SecurityPkg/RngDxe: Check before advertising Cpu Rng algo

RngGetBytes() relies on the RngLib. The RngLib might use the RNDR
instruction if the FEAT_RNG feature is present. RngGetInfo and
RngGetRNG both must check that RngGetBytes() is working before
advertising/using it.

To do so, allocate an array storing the available algorithms.
The Rng algorithm at the lowest index will be the default Rng
algorithm. The array is shared between RngGetInfo and RngGetRNG.

This array is allocated when the driver is loaded, and freed
when unloaded.

This patch also prevents from having PcdCpuRngSupportedAlgorithm
let to a zero GUID, but let the possibility to have no valid Rng
algorithm in such case.

Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>
Acked-by: Jiewen Yao <jiewen.yao@intel.com>

SecurityPkg/RandomNumberGenerator/RngDxe/RngDxe.c

@@ -27,6 +27,13 @@ SPDX-License-Identifier: BSD-2-Clause-Patent
 
 #include "RngDxeInternals.h"
 
+//
+// Array containing the validated Rng algorithm.
+// The entry with the lowest index will be the default algorithm.
+//
+UINTN              mAvailableAlgoArrayCount;
+EFI_RNG_ALGORITHM  *mAvailableAlgoArray;
+
 //
 // The Random Number Generator (RNG) protocol
 //
@@ -66,8 +73,39 @@ RngDriverEntry (
                   &mRngRdRand,
                   NULL
                   );
+  if (EFI_ERROR (Status)) {
+    return Status;
+  }
 
-  return Status;
+  //
+  // Get the list of available algorithm.
+  //
+  return GetAvailableAlgorithms ();
+}
+
+/**
+  This is the unload handle for RndgDxe module.
+
+  Disconnect the driver specified by ImageHandle from all the devices in the handle database.
+  Uninstall all the protocols installed in the driver entry point.
+
+  @param[in] ImageHandle           The drivers' driver image.
+
+  @retval    EFI_SUCCESS           The image is unloaded.
+  @retval    Others                Failed to unload the image.
+
+**/
+EFI_STATUS
+EFIAPI
+RngDriverUnLoad (
+  IN EFI_HANDLE  ImageHandle
+  )
+{
+  //
+  // Free the list of available algorithm.
+  //
+  FreeAvailableAlgorithms ();
+  return EFI_SUCCESS;
 }
 
 /**