From 4d36b0287d3ab0912ba8a4790340ca099960b2b0 Mon Sep 17 00:00:00 2001
From: Tudor Brindus <me@tbrindus.ca>
Date: Fri, 1 May 2020 21:09:22 -0400
Subject: [PATCH] Detect broken RDRAND during initialization

Some CPUs advertise RDRAND in CPUID, but return 0xFFFFFFFF
unconditionally. To avoid locking up later, test RDRAND during
initialization, and if it returns 0xFFFFFFFF, mark it as nonexistent.

Fixes #588.
---
 random_seed.c | 37 +++++++++++++++++++++++++++++++++----
 1 file changed, 33 insertions(+), 4 deletions(-)

diff --git a/random_seed.c b/random_seed.c
index c459f0f..4ddcb07 100644
--- a/random_seed.c
+++ b/random_seed.c
@@ -43,12 +43,41 @@ 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
-	int regs[4];
-	do_cpuid(regs, 1);
-	return (regs[2] & (1 << 30)) != 0;
+	if (_has_rdrand == -1)
+	{
+		// CPUID.01H:ECX.RDRAND[bit 30] == 1
+		int regs[4];
+		do_cpuid(regs, 1);
+		if (!(regs[2] & (1 << 30)))
+		{
+			_has_rdrand = 0;
+		} else
+		{
+			// Some CPUs advertise RDRAND in CPUID, but return 0xFFFFFFFF
+			// unconditionally. To avoid locking up later, test RDRAND here. If over
+			// 10 trials RDRAND has returned the same value, declare it broken.
+			_has_rdrand = 0;
+			int prev = get_rdrand_seed();
+			for (int i = 0; i < 10; i++) {
+				int temp = get_rdrand_seed();
+				if (temp != prev) {
+					_has_rdrand = 1;
+					break;
+				}
+
+				prev = temp;
+			}
+		}
+	}
+
+	return _has_rdrand;
 }
 
 #endif
-- 
2.50.0