From 26873a4bf8d12bc5bc292c51030559e8ed353599 Mon Sep 17 00:00:00 2001
From: Ard Biesheuvel <workofard@gmail.com>
Date: Thu, 13 Jun 2024 19:44:43 +0200
Subject: Revert "MdePkg/BaseRngLib: Add a smoketest for RDRAND and check
 CPUID"

This reverts commit c3a8ca7b54a9fd17acdf16c6282a92cc989fa92a.
---
 MdePkg/Library/BaseRngLib/Rand/RdRand.c | 99 +++------------------------------
 1 file changed, 8 insertions(+), 91 deletions(-)

diff --git a/MdePkg/Library/BaseRngLib/Rand/RdRand.c b/MdePkg/Library/BaseRngLib/Rand/RdRand.c
index 06d2a6f..9bd6835 100644
--- a/MdePkg/Library/BaseRngLib/Rand/RdRand.c
+++ b/MdePkg/Library/BaseRngLib/Rand/RdRand.c
@@ -3,7 +3,6 @@
   to provide high-quality random numbers.
 
 Copyright (c) 2023, Arm Limited. All rights reserved.<BR>
-Copyright (c) 2022, Pedro Falcato. All rights reserved.<BR>
 Copyright (c) 2021, NUVIA Inc. All rights reserved.<BR>
 Copyright (c) 2015, Intel Corporation. All rights reserved.<BR>
 
@@ -25,88 +24,6 @@ SPDX-License-Identifier: BSD-2-Clause-Patent
 
 STATIC BOOLEAN  mRdRandSupported;
 
-//
-// Intel SDM says 10 tries is good enough for reliable RDRAND usage.
-//
-#define RDRAND_RETRIES  10
-
-#define RDRAND_TEST_SAMPLES  8
-
-#define RDRAND_MIN_CHANGE  5
-
-//
-// Add a define for native-word RDRAND, just for the test.
-//
-#ifdef MDE_CPU_X64
-#define ASM_RDRAND  AsmRdRand64
-#else
-#define ASM_RDRAND  AsmRdRand32
-#endif
-
-/**
-  Tests RDRAND for broken implementations.
-
-  @retval TRUE         RDRAND is reliable (and hopefully safe).
-  @retval FALSE        RDRAND is unreliable and should be disabled, despite CPUID.
-
-**/
-STATIC
-BOOLEAN
-TestRdRand (
-  VOID
-  )
-{
-  //
-  // Test for notoriously broken rdrand implementations that always return the same
-  // value, like the Zen 3 uarch (all-1s) or other several AMD families on suspend/resume (also all-1s).
-  // Note that this should be expanded to extensively test for other sorts of possible errata.
-  //
-
-  //
-  // Our algorithm samples rdrand $RDRAND_TEST_SAMPLES times and expects
-  // a different result $RDRAND_MIN_CHANGE times for reliable RDRAND usage.
-  //
-  UINTN   Prev;
-  UINT8   Idx;
-  UINT8   TestIteration;
-  UINT32  Changed;
-
-  Changed = 0;
-
-  for (TestIteration = 0; TestIteration < RDRAND_TEST_SAMPLES; TestIteration++) {
-    UINTN  Sample;
-    //
-    // Note: We use a retry loop for rdrand. Normal users get this in BaseRng.c
-    // Any failure to get a random number will assume RDRAND does not work.
-    //
-    for (Idx = 0; Idx < RDRAND_RETRIES; Idx++) {
-      if (ASM_RDRAND (&Sample)) {
-        break;
-      }
-    }
-
-    if (Idx == RDRAND_RETRIES) {
-      DEBUG ((DEBUG_ERROR, "BaseRngLib/x86: CPU BUG: Failed to get an RDRAND random number - disabling\n"));
-      return FALSE;
-    }
-
-    if (TestIteration != 0) {
-      Changed += Sample != Prev;
-    }
-
-    Prev = Sample;
-  }
-
-  if (Changed < RDRAND_MIN_CHANGE) {
-    DEBUG ((DEBUG_ERROR, "BaseRngLib/x86: CPU BUG: RDRAND not reliable - disabling\n"));
-    return FALSE;
-  }
-
-  return TRUE;
-}
-
-#undef ASM_RDRAND
-
 /**
   The constructor function checks whether or not RDRAND instruction is supported
   by the host hardware.
@@ -131,13 +48,10 @@ BaseRngLibConstructor (
   // CPUID. A value of 1 indicates that processor support RDRAND instruction.
   //
   AsmCpuid (1, 0, 0, &RegEcx, 0);
+  ASSERT ((RegEcx & RDRAND_MASK) == RDRAND_MASK);
 
   mRdRandSupported = ((RegEcx & RDRAND_MASK) == RDRAND_MASK);
 
-  if (mRdRandSupported) {
-    mRdRandSupported = TestRdRand ();
-  }
-
   return EFI_SUCCESS;
 }
 
@@ -156,7 +70,6 @@ ArchGetRandomNumber16 (
   OUT     UINT16  *Rand
   )
 {
-  ASSERT (mRdRandSupported);
   return AsmRdRand16 (Rand);
 }
 
@@ -175,7 +88,6 @@ ArchGetRandomNumber32 (
   OUT     UINT32  *Rand
   )
 {
-  ASSERT (mRdRandSupported);
   return AsmRdRand32 (Rand);
 }
 
@@ -194,7 +106,6 @@ ArchGetRandomNumber64 (
   OUT     UINT64  *Rand
   )
 {
-  ASSERT (mRdRandSupported);
   return AsmRdRand64 (Rand);
 }
 
@@ -211,7 +122,13 @@ ArchIsRngSupported (
   VOID
   )
 {
-  return mRdRandSupported;
+  /*
+     Existing software depends on this always returning TRUE, so for
+     now hard-code it.
+
+     return mRdRandSupported;
+  */
+  return TRUE;
 }
 
 /**
-- 
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