Hi Pierre,

Thank you for this patch.

I think we should treat algorithms that have a zero GUID as unsafe. I 
have made some suggestions on those lines marked inline as [SAMI].

Regards,

Sami Mujawar

On 09/05/2023 08:40 am, pierre.gondois@arm.com wrote:
> From: Pierre Gondois<pierre.gondois@arm.com>
>
> 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.
>
> Signed-off-by: Pierre Gondois<pierre.gondois@arm.com>
> ---
>   .../RngDxe/AArch64/AArch64Algo.c              | 24 +++++++++----------
>   .../RandomNumberGenerator/RngDxe/ArmRngDxe.c  |  6 ++++-
>   .../RandomNumberGenerator/RngDxe/RngDxe.inf   |  5 ++--
>   3 files changed, 19 insertions(+), 16 deletions(-)
>
> diff --git a/SecurityPkg/RandomNumberGenerator/RngDxe/AArch64/AArch64Algo.c b/SecurityPkg/RandomNumberGenerator/RngDxe/AArch64/AArch64Algo.c
> index e8be217f8a8c..a1ff7bd58fda 100644
> --- a/SecurityPkg/RandomNumberGenerator/RngDxe/AArch64/AArch64Algo.c
> +++ b/SecurityPkg/RandomNumberGenerator/RngDxe/AArch64/AArch64Algo.c
> @@ -10,6 +10,7 @@
>   #include <Library/DebugLib.h>
>   #include <Library/MemoryAllocationLib.h>
>   #include <Library/ArmTrngLib.h>
> +#include <Library/RngLib.h>
>   
>   #include "RngDxeInternals.h"
>   
> @@ -28,9 +29,10 @@ GetAvailableAlgorithms (
>     VOID
>     )
>   {
> -  UINT64  DummyRand;
> -  UINT16  MajorRevision;
> -  UINT16  MinorRevision;
> +  EFI_STATUS  Status;
> +  UINT16      MajorRevision;
> +  UINT16      MinorRevision;
> +  GUID        RngGuid;
>   
>     // Rng algorithms 2 times, one for the allocation, one to populate.
>     mAvailableAlgoArray = AllocateZeroPool (RNG_AVAILABLE_ALGO_MAX);
> @@ -38,24 +40,22 @@ GetAvailableAlgorithms (
>       return EFI_OUT_OF_RESOURCES;
>     }
>   
> -  // Check RngGetBytes() before advertising PcdCpuRngSupportedAlgorithm.
> -  if (!EFI_ERROR (RngGetBytes (sizeof (DummyRand), (UINT8 *)&DummyRand))) {
<SNIP>
> +  // Identify RngLib algorithm.
> +  Status = GetRngGuid (&RngGuid);
> +  if (!EFI_ERROR (Status)) {
>       CopyMem (
>         &mAvailableAlgoArray[mAvailableAlgoArrayCount],
> -      PcdGetPtr (PcdCpuRngSupportedAlgorithm),
> -      sizeof (EFI_RNG_ALGORITHM)
> +      RngGuid,
> +      sizeof (RngGuid)
>         );
>       mAvailableAlgoArrayCount++;
>   
> -    DEBUG_CODE_BEGIN ();
> -    if (IsZeroGuid (PcdGetPtr (PcdCpuRngSupportedAlgorithm))) {
> +    if (IsZeroGuid (&RngGuid)) {
>         DEBUG ((
>           DEBUG_WARN,
> -        "PcdCpuRngSupportedAlgorithm should be a non-zero GUID\n"
> +        "RngLib should have a non-zero GUID\n"
>           ));
>       }
> -
> -    DEBUG_CODE_END ();
>     }
>   
>     // Raw algorithm (Trng)

...

</SNIP>

[SAMI] I think this code should do the following:

---

   BOOLEAN UnSafeAlgo = FALSE;
   mAvailableAlgoArrayCount = 0;
// Identify RngLib algorithm.
   Status = GetRngGuid (&RngGuid);
if(!EFI_ERROR (Status)) {
if((IsZeroGuid (&RngGuid)) ||
         (CompareGuid (&RngGuid, &gEfiRngAlgorithmUnSafe))) {
// Treat zero GUID as an unsafe algorithm
       DEBUG ((
         DEBUG_WARN,
"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++;
     }
   }
// Raw algorithm (Trng)
if(!EFI_ERROR (GetArmTrngVersion (&MajorRevision, &MinorRevision))) {
     CopyMem (
       &mAvailableAlgoArray[mAvailableAlgoArrayCount],
       &gEfiRngAlgorithmRaw,
sizeof(EFI_RNG_ALGORITHM)
       );
     mAvailableAlgoArrayCount++;
   }
// Add unsafe algorithms at the end of the list.
if(UnSafeAlgo) {
     CopyMem (
       &mAvailableAlgoArray[mAvailableAlgoArrayCount],
       &gEfiRngAlgorithmUnSafe,
sizeof(EFI_RNG_ALGORITHM)
       );
     mAvailableAlgoArrayCount++;
   }
---

Doing this will not need the call to RngFindDefaultAlgo () introduced in 
the next patch "[PATCH v1 7/8] SecurityPkg/RngDxe: Select safe default 
Rng algorithm", which can be dropped.

[SAMI]

> diff --git a/SecurityPkg/RandomNumberGenerator/RngDxe/ArmRngDxe.c b/SecurityPkg/RandomNumberGenerator/RngDxe/ArmRngDxe.c
> index ce49ff7ae661..78a18c5e1177 100644
> --- a/SecurityPkg/RandomNumberGenerator/RngDxe/ArmRngDxe.c
> +++ b/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;
>     }
> diff --git a/SecurityPkg/RandomNumberGenerator/RngDxe/RngDxe.inf b/SecurityPkg/RandomNumberGenerator/RngDxe/RngDxe.inf
> index d6c2d30195bf..aa5743387ed9 100644
> --- a/SecurityPkg/RandomNumberGenerator/RngDxe/RngDxe.inf
> +++ b/SecurityPkg/RandomNumberGenerator/RngDxe/RngDxe.inf
> @@ -75,13 +75,12 @@ [Guids]
>     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
> +  gEfiRngAlgorithmArmRndr             ## SOMETIMES_PRODUCES    ## GUID        # Unique ID of the algorithm for RNG
[SAMI] Can the [Guids] section be split to relect the usage across 
architectures, please?
> +  gEfiRngAlgorithmUnSafe              ## SOMETIMES_PRODUCES    ## GUID        # Unique ID of the algorithm for RNG
>   
>   [Protocols]
>     gEfiRngProtocolGuid                ## PRODUCES
>   
> -[Pcd.AARCH64]
> -  gEfiMdePkgTokenSpaceGuid.PcdCpuRngSupportedAlgorithm      ## CONSUMES
> -
>   [Depex]
>     TRUE
>