[edk2-devel] [PATCH v4 1/1] MdePkg/BaseRngLib: Add a smoketest for RDRAND and check CPUID

[Pedro Falcato]

RDRAND has notoriously been broken many times over its lifespan.
Add a smoketest to RDRAND, in order to better sniff out potential
security concerns.

Also add a proper CPUID test in order to support older CPUs which may
not have it; it was previously being tested but then promptly ignored.

Testing algorithm inspired by linux's arch/x86/kernel/cpu/rdrand.c
:x86_init_rdrand() per commit 049f9ae9..

Many thanks to Jason Donenfeld for relicensing his linux RDRAND detection
code to MIT and the public domain.

>On Tue, Nov 22, 2022 at 2:21 PM Jason A. Donenfeld <Jason at zx2c4.com> wrote:
  <..>
>    I (re)wrote that function in Linux. I hereby relicense it as MIT, and
>    also place it into public domain. Do with it what you will now.
>
>    Jason

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=4163

Signed-off-by: Pedro Falcato <pedro.falcato at gmail.com>
Cc: Michael D Kinney <michael.d.kinney at intel.com>
Cc: Liming Gao <gaoliming at byosoft.com.cn>
Cc: Zhiguang Liu <zhiguang.liu at intel.com>
Cc: Jason A. Donenfeld <Jason at zx2c4.com>
---
v4: Added a doxygen comment to the TestRdRand() function
v3: Addressed mailing list comments
v2: Replaced the original v1 naive detection with a better, although still not great, detection. 
 MdePkg/Library/BaseRngLib/Rand/RdRand.c | 99 +++++++++++++++++++++++--
 1 file changed, 91 insertions(+), 8 deletions(-)

diff --git a/MdePkg/Library/BaseRngLib/Rand/RdRand.c b/MdePkg/Library/BaseRngLib/Rand/RdRand.c
index 070d41e2555f..ff99436dbbbd 100644
--- a/MdePkg/Library/BaseRngLib/Rand/RdRand.c
+++ b/MdePkg/Library/BaseRngLib/Rand/RdRand.c
@@ -2,6 +2,7 @@
   Random number generator services that uses RdRand instruction access
   to provide high-quality random numbers.
 
+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>
 
@@ -22,6 +23,88 @@ 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 AsmRdRand  AsmRdRand64
+#else
+#define AsmRdRand  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 (AsmRdRand (&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 AsmRdRand
+
 /**
   The constructor function checks whether or not RDRAND instruction is supported
   by the host hardware.
@@ -46,10 +129,13 @@ 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;
 }
 
@@ -68,6 +154,7 @@ ArchGetRandomNumber16 (
   OUT     UINT16  *Rand
   )
 {
+  ASSERT (mRdRandSupported);
   return AsmRdRand16 (Rand);
 }
 
@@ -86,6 +173,7 @@ ArchGetRandomNumber32 (
   OUT     UINT32  *Rand
   )
 {
+  ASSERT (mRdRandSupported);
   return AsmRdRand32 (Rand);
 }
 
@@ -104,6 +192,7 @@ ArchGetRandomNumber64 (
   OUT     UINT64  *Rand
   )
 {
+  ASSERT (mRdRandSupported);
   return AsmRdRand64 (Rand);
 }
 
@@ -120,11 +209,5 @@ ArchIsRngSupported (
   VOID
   )
 {
-  /*
-     Existing software depends on this always returning TRUE, so for
-     now hard-code it.
-
-     return mRdRandSupported;
-  */
-  return TRUE;
+  return mRdRandSupported;
 }
-- 
2.38.1



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