Merge pull request #624 from besser82/topic/besser82/json-c-0.14/rdrand_fixes

json-c-0.14: Detect broken RDRAND during initialization.

random_seed.c

@@ -26,19 +26,8 @@
 static void do_cpuid(int regs[], int h)
 {
 	/* clang-format off */
-    __asm__ __volatile__(
-#if defined __x86_64__
-                         "pushq %%rbx;\n"
-#else
-                         "pushl %%ebx;\n"
-#endif
-                         "cpuid;\n"
-#if defined __x86_64__
-                         "popq %%rbx;\n"
-#else
-                         "popl %%ebx;\n"
-#endif
-                         : "=a"(regs[0]), [ebx] "=r"(regs[1]), "=c"(regs[2]), "=d"(regs[3])
+    __asm__ __volatile__("cpuid"
+                         : "=a"(regs[0]), "=b"(regs[1]), "=c"(regs[2]), "=d"(regs[3])
                          : "a"(h));
 	/* clang-format on */
 }
@@ -54,12 +43,51 @@ static void do_cpuid(int regs[], int h)
 
 #if HAS_X86_CPUID
 
+static int get_rdrand_seed(void);
+
+/* Valid values are -1 (haven't tested), 0 (no), and 1 (yes). */
+static int _has_rdrand = -1;
+
 static int has_rdrand(void)
 {
-	// CPUID.01H:ECX.RDRAND[bit 30] == 1
+	if (_has_rdrand != -1)
+	{
+		return _has_rdrand;
+	}
+
+	/* CPUID.01H:ECX.RDRAND[bit 30] == 1 */
 	int regs[4];
 	do_cpuid(regs, 1);
-	return (regs[2] & (1 << 30)) != 0;
+	if (!(regs[2] & (1 << 30)))
+	{
+		_has_rdrand = 0;
+		return 0;
+	}
+
+	/*
+	 * Some CPUs advertise RDRAND in CPUID, but return 0xFFFFFFFF
+	 * unconditionally. To avoid locking up later, test RDRAND here. If over
+	 * 3 trials RDRAND has returned the same value, declare it broken.
+	 * Example CPUs are AMD Ryzen 3000 series
+	 * and much older AMD APUs, such as the E1-1500
+	 * https://github.com/systemd/systemd/issues/11810
+	 * https://linuxreviews.org/RDRAND_stops_returning_random_values_on_older_AMD_CPUs_after_suspend
+	 */
+	_has_rdrand = 0;
+	int prev = get_rdrand_seed();
+	for (int i = 0; i < 3; i++)
+	{
+		int temp = get_rdrand_seed();
+		if (temp != prev)
+		{
+			_has_rdrand = 1;
+			break;
+		}
+
+		prev = temp;
+	}
+
+	return _has_rdrand;
 }
 
 #endif
@@ -74,7 +102,7 @@ static int get_rdrand_seed(void)
 {
 	DEBUG_SEED("get_rdrand_seed");
 	int _eax;
-	// rdrand eax
+	/* rdrand eax */
 	/* clang-format off */
 	__asm__ __volatile__("1: .byte 0x0F\n"
 	                     "   .byte 0xC7\n"
@@ -114,7 +142,7 @@ static int get_rdrand_seed(void)
 	DEBUG_SEED("get_rdrand_seed");
 	int _eax;
 retry:
-	// rdrand eax
+	/* rdrand eax */
 	__asm _emit 0x0F __asm _emit 0xC7 __asm _emit 0xF0
 	__asm jnc retry
 	__asm mov _eax, eax
@@ -188,6 +216,10 @@ static int get_dev_random_seed(void)
 
 /* clang-format off */
 #include <windows.h>
+
+/* Caution: these blank lines must remain so clang-format doesn't reorder
+   includes to put windows.h after wincrypt.h */
+
 #include <wincrypt.h>
 /* clang-format on */
 #ifndef __GNUC__