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, 2018
   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 <asm/cpacf.h>
  37
  38 DEFINE_STATIC_KEY_FALSE(s390_arch_random_available);
  39
  40 atomic64_t s390_arch_random_counter = ATOMIC64_INIT(0);
  41 EXPORT_SYMBOL(s390_arch_random_counter);
  42
  43 #define ARCH_REFILL_TICKS (HZ/2)
  44 #define ARCH_PRNG_SEED_SIZE 32
  45 #define ARCH_RNG_BUF_SIZE 2048
  46
  47 static DEFINE_SPINLOCK(arch_rng_lock);
  48 static u8 *arch_rng_buf;
  49 static unsigned int arch_rng_buf_idx;
  50
  51 static void arch_rng_refill_buffer(struct work_struct *);
  52 static DECLARE_DELAYED_WORK(arch_rng_work, arch_rng_refill_buffer);
  53
  54 bool s390_arch_random_generate(u8 *buf, unsigned int nbytes)
  55 {
  56         /* lock rng buffer */
  57         if (!spin_trylock(&arch_rng_lock))
  58                 return false;
  59
  60         /* try to resolve the requested amount of bytes from the buffer */
  61         arch_rng_buf_idx -= nbytes;
  62         if (arch_rng_buf_idx < ARCH_RNG_BUF_SIZE) {
  63                 memcpy(buf, arch_rng_buf + arch_rng_buf_idx, nbytes);
  64                 atomic64_add(nbytes, &s390_arch_random_counter);
  65                 spin_unlock(&arch_rng_lock);
  66                 return true;
  67         }
  68
  69         /* not enough bytes in rng buffer, refill is done asynchronously */
  70         spin_unlock(&arch_rng_lock);
  71
  72         return false;
  73 }
  74 EXPORT_SYMBOL(s390_arch_random_generate);
  75
  76 static void arch_rng_refill_buffer(struct work_struct *unused)
  77 {
  78         unsigned int delay = ARCH_REFILL_TICKS;
  79
  80         spin_lock(&arch_rng_lock);
  81         if (arch_rng_buf_idx > ARCH_RNG_BUF_SIZE) {
  82                 /* buffer is exhausted and needs refill */
  83                 u8 seed[ARCH_PRNG_SEED_SIZE];
  84                 u8 prng_wa[240];
  85                 /* fetch ARCH_PRNG_SEED_SIZE bytes of entropy */
  86                 cpacf_trng(NULL, 0, seed, sizeof(seed));
  87                 /* blow this entropy up to ARCH_RNG_BUF_SIZE with PRNG */
  88                 memset(prng_wa, 0, sizeof(prng_wa));
  89                 cpacf_prno(CPACF_PRNO_SHA512_DRNG_SEED,
  90                            &prng_wa, NULL, 0, seed, sizeof(seed));
  91                 cpacf_prno(CPACF_PRNO_SHA512_DRNG_GEN,
  92                            &prng_wa, arch_rng_buf, ARCH_RNG_BUF_SIZE, NULL, 0);
  93                 arch_rng_buf_idx = ARCH_RNG_BUF_SIZE;
  94         }
  95         delay += (ARCH_REFILL_TICKS * arch_rng_buf_idx) / ARCH_RNG_BUF_SIZE;
  96         spin_unlock(&arch_rng_lock);
  97
  98         /* kick next check */
  99         queue_delayed_work(system_long_wq, &arch_rng_work, delay);
 100 }
 101
 102 static int __init s390_arch_random_init(void)
 103 {
 104         /* all the needed PRNO subfunctions available ? */
 105         if (cpacf_query_func(CPACF_PRNO, CPACF_PRNO_TRNG) &&
 106             cpacf_query_func(CPACF_PRNO, CPACF_PRNO_SHA512_DRNG_GEN)) {
 107
 108                 /* alloc arch random working buffer */
 109                 arch_rng_buf = kmalloc(ARCH_RNG_BUF_SIZE, GFP_KERNEL);
 110                 if (!arch_rng_buf)
 111                         return -ENOMEM;
 112
 113                 /* kick worker queue job to fill the random buffer */
 114                 queue_delayed_work(system_long_wq,
 115                                    &arch_rng_work, ARCH_REFILL_TICKS);
 116
 117                 /* enable arch random to the outside world */
 118                 static_branch_enable(&s390_arch_random_available);
 119         }
 120
 121         return 0;
 122 }
 123 arch_initcall(s390_arch_random_init);