arch/arm64/include/asm/archrandom.h

   1 /* SPDX-License-Identifier: GPL-2.0 */
   2 #ifndef _ASM_ARCHRANDOM_H
   3 #define _ASM_ARCHRANDOM_H
   4
   5 #include <linux/arm-smccc.h>
   6 #include <linux/bug.h>
   7 #include <linux/kernel.h>
   8 #include <linux/irqflags.h>
   9 #include <asm/cpufeature.h>
  10
  11 #define ARM_SMCCC_TRNG_MIN_VERSION      0x10000UL
  12
  13 extern bool smccc_trng_available;
  14
  15 static inline bool __init smccc_probe_trng(void)
  16 {
  17         struct arm_smccc_res res;
  18
  19         arm_smccc_1_1_invoke(ARM_SMCCC_TRNG_VERSION, &res);
  20         if ((s32)res.a0 < 0)
  21                 return false;
  22
  23         return res.a0 >= ARM_SMCCC_TRNG_MIN_VERSION;
  24 }
  25
  26 static inline bool __arm64_rndr(unsigned long *v)
  27 {
  28         bool ok;
  29
  30         /*
  31          * Reads of RNDR set PSTATE.NZCV to 0b0000 on success,
  32          * and set PSTATE.NZCV to 0b0100 otherwise.
  33          */
  34         asm volatile(
  35                 __mrs_s("%0", SYS_RNDR_EL0) "\n"
  36         "       cset %w1, ne\n"
  37         : "=r" (*v), "=r" (ok)
  38         :
  39         : "cc");
  40
  41         return ok;
  42 }
  43
  44 static inline bool __arm64_rndrrs(unsigned long *v)
  45 {
  46         bool ok;
  47
  48         /*
  49          * Reads of RNDRRS set PSTATE.NZCV to 0b0000 on success,
  50          * and set PSTATE.NZCV to 0b0100 otherwise.
  51          */
  52         asm volatile(
  53                 __mrs_s("%0", SYS_RNDRRS_EL0) "\n"
  54         "       cset %w1, ne\n"
  55         : "=r" (*v), "=r" (ok)
  56         :
  57         : "cc");
  58
  59         return ok;
  60 }
  61
  62 static __always_inline bool __cpu_has_rng(void)
  63 {
  64         if (unlikely(!system_capabilities_finalized() && !preemptible()))
  65                 return this_cpu_has_cap(ARM64_HAS_RNG);
  66         return alternative_has_cap_unlikely(ARM64_HAS_RNG);
  67 }
  68
  69 static inline size_t __must_check arch_get_random_longs(unsigned long *v, size_t max_longs)
  70 {
  71         /*
  72          * Only support the generic interface after we have detected
  73          * the system wide capability, avoiding complexity with the
  74          * cpufeature code and with potential scheduling between CPUs
  75          * with and without the feature.
  76          */
  77         if (max_longs && __cpu_has_rng() && __arm64_rndr(v))
  78                 return 1;
  79         return 0;
  80 }
  81
  82 static inline size_t __must_check arch_get_random_seed_longs(unsigned long *v, size_t max_longs)
  83 {
  84         if (!max_longs)
  85                 return 0;
  86
  87         /*
  88          * We prefer the SMCCC call, since its semantics (return actual
  89          * hardware backed entropy) is closer to the idea behind this
  90          * function here than what even the RNDRSS register provides
  91          * (the output of a pseudo RNG freshly seeded by a TRNG).
  92          */
  93         if (smccc_trng_available) {
  94                 struct arm_smccc_res res;
  95
  96                 max_longs = min_t(size_t, 3, max_longs);
  97                 arm_smccc_1_1_invoke(ARM_SMCCC_TRNG_RND64, max_longs * 64, &res);
  98                 if ((int)res.a0 >= 0) {
  99                         switch (max_longs) {
 100                         case 3:
 101                                 *v++ = res.a1;
 102                                 fallthrough;
 103                         case 2:
 104                                 *v++ = res.a2;
 105                                 fallthrough;
 106                         case 1:
 107                                 *v++ = res.a3;
 108                                 break;
 109                         }
 110                         return max_longs;
 111                 }
 112         }
 113
 114         /*
 115          * RNDRRS is not backed by an entropy source but by a DRBG that is
 116          * reseeded after each invocation. This is not a 100% fit but good
 117          * enough to implement this API if no other entropy source exists.
 118          */
 119         if (__cpu_has_rng() && __arm64_rndrrs(v))
 120                 return 1;
 121
 122         return 0;
 123 }
 124
 125 static inline bool __init __early_cpu_has_rndr(void)
 126 {
 127         /* Open code as we run prior to the first call to cpufeature. */
 128         unsigned long ftr = read_sysreg_s(SYS_ID_AA64ISAR0_EL1);
 129         return (ftr >> ID_AA64ISAR0_EL1_RNDR_SHIFT) & 0xf;
 130 }
 131
 132 #endif /* _ASM_ARCHRANDOM_H */