[PATCH 1/1][edk2-stable202211] SecurityPkg/RngDxe: Fix Rng algo selection for Arm

["PierreGondois" <pierre.gondois@arm.com>]

From: Pierre Gondois <pierre.gondois@arm.com>

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=4151

PcdCpuRngSupportedAlgorithm is set to the Zero Guid for KvmTool
since it is not possible to predict which algorithm will be
implemented for Arm's FEAT_RNG in the host. Current handling is:
- skipping the Zero Guid, which should not happen to handle
  KvmTool's case,
- triggering an ASSERT if no algorithm was found. However having
  no Rng algorithm is a valid case,

Correctly handle the Zero Guid case and replace the ASSERT by a
warning message when no Rng algorithm is found.
Also simplify the selection of the Rng algorithm when the default
one is selected by just picking up the first element of
mAvailableAlgoArray.

Reported-by: Sami Mujawar <sami.mujawar@arm.com>
Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>
---
 .../RandomNumberGenerator/RngDxe/ArmRngDxe.c      | 15 +++------------
 SecurityPkg/RandomNumberGenerator/RngDxe/RngDxe.c |  8 +++++++-
 2 files changed, 10 insertions(+), 13 deletions(-)

diff --git a/SecurityPkg/RandomNumberGenerator/RngDxe/ArmRngDxe.c b/SecurityPkg/RandomNumberGenerator/RngDxe/ArmRngDxe.c
index 5ba319899ce9..722d53386373 100644
--- a/SecurityPkg/RandomNumberGenerator/RngDxe/ArmRngDxe.c
+++ b/SecurityPkg/RandomNumberGenerator/RngDxe/ArmRngDxe.c
@@ -76,7 +76,6 @@ RngGetRNG (
   )
 {
   EFI_STATUS  Status;
-  UINTN       Index;
 
   if ((This == NULL) || (RNGValueLength == 0) || (RNGValue == NULL)) {
     return EFI_INVALID_PARAMETER;
@@ -86,21 +85,13 @@ RngGetRNG (
     //
     // Use the default RNG algorithm if RNGAlgorithm is NULL.
     //
-    for (Index = 0; Index < mAvailableAlgoArrayCount; Index++) {
-      if (!IsZeroGuid (&mAvailableAlgoArray[Index])) {
-        RNGAlgorithm = &mAvailableAlgoArray[Index];
-        goto FoundAlgo;
-      }
-    }
-
-    if (Index == mAvailableAlgoArrayCount) {
-      // No algorithm available.
-      ASSERT (Index != mAvailableAlgoArrayCount);
+    if (mAvailableAlgoArrayCount != 0) {
+      RNGAlgorithm = &mAvailableAlgoArray[0];
+    } else {
       return EFI_DEVICE_ERROR;
     }
   }
 
-FoundAlgo:
   if (CompareGuid (RNGAlgorithm, PcdGetPtr (PcdCpuRngSupportedAlgorithm))) {
     Status = RngGetBytes (RNGValueLength, RNGValue);
     return Status;
diff --git a/SecurityPkg/RandomNumberGenerator/RngDxe/RngDxe.c b/SecurityPkg/RandomNumberGenerator/RngDxe/RngDxe.c
index 421abb52b8bf..403b31b73609 100644
--- a/SecurityPkg/RandomNumberGenerator/RngDxe/RngDxe.c
+++ b/SecurityPkg/RandomNumberGenerator/RngDxe/RngDxe.c
@@ -21,6 +21,7 @@ SPDX-License-Identifier: BSD-2-Clause-Patent
 
 #include <Library/BaseLib.h>
 #include <Library/BaseMemoryLib.h>
+#include <Library/DebugLib.h>
 #include <Library/UefiBootServicesTableLib.h>
 #include <Library/RngLib.h>
 #include <Protocol/Rng.h>
@@ -80,7 +81,12 @@ RngDriverEntry (
   //
   // Get the list of available algorithm.
   //
-  return GetAvailableAlgorithms ();
+  Status = GetAvailableAlgorithms ();
+  if (mAvailableAlgoArrayCount == 0) {
+    DEBUG ((DEBUG_WARN, "No Rng algorithm found in RngDxe.\n"));
+  }
+
+  return Status;
 }
 
 /**
-- 
2.25.1