SecurityPkg/RngDxe: Use GetRngGuid() when probing RngLib

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

The EFI_RNG_PROTOCOL can rely on the RngLib. The RngLib has multiple
implementations, some of them are unsafe (e.g. BaseRngLibTimerLib).
To allow the RngDxe to detect when such implementation is used,
a GetRngGuid() function was added in a previous patch.

The EFI_RNG_PROTOCOL can advertise multiple algorithms through
Guids. The PcdCpuRngSupportedAlgorithm is currently used to
advertise the RngLib in the Arm implementation.

The issues of doing that are:
- the RngLib implementation might not use CPU instructions,
  cf. the BaseRngLibTimerLib
- most platforms don't set PcdCpuRngSupportedAlgorithm

A GetRngGuid() was added to the RngLib in a previous patch,
allowing to identify the algorithm implemented by the RngLib.
Make use of this function and place the unsage algorithm
at the last position in the mAvailableAlgoArray.

Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Jiewen Yao <Jiewen.yao@intel.com>
Tested-by: Kun Qin <kun.qin@microsoft.com>

SecurityPkg/RandomNumberGenerator/RngDxe/AArch64/AArch64Algo.c

@@ -10,6 +10,8 @@
 #include <Library/DebugLib.h>
 #include <Library/MemoryAllocationLib.h>
 #include <Library/ArmTrngLib.h>
+#include <Library/RngLib.h>
+#include <Guid/RngAlgorithm.h>
 
 #include "RngDxeInternals.h"
 
@@ -28,9 +30,13 @@ GetAvailableAlgorithms (
   VOID
   )
 {
-  UINT64  DummyRand;
+  EFI_STATUS  Status;
   UINT16      MajorRevision;
   UINT16      MinorRevision;
+  GUID        RngGuid;
+  BOOLEAN     UnSafeAlgo;
+
+  UnSafeAlgo = FALSE;
 
   // Rng algorithms 2 times, one for the allocation, one to populate.
   mAvailableAlgoArray = AllocateZeroPool (RNG_AVAILABLE_ALGO_MAX);
@@ -38,24 +44,29 @@ GetAvailableAlgorithms (
     return EFI_OUT_OF_RESOURCES;
   }
 
-  // Check RngGetBytes() before advertising PcdCpuRngSupportedAlgorithm.
-  if (!EFI_ERROR (RngGetBytes (sizeof (DummyRand), (UINT8 *)&DummyRand))) {
-    CopyMem (
-      &mAvailableAlgoArray[mAvailableAlgoArrayCount],
-      PcdGetPtr (PcdCpuRngSupportedAlgorithm),
-      sizeof (EFI_RNG_ALGORITHM)
-      );
-    mAvailableAlgoArrayCount++;
-
-    DEBUG_CODE_BEGIN ();
-    if (IsZeroGuid (PcdGetPtr (PcdCpuRngSupportedAlgorithm))) {
+  // Identify RngLib algorithm.
+  Status = GetRngGuid (&RngGuid);
+  if (!EFI_ERROR (Status)) {
+    if (IsZeroGuid (&RngGuid) ||
+        CompareGuid (&RngGuid, &gEdkiiRngAlgorithmUnSafe))
+    {
+      // Treat zero GUID as an unsafe algorithm
       DEBUG ((
         DEBUG_WARN,
-        "PcdCpuRngSupportedAlgorithm should be a non-zero GUID\n"
+        "RngLib uses an Unsafe algorithm and "
+        "must not be used for production builds.\n"
         ));
+      // Set the UnSafeAlgo flag to indicate an unsafe algorithm was found
+      // so that it can be added at the end of the algorithm list.
+      UnSafeAlgo = TRUE;
+    } else {
+      CopyMem (
+        &mAvailableAlgoArray[mAvailableAlgoArrayCount],
+        &RngGuid,
+        sizeof (RngGuid)
+        );
+      mAvailableAlgoArrayCount++;
     }
-
-    DEBUG_CODE_END ();
   }
 
   // Raw algorithm (Trng)
@@ -68,5 +79,15 @@ GetAvailableAlgorithms (
     mAvailableAlgoArrayCount++;
   }
 
+  // Add unsafe algorithm at the end of the list.
+  if (UnSafeAlgo) {
+    CopyMem (
+      &mAvailableAlgoArray[mAvailableAlgoArrayCount],
+      &gEdkiiRngAlgorithmUnSafe,
+      sizeof (EFI_RNG_ALGORITHM)
+      );
+    mAvailableAlgoArrayCount++;
+  }
+
   return EFI_SUCCESS;
 }

SecurityPkg/RandomNumberGenerator/RngDxe/ArmRngDxe.c

@@ -78,6 +78,7 @@ RngGetRNG (
 {
   EFI_STATUS  Status;
   UINTN       Index;
+  GUID        RngGuid;
 
   if ((This == NULL) || (RNGValueLength == 0) || (RNGValue == NULL)) {
     return EFI_INVALID_PARAMETER;
@@ -102,7 +103,10 @@ RngGetRNG (
   }
 
 FoundAlgo:
-  if (CompareGuid (RNGAlgorithm, PcdGetPtr (PcdCpuRngSupportedAlgorithm))) {
+  Status = GetRngGuid (&RngGuid);
+  if (!EFI_ERROR (Status) &&
+      CompareGuid (RNGAlgorithm, &RngGuid))
+  {
     Status = RngGetBytes (RNGValueLength, RNGValue);
     return Status;
   }

SecurityPkg/RandomNumberGenerator/RngDxe/RngDxe.inf

@@ -75,13 +75,11 @@
   gEfiRngAlgorithmX9313DesGuid        ## SOMETIMES_PRODUCES    ## GUID        # Unique ID of the algorithm for RNG
   gEfiRngAlgorithmX931AesGuid         ## SOMETIMES_PRODUCES    ## GUID        # Unique ID of the algorithm for RNG
   gEfiRngAlgorithmRaw                 ## SOMETIMES_PRODUCES    ## GUID        # Unique ID of the algorithm for RNG
+  gEdkiiRngAlgorithmUnSafe            ## SOMETIMES_PRODUCES    ## GUID        # Unique ID of the algorithm for RNG
 
 [Protocols]
   gEfiRngProtocolGuid                ## PRODUCES
 
-[Pcd.AARCH64]
-  gEfiMdePkgTokenSpaceGuid.PcdCpuRngSupportedAlgorithm      ## CONSUMES
-
 [Depex]
   TRUE