arch/s390/crypto/arch_random.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * s390 arch random implementation.
   4  *
   5  * Copyright IBM Corp. 2017, 2020
   6  * Author(s): Harald Freudenberger
   7  *
   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.
  28  */
  29
  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>
  38
  39 DEFINE_STATIC_KEY_FALSE(s390_arch_random_available);
  40
  41 atomic64_t s390_arch_random_counter = ATOMIC64_INIT(0);
  42 EXPORT_SYMBOL(s390_arch_random_counter);
  43
  44 #define ARCH_REFILL_TICKS (HZ/2)
  45 #define ARCH_PRNG_SEED_SIZE 32
  46 #define ARCH_RNG_BUF_SIZE 2048
  47
  48 static DEFINE_SPINLOCK(arch_rng_lock);
  49 static u8 *arch_rng_buf;
  50 static unsigned int arch_rng_buf_idx;
  51
  52 static void arch_rng_refill_buffer(struct work_struct *);
  53 static DECLARE_DELAYED_WORK(arch_rng_work, arch_rng_refill_buffer);
  54
  55 bool s390_arch_random_generate(u8 *buf, unsigned int nbytes)
  56 {
  57         /* lock rng buffer */
  58         if (!spin_trylock(&arch_rng_lock))
  59                 return false;
  60
  61         /* try to resolve the requested amount of bytes from the buffer */
  62         arch_rng_buf_idx -= nbytes;
  63         if (arch_rng_buf_idx < ARCH_RNG_BUF_SIZE) {
  64                 memcpy(buf, arch_rng_buf + arch_rng_buf_idx, nbytes);
  65                 atomic64_add(nbytes, &s390_arch_random_counter);
  66                 spin_unlock(&arch_rng_lock);
  67                 return true;
  68         }
  69
  70         /* not enough bytes in rng buffer, refill is done asynchronously */
  71         spin_unlock(&arch_rng_lock);
  72
  73         return false;
  74 }
  75 EXPORT_SYMBOL(s390_arch_random_generate);
  76
  77 static void arch_rng_refill_buffer(struct work_struct *unused)
  78 {
  79         unsigned int delay = ARCH_REFILL_TICKS;
  80
  81         spin_lock(&arch_rng_lock);
  82         if (arch_rng_buf_idx > ARCH_RNG_BUF_SIZE) {
  83                 /* buffer is exhausted and needs refill */
  84                 u8 seed[ARCH_PRNG_SEED_SIZE];
  85                 u8 prng_wa[240];
  86                 /* fetch ARCH_PRNG_SEED_SIZE bytes of entropy */
  87                 cpacf_trng(NULL, 0, seed, sizeof(seed));
  88                 /* blow this entropy up to ARCH_RNG_BUF_SIZE with PRNG */
  89                 memset(prng_wa, 0, sizeof(prng_wa));
  90                 cpacf_prno(CPACF_PRNO_SHA512_DRNG_SEED,
  91                            &prng_wa, NULL, 0, seed, sizeof(seed));
  92                 cpacf_prno(CPACF_PRNO_SHA512_DRNG_GEN,
  93                            &prng_wa, arch_rng_buf, ARCH_RNG_BUF_SIZE, NULL, 0);
  94                 arch_rng_buf_idx = ARCH_RNG_BUF_SIZE;
  95         }
  96         delay += (ARCH_REFILL_TICKS * arch_rng_buf_idx) / ARCH_RNG_BUF_SIZE;
  97         spin_unlock(&arch_rng_lock);
  98
  99         /* kick next check */
 100         queue_delayed_work(system_long_wq, &arch_rng_work, delay);
 101 }
 102
 103 /*
 104  * Here follows the implementation of s390_arch_get_random_long().
 105  *
 106  * The random longs to be pulled by arch_get_random_long() are
 107  * prepared in an 4K buffer which is filled from the NIST 800-90
 108  * compliant s390 drbg. By default the random long buffer is refilled
 109  * 256 times before the drbg itself needs a reseed. The reseed of the
 110  * drbg is done with 32 bytes fetched from the high quality (but slow)
 111  * trng which is assumed to deliver 100% entropy. So the 32 * 8 = 256
 112  * bits of entropy are spread over 256 * 4KB = 1MB serving 131072
 113  * arch_get_random_long() invocations before reseeded.
 114  *
 115  * How often the 4K random long buffer is refilled with the drbg
 116  * before the drbg is reseeded can be adjusted. There is a module
 117  * parameter 's390_arch_rnd_long_drbg_reseed' accessible via
 118  *   /sys/module/arch_random/parameters/rndlong_drbg_reseed
 119  * or as kernel command line parameter
 120  *   arch_random.rndlong_drbg_reseed=<value>
 121  * This parameter tells how often the drbg fills the 4K buffer before
 122  * it is re-seeded by fresh entropy from the trng.
 123  * A value of 16 results in reseeding the drbg at every 16 * 4 KB = 64
 124  * KB with 32 bytes of fresh entropy pulled from the trng. So a value
 125  * of 16 would result in 256 bits entropy per 64 KB.
 126  * A value of 256 results in 1MB of drbg output before a reseed of the
 127  * drbg is done. So this would spread the 256 bits of entropy among 1MB.
 128  * Setting this parameter to 0 forces the reseed to take place every
 129  * time the 4K buffer is depleted, so the entropy rises to 256 bits
 130  * entropy per 4K or 0.5 bit entropy per arch_get_random_long().  With
 131  * setting this parameter to negative values all this effort is
 132  * disabled, arch_get_random long() returns false and thus indicating
 133  * that the arch_get_random_long() feature is disabled at all.
 134  */
 135
 136 static unsigned long rndlong_buf[512];
 137 static DEFINE_SPINLOCK(rndlong_lock);
 138 static int rndlong_buf_index;
 139
 140 static int rndlong_drbg_reseed = 256;
 141 module_param_named(rndlong_drbg_reseed, rndlong_drbg_reseed, int, 0600);
 142 MODULE_PARM_DESC(rndlong_drbg_reseed, "s390 arch_get_random_long() drbg reseed");
 143
 144 static inline void refill_rndlong_buf(void)
 145 {
 146         static u8 prng_ws[240];
 147         static int drbg_counter;
 148
 149         if (--drbg_counter < 0) {
 150                 /* need to re-seed the drbg */
 151                 u8 seed[32];
 152
 153                 /* fetch seed from trng */
 154                 cpacf_trng(NULL, 0, seed, sizeof(seed));
 155                 /* seed drbg */
 156                 memset(prng_ws, 0, sizeof(prng_ws));
 157                 cpacf_prno(CPACF_PRNO_SHA512_DRNG_SEED,
 158                            &prng_ws, NULL, 0, seed, sizeof(seed));
 159                 /* re-init counter for drbg */
 160                 drbg_counter = rndlong_drbg_reseed;
 161         }
 162
 163         /* fill the arch_get_random_long buffer from drbg */
 164         cpacf_prno(CPACF_PRNO_SHA512_DRNG_GEN, &prng_ws,
 165                    (u8 *) rndlong_buf, sizeof(rndlong_buf),
 166                    NULL, 0);
 167 }
 168
 169 bool s390_arch_get_random_long(unsigned long *v)
 170 {
 171         bool rc = false;
 172         unsigned long flags;
 173
 174         /* arch_get_random_long() disabled ? */
 175         if (rndlong_drbg_reseed < 0)
 176                 return false;
 177
 178         /* try to lock the random long lock */
 179         if (!spin_trylock_irqsave(&rndlong_lock, flags))
 180                 return false;
 181
 182         if (--rndlong_buf_index >= 0) {
 183                 /* deliver next long value from the buffer */
 184                 *v = rndlong_buf[rndlong_buf_index];
 185                 rc = true;
 186                 goto out;
 187         }
 188
 189         /* buffer is depleted and needs refill */
 190         if (in_interrupt()) {
 191                 /* delay refill in interrupt context to next caller */
 192                 rndlong_buf_index = 0;
 193                 goto out;
 194         }
 195
 196         /* refill random long buffer */
 197         refill_rndlong_buf();
 198         rndlong_buf_index = ARRAY_SIZE(rndlong_buf);
 199
 200         /* and provide one random long */
 201         *v = rndlong_buf[--rndlong_buf_index];
 202         rc = true;
 203
 204 out:
 205         spin_unlock_irqrestore(&rndlong_lock, flags);
 206         return rc;
 207 }
 208 EXPORT_SYMBOL(s390_arch_get_random_long);
 209
 210 static int __init s390_arch_random_init(void)
 211 {
 212         /* all the needed PRNO subfunctions available ? */
 213         if (cpacf_query_func(CPACF_PRNO, CPACF_PRNO_TRNG) &&
 214             cpacf_query_func(CPACF_PRNO, CPACF_PRNO_SHA512_DRNG_GEN)) {
 215
 216                 /* alloc arch random working buffer */
 217                 arch_rng_buf = kmalloc(ARCH_RNG_BUF_SIZE, GFP_KERNEL);
 218                 if (!arch_rng_buf)
 219                         return -ENOMEM;
 220
 221                 /* kick worker queue job to fill the random buffer */
 222                 queue_delayed_work(system_long_wq,
 223                                    &arch_rng_work, ARCH_REFILL_TICKS);
 224
 225                 /* enable arch random to the outside world */
 226                 static_branch_enable(&s390_arch_random_available);
 227         }
 228
 229         return 0;
 230 }
 231 arch_initcall(s390_arch_random_init);