Use RdSeed when available, and reduce RdRand load

This introduces support for autodetecting and using the RdSeed instruction.

In addition:
* In SeedFast, only 64 bits of entropy are generated through RdRand (256 was relatively slow).
* In SeedStartup, 256 bits of entropy are generated, using RdSeed (preferably) or RdRand (otherwise).

src/random.cpp

@@ -78,25 +78,119 @@ static inline int64_t GetPerformanceCounter() noexcept
 }
 
 #if defined(__x86_64__) || defined(__amd64__) || defined(__i386__)
-static bool rdrand_supported = false;
+static bool g_rdrand_supported = false;
+static bool g_rdseed_supported = false;
 static constexpr uint32_t CPUID_F1_ECX_RDRAND = 0x40000000;
+static constexpr uint32_t CPUID_F7_EBX_RDSEED = 0x00040000;
+#ifdef bit_RDRND
+static_assert(CPUID_F1_ECX_RDRAND == bit_RDRND, "Unexpected value for bit_RDRND");
+#endif
+#ifdef bit_RDSEED
+static_assert(CPUID_F7_EBX_RDSEED == bit_RDSEED, "Unexpected value for bit_RDSEED");
+#endif
+static void inline GetCPUID(uint32_t leaf, uint32_t subleaf, uint32_t& a, uint32_t& b, uint32_t& c, uint32_t& d)
+{
+ // We can't use __get_cpuid as it doesn't support subleafs.
+#ifdef __GNUC__
+ __cpuid_count(leaf, subleaf, a, b, c, d);
+#else
+ __asm__ ("cpuid" : "=a"(a), "=b"(b), "=c"(c), "=d"(d) : "0"(leaf), "2"(subleaf));
+#endif
+}
+
 static void InitHardwareRand()
 {
  uint32_t eax, ebx, ecx, edx;
- if (__get_cpuid(1, &eax, &ebx, &ecx, &edx) && (ecx & CPUID_F1_ECX_RDRAND)) {
- rdrand_supported = true;
+ GetCPUID(1, 0, eax, ebx, ecx, edx);
+ if (ecx & CPUID_F1_ECX_RDRAND) {
+ g_rdrand_supported = true;
+ }
+ GetCPUID(7, 0, eax, ebx, ecx, edx);
+ if (ebx & CPUID_F7_EBX_RDSEED) {
+ g_rdseed_supported = true;
  }
 }
 
 static void ReportHardwareRand()
 {
- if (rdrand_supported) {
- // This must be done in a separate function, as HWRandInit() may be indirectly called
- // from global constructors, before logging is initialized.
+ // This must be done in a separate function, as HWRandInit() may be indirectly called
+ // from global constructors, before logging is initialized.
+ if (g_rdseed_supported) {
+ LogPrintf("Using RdSeed as additional entropy source\n");
+ }
+ if (g_rdrand_supported) {
  LogPrintf("Using RdRand as an additional entropy source\n");
  }
 }
 
+/** Read 64 bits of entropy using rdrand.
+ *
+ * Must only be called when RdRand is supported.
+ */
+static uint64_t GetRdRand() noexcept
+{
+ // RdRand may very rarely fail. Invoke it up to 10 times in a loop to reduce this risk.
+#ifdef __i386__
+ uint8_t ok;
+ uint32_t r1, r2;
+ for (int i = 0; i < 10; ++i) {
+ __asm__ volatile (".byte 0x0f, 0xc7, 0xf0; setc %1" : "=a"(r1), "=q"(ok) :: "cc"); // rdrand %eax
+ if (ok) break;
+ }
+ for (int i = 0; i < 10; ++i) {
+ __asm__ volatile (".byte 0x0f, 0xc7, 0xf0; setc %1" : "=a"(r2), "=q"(ok) :: "cc"); // rdrand %eax
+ if (ok) break;
+ }
+ return (((uint64_t)r2) << 32) | r1;
+#elif defined(__x86_64__) || defined(__amd64__)
+ uint8_t ok;
+ uint64_t r1;
+ for (int i = 0; i < 10; ++i) {
+ __asm__ volatile (".byte 0x48, 0x0f, 0xc7, 0xf0; setc %1" : "=a"(r1), "=q"(ok) :: "cc"); // rdrand %rax
+ if (ok) break;
+ }
+ return r1;
+#else
+#error "RdRand is only supported on x86 and x86_64"
+#endif
+}
+
+/** Read 64 bits of entropy using rdseed.
+ *
+ * Must only be called when RdSeed is supported.
+ */
+static uint64_t GetRdSeed() noexcept
+{
+ // RdSeed may fail when the HW RNG is overloaded. Loop indefinitely until enough entropy is gathered,
+ // but pause after every failure.
+#ifdef __i386__
+ uint8_t ok;
+ uint32_t r1, r2;
+ do {
+ __asm__ volatile (".byte 0x0f, 0xc7, 0xf8; setc %1" : "=a"(r1), "=q"(ok) :: "cc"); // rdseed %eax
+ if (ok) break;
+ __asm__ volatile ("pause");
+ } while(true);
+ do {
+ __asm__ volatile (".byte 0x0f, 0xc7, 0xf8; setc %1" : "=a"(r2), "=q"(ok) :: "cc"); // rdseed %eax
+ if (ok) break;
+ __asm__ volatile ("pause");
+ } while(true);
+ return (((uint64_t)r2) << 32) | r1;
+#elif defined(__x86_64__) || defined(__amd64__)
+ uint8_t ok;
+ uint64_t r1;
+ do {
+ __asm__ volatile (".byte 0x48, 0x0f, 0xc7, 0xf8; setc %1" : "=a"(r1), "=q"(ok) :: "cc"); // rdseed %rax
+ if (ok) break;
+ __asm__ volatile ("pause");
+ } while(true);
+ return r1;
+#else
+#error "RdSeed is only supported on x86 and x86_64"
+#endif
+}
+
 #else
 /* Access to other hardware random number generators could be added here later,
  * assuming it is sufficiently fast (in the order of a few hundred CPU cycles).
@@ -107,40 +201,40 @@ static void InitHardwareRand() {}
 static void ReportHardwareRand() {}
 #endif
 
-static bool GetHardwareRand(unsigned char* ent32) noexcept {
+/** Add 64 bits of entropy gathered from hardware to hasher. Do nothing if not supported. */
+static void SeedHardwareFast(CSHA512& hasher) noexcept {
 #if defined(__x86_64__) || defined(__amd64__) || defined(__i386__)
- if (rdrand_supported) {
- uint8_t ok;
- // Not all assemblers support the rdrand instruction, write it in hex.
-#ifdef __i386__
- for (int iter = 0; iter < 4; ++iter) {
- uint32_t r1, r2;
- __asm__ volatile (".byte 0x0f, 0xc7, 0xf0;" // rdrand %eax
- ".byte 0x0f, 0xc7, 0xf2;" // rdrand %edx
- "setc %2" :
- "=a"(r1), "=d"(r2), "=q"(ok) :: "cc");
- if (!ok) return false;
- WriteLE32(ent32 + 8 * iter, r1);
- WriteLE32(ent32 + 8 * iter + 4, r2);
- }
-#else
- uint64_t r1, r2, r3, r4;
- __asm__ volatile (".byte 0x48, 0x0f, 0xc7, 0xf0, " // rdrand %rax
- "0x48, 0x0f, 0xc7, 0xf3, " // rdrand %rbx
- "0x48, 0x0f, 0xc7, 0xf1, " // rdrand %rcx
- "0x48, 0x0f, 0xc7, 0xf2; " // rdrand %rdx
- "setc %4" :
- "=a"(r1), "=b"(r2), "=c"(r3), "=d"(r4), "=q"(ok) :: "cc");
- if (!ok) return false;
- WriteLE64(ent32, r1);
- WriteLE64(ent32 + 8, r2);
- WriteLE64(ent32 + 16, r3);
- WriteLE64(ent32 + 24, r4);
+ if (g_rdrand_supported) {
+ uint64_t out = GetRdRand();
+ hasher.Write((const unsigned char*)&out, sizeof(out));
+ return;
+ }
 #endif
- return true;
+}
+
+/** Add 256 bits of entropy gathered from hardware to hasher. Do nothing if not supported. */
+static void SeedHardwareSlow(CSHA512& hasher) noexcept {
+#if defined(__x86_64__) || defined(__amd64__) || defined(__i386__)
+ // When we want 256 bits of entropy, prefer RdSeed over RdRand, as it's
+ // guaranteed to produce independent randomness on every call.
+ if (g_rdseed_supported) {
+ for (int i = 0; i < 4; ++i) {
+ uint64_t out = GetRdSeed();
+ hasher.Write((const unsigned char*)&out, sizeof(out));
+ }
+ return;
+ }
+ // When falling back to RdRand, XOR the result of 1024 results.
+ // This guarantees a reseeding occurs between each.
+ if (g_rdrand_supported) {
+ for (int i = 0; i < 4; ++i) {
+ uint64_t out = 0;
+ for (int j = 0; j < 1024; ++j) out ^= GetRdRand();
+ hasher.Write((const unsigned char*)&out, sizeof(out));
+ }
+ return;
  }
 #endif
- return false;
 }
 
 static void RandAddSeedPerfmon(CSHA512& hasher)
@@ -407,8 +501,7 @@ static void SeedFast(CSHA512& hasher) noexcept
  hasher.Write((const unsigned char*)&ptr, sizeof(ptr));
 
  // Hardware randomness is very fast when available; use it always.
- bool have_hw_rand = GetHardwareRand(buffer);
- if (have_hw_rand) hasher.Write(buffer, sizeof(buffer));
+ SeedHardwareFast(hasher);
 
  // High-precision timestamp
  SeedTimestamp(hasher);
@@ -460,6 +553,9 @@ static void SeedStartup(CSHA512& hasher) noexcept
  RAND_screen();
 #endif
 
+ // Gather 256 bits of hardware randomness, if available
+ SeedHardwareSlow(hasher);
+
  // Everything that the 'slow' seeder includes.
  SeedSlow(hasher);
 

src/random.h

@@ -24,7 +24,7 @@
  * perform 'fast' seeding, consisting of mixing in:
  * - A stack pointer (indirectly committing to calling thread and call stack)
  * - A high-precision timestamp (rdtsc when available, c++ high_resolution_clock otherwise)
- * - Hardware RNG (rdrand) when available.
+ * - 64 bits from the hardware RNG (rdrand) when available.
  * These entropy sources are very fast, and only designed to protect against situations
  * where a VM state restore/copy results in multiple systems with the same randomness.
  * FastRandomContext on the other hand does not protect against this once created, but
@@ -48,6 +48,7 @@
  *
  * On first use of the RNG (regardless of what function is called first), all entropy
  * sources used in the 'slow' seeder are included, but also:
+ * - 256 bits from the hardware RNG (rdseed or rdrand) when available.
  * - (On Windows) Performance monitoring data from the OS.
  * - (On Windows) Through OpenSSL, the screen contents.
  *