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/RngDxeInternals.h

@@ -12,6 +12,33 @@
 
 #include <Protocol/Rng.h>
 
+//
+// Array containing the validated Rng algorithm.
+// The entry with the lowest index will be the default algorithm.
+//
+extern UINTN              mAvailableAlgoArrayCount;
+extern EFI_RNG_ALGORITHM  *mAvailableAlgoArray;
+
+/** Allocate and initialize mAvailableAlgoArray with the available
+    Rng algorithms. Also update mAvailableAlgoArrayCount.
+
+  @retval EFI_SUCCESS             The function completed successfully.
+  @retval EFI_OUT_OF_RESOURCES    Could not allocate memory.
+**/
+EFI_STATUS
+EFIAPI
+GetAvailableAlgorithms (
+  VOID
+  );
+
+/** Free mAvailableAlgoArray.
+**/
+VOID
+EFIAPI
+FreeAvailableAlgorithms (
+  VOID
+  );
+
 /**
   Returns information about the random number generation implementation.