SecurityPkg/RngDxe: Conditionally install EFI_RNG_PROTOCOL

On Arm platforms, the number of available RNG algorithms is
dynamically detected and can be 0 in the absence of FEAT_RNG
and firmware TRNG.
In this case, the EFI_RNG_PROTOCOL should not be installed to
prevent from installing an empty protocol.

Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>
[ardb: return EFI_REQUEST_UNLOAD_IMAGE instead of an error]
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
(cherry picked from commit 75fb0cfc82)

SecurityPkg/RandomNumberGenerator/RngDxe/RngDxe.c

@@ -63,6 +63,18 @@ RngDriverEntry (
   EFI_STATUS  Status;
   EFI_HANDLE  Handle;
 
+  //
+  // Get the list of available algorithm.
+  //
+  Status = GetAvailableAlgorithms ();
+  if (EFI_ERROR (Status)) {
+    return Status;
+  }
+
+  if (mAvailableAlgoArrayCount == 0) {
+    return EFI_REQUEST_UNLOAD_IMAGE;
+  }
+
   //
   // Install UEFI RNG (Random Number Generator) Protocol
   //
@@ -74,13 +86,10 @@ RngDriverEntry (
                   NULL
                   );
   if (EFI_ERROR (Status)) {
-    return Status;
+    FreeAvailableAlgorithms ();
   }
 
-  //
-  // Get the list of available algorithm.
-  //
-  return GetAvailableAlgorithms ();
+  return Status;
 }
 
 /**