != READ_ONCE(rng_info->generation))) {
212 /*
213 * Prevent this from looping forever in case of low memory or racing with a
214 * user force-reseeding the kernel's RNG using the ioctl.
215 */
216 if (have_retried) {
217 WRITE_ONCE(state->in_use, false);
218 goto fallback_syscall;
219 }
220
221 have_retried = true;
222 buffer = orig_buffer;
223 goto retry_generation;
224 }
225
226 /*
227 * Set @state->in_use to false only when there will be no more reads or writes of
228 * @state.
229 */
230 WRITE_ONCE(state->in_use, false);
231 return ret;
232 }
233
234 /* Generate blocks of RNG output directly into @buffer while there's enough room left. */
235 nblocks = len / CHACHA_BLOCK_SIZE;
236 if (nblocks) {
237 __arch_chacha20_blocks_nostack(buffer, state->key, counter, nblocks);
238 buffer += nblocks * CHACHA_BLOCK_SIZE;
239 len -= nblocks * CHACHA_BLOCK_SIZE;
240 }
241
242 BUILD_BUG_ON(sizeof(state->batch_key) % CHACHA_BLOCK_SIZE != 0);
243
244 /* Refill the batch and overwrite the key, in order to preserve forward secrecy. */
245 __arch_chacha20_blocks_nostack(state->batch_key, state->key, counter,
246 sizeof(state->batch_key) / CHACHA_BLOCK_SIZE);
247
248 /* Since the batch was just refilled, set the position back to 0 to indicate a full batch. */
249 state->pos = 0;
250 goto more_batch;
251
252 fallback_syscall:
253 return getrandom_syscall(orig_buffer, orig_len, flags);
254 }
255
256 static __always_inline ssize_t
__cvdso_getrandom(void * buffer,size_t len,unsigned int flags,void * opaque_state,size_t opaque_len)257 __cvdso_getrandom(void *buffer, size_t len, unsigned int flags, void *opaque_state, size_t opaque_len)
258 {
259 return __cvdso_getrandom_data(__arch_get_vdso_rng_data(), buffer, len, flags, opaque_state, opaque_len);
260 }
261