1 // SPDX-License-Identifier: GPL-2.0
3 * s390 arch random implementation.
5 * Copyright IBM Corp. 2017, 2020
6 * Author(s): Harald Freudenberger
8 * The s390_arch_random_generate() function may be called from random.c
9 * in interrupt context. So this implementation does the best to be very
10 * fast. There is a buffer of random data which is asynchronously checked
11 * and filled by a workqueue thread.
12 * If there are enough bytes in the buffer the s390_arch_random_generate()
13 * just delivers these bytes. Otherwise false is returned until the
14 * worker thread refills the buffer.
15 * The worker fills the rng buffer by pulling fresh entropy from the
16 * high quality (but slow) true hardware random generator. This entropy
17 * is then spread over the buffer with an pseudo random generator PRNG.
18 * As the arch_get_random_seed_long() fetches 8 bytes and the calling
19 * function add_interrupt_randomness() counts this as 1 bit entropy the
20 * distribution needs to make sure there is in fact 1 bit entropy contained
21 * in 8 bytes of the buffer. The current values pull 32 byte entropy
22 * and scatter this into a 2048 byte buffer. So 8 byte in the buffer
23 * will contain 1 bit of entropy.
24 * The worker thread is rescheduled based on the charge level of the
25 * buffer but at least with 500 ms delay to avoid too much CPU consumption.
26 * So the max. amount of rng data delivered via arch_get_random_seed is
27 * limited to 4k bytes per second.
30 #include <linux/kernel.h>
31 #include <linux/atomic.h>
32 #include <linux/random.h>
33 #include <linux/slab.h>
34 #include <linux/static_key.h>
35 #include <linux/workqueue.h>
36 #include <linux/moduleparam.h>
37 #include <asm/cpacf.h>
39 DEFINE_STATIC_KEY_FALSE(s390_arch_random_available);
41 atomic64_t s390_arch_random_counter = ATOMIC64_INIT(0);
42 EXPORT_SYMBOL(s390_arch_random_counter);
44 #define ARCH_REFILL_TICKS (HZ/2)
45 #define ARCH_PRNG_SEED_SIZE 32
46 #define ARCH_RNG_BUF_SIZE 2048
48 static DEFINE_SPINLOCK(arch_rng_lock);
49 static u8 *arch_rng_buf;
50 static unsigned int arch_rng_buf_idx;
52 static void arch_rng_refill_buffer(struct work_struct *);
53 static DECLARE_DELAYED_WORK(arch_rng_work, arch_rng_refill_buffer);
55 bool s390_arch_random_generate(u8 *buf, unsigned int nbytes)
57 /* max hunk is ARCH_RNG_BUF_SIZE */
58 if (nbytes > ARCH_RNG_BUF_SIZE)
62 if (!spin_trylock(&arch_rng_lock))
65 /* try to resolve the requested amount of bytes from the buffer */
66 arch_rng_buf_idx -= nbytes;
67 if (arch_rng_buf_idx < ARCH_RNG_BUF_SIZE) {
68 memcpy(buf, arch_rng_buf + arch_rng_buf_idx, nbytes);
69 atomic64_add(nbytes, &s390_arch_random_counter);
70 spin_unlock(&arch_rng_lock);
74 /* not enough bytes in rng buffer, refill is done asynchronously */
75 spin_unlock(&arch_rng_lock);
79 EXPORT_SYMBOL(s390_arch_random_generate);
81 static void arch_rng_refill_buffer(struct work_struct *unused)
83 unsigned int delay = ARCH_REFILL_TICKS;
85 spin_lock(&arch_rng_lock);
86 if (arch_rng_buf_idx > ARCH_RNG_BUF_SIZE) {
87 /* buffer is exhausted and needs refill */
88 u8 seed[ARCH_PRNG_SEED_SIZE];
90 /* fetch ARCH_PRNG_SEED_SIZE bytes of entropy */
91 cpacf_trng(NULL, 0, seed, sizeof(seed));
92 /* blow this entropy up to ARCH_RNG_BUF_SIZE with PRNG */
93 memset(prng_wa, 0, sizeof(prng_wa));
94 cpacf_prno(CPACF_PRNO_SHA512_DRNG_SEED,
95 &prng_wa, NULL, 0, seed, sizeof(seed));
96 cpacf_prno(CPACF_PRNO_SHA512_DRNG_GEN,
97 &prng_wa, arch_rng_buf, ARCH_RNG_BUF_SIZE, NULL, 0);
98 arch_rng_buf_idx = ARCH_RNG_BUF_SIZE;
100 delay += (ARCH_REFILL_TICKS * arch_rng_buf_idx) / ARCH_RNG_BUF_SIZE;
101 spin_unlock(&arch_rng_lock);
103 /* kick next check */
104 queue_delayed_work(system_long_wq, &arch_rng_work, delay);
108 * Here follows the implementation of s390_arch_get_random_long().
110 * The random longs to be pulled by arch_get_random_long() are
111 * prepared in an 4K buffer which is filled from the NIST 800-90
112 * compliant s390 drbg. By default the random long buffer is refilled
113 * 256 times before the drbg itself needs a reseed. The reseed of the
114 * drbg is done with 32 bytes fetched from the high quality (but slow)
115 * trng which is assumed to deliver 100% entropy. So the 32 * 8 = 256
116 * bits of entropy are spread over 256 * 4KB = 1MB serving 131072
117 * arch_get_random_long() invocations before reseeded.
119 * How often the 4K random long buffer is refilled with the drbg
120 * before the drbg is reseeded can be adjusted. There is a module
121 * parameter 's390_arch_rnd_long_drbg_reseed' accessible via
122 * /sys/module/arch_random/parameters/rndlong_drbg_reseed
123 * or as kernel command line parameter
124 * arch_random.rndlong_drbg_reseed=<value>
125 * This parameter tells how often the drbg fills the 4K buffer before
126 * it is re-seeded by fresh entropy from the trng.
127 * A value of 16 results in reseeding the drbg at every 16 * 4 KB = 64
128 * KB with 32 bytes of fresh entropy pulled from the trng. So a value
129 * of 16 would result in 256 bits entropy per 64 KB.
130 * A value of 256 results in 1MB of drbg output before a reseed of the
131 * drbg is done. So this would spread the 256 bits of entropy among 1MB.
132 * Setting this parameter to 0 forces the reseed to take place every
133 * time the 4K buffer is depleted, so the entropy rises to 256 bits
134 * entropy per 4K or 0.5 bit entropy per arch_get_random_long(). With
135 * setting this parameter to negative values all this effort is
136 * disabled, arch_get_random long() returns false and thus indicating
137 * that the arch_get_random_long() feature is disabled at all.
140 static unsigned long rndlong_buf[512];
141 static DEFINE_SPINLOCK(rndlong_lock);
142 static int rndlong_buf_index;
144 static int rndlong_drbg_reseed = 256;
145 module_param_named(rndlong_drbg_reseed, rndlong_drbg_reseed, int, 0600);
146 MODULE_PARM_DESC(rndlong_drbg_reseed, "s390 arch_get_random_long() drbg reseed");
148 static inline void refill_rndlong_buf(void)
150 static u8 prng_ws[240];
151 static int drbg_counter;
153 if (--drbg_counter < 0) {
154 /* need to re-seed the drbg */
157 /* fetch seed from trng */
158 cpacf_trng(NULL, 0, seed, sizeof(seed));
160 memset(prng_ws, 0, sizeof(prng_ws));
161 cpacf_prno(CPACF_PRNO_SHA512_DRNG_SEED,
162 &prng_ws, NULL, 0, seed, sizeof(seed));
163 /* re-init counter for drbg */
164 drbg_counter = rndlong_drbg_reseed;
167 /* fill the arch_get_random_long buffer from drbg */
168 cpacf_prno(CPACF_PRNO_SHA512_DRNG_GEN, &prng_ws,
169 (u8 *) rndlong_buf, sizeof(rndlong_buf),
173 bool s390_arch_get_random_long(unsigned long *v)
178 /* arch_get_random_long() disabled ? */
179 if (rndlong_drbg_reseed < 0)
182 /* try to lock the random long lock */
183 if (!spin_trylock_irqsave(&rndlong_lock, flags))
186 if (--rndlong_buf_index >= 0) {
187 /* deliver next long value from the buffer */
188 *v = rndlong_buf[rndlong_buf_index];
193 /* buffer is depleted and needs refill */
194 if (in_interrupt()) {
195 /* delay refill in interrupt context to next caller */
196 rndlong_buf_index = 0;
200 /* refill random long buffer */
201 refill_rndlong_buf();
202 rndlong_buf_index = ARRAY_SIZE(rndlong_buf);
204 /* and provide one random long */
205 *v = rndlong_buf[--rndlong_buf_index];
209 spin_unlock_irqrestore(&rndlong_lock, flags);
212 EXPORT_SYMBOL(s390_arch_get_random_long);
214 static int __init s390_arch_random_init(void)
216 /* all the needed PRNO subfunctions available ? */
217 if (cpacf_query_func(CPACF_PRNO, CPACF_PRNO_TRNG) &&
218 cpacf_query_func(CPACF_PRNO, CPACF_PRNO_SHA512_DRNG_GEN)) {
220 /* alloc arch random working buffer */
221 arch_rng_buf = kmalloc(ARCH_RNG_BUF_SIZE, GFP_KERNEL);
225 /* kick worker queue job to fill the random buffer */
226 queue_delayed_work(system_long_wq,
227 &arch_rng_work, ARCH_REFILL_TICKS);
229 /* enable arch random to the outside world */
230 static_branch_enable(&s390_arch_random_available);
235 arch_initcall(s390_arch_random_init);