Re: [PATCH v2 2/5] MdeModulePkg/FaultTolerantWrite:[CVE-2017-5753]Fix bounds check bypass

["Zeng, Star" <star.zeng@intel.com>]

Got it, thanks.

Reviewed-by: Star Zeng <star.zeng@intel.com>

Star
-----Original Message-----
From: Wu, Hao A 
Sent: Saturday, September 29, 2018 2:21 PM
To: Zeng, Star <star.zeng@intel.com>; edk2-devel@lists.01.org
Cc: Yao, Jiewen <jiewen.yao@intel.com>
Subject: RE: [PATCH v2 2/5] MdeModulePkg/FaultTolerantWrite:[CVE-2017-5753]Fix bounds check bypass

> -----Original Message-----
> From: Zeng, Star
> Sent: Saturday, September 29, 2018 2:11 PM
> To: Wu, Hao A; edk2-devel@lists.01.org
> Cc: Yao, Jiewen; Zeng, Star
> Subject: RE: [PATCH v2 2/5] MdeModulePkg/FaultTolerantWrite:[CVE-2017-
> 5753]Fix bounds check bypass
> 
> Please double check whether the AsmLfence calling should be before the 
> line below.
> 
>         PrivateData = (VOID *)&SmmFtwWriteHeader->Data[Length];

Hi,

The above code is getting the address of a possible cross bounday access during the speculative execution.

I also checked that the subsequent usage of 'PrivateData' does not have a code pattern of the 'Bounds check bypass' issue. So I think the
AsmLfence() is not needed here.

Best Regards,
Hao Wu

> 
> 
> Thanks,
> Star
> -----Original Message-----
> From: Wu, Hao A
> Sent: Tuesday, September 25, 2018 2:13 PM
> To: edk2-devel@lists.01.org
> Cc: Wu, Hao A <hao.a.wu@intel.com>; Yao, Jiewen 
> <jiewen.yao@intel.com>; Zeng, Star <star.zeng@intel.com>
> Subject: [PATCH v2 2/5] MdeModulePkg/FaultTolerantWrite:[CVE-2017-
> 5753]Fix bounds check bypass
> 
> REF:https://bugzilla.tianocore.org/show_bug.cgi?id=1194
> 
> Speculative execution is used by processor to avoid having to wait for 
> data to arrive from memory, or for previous operations to finish, the 
> processor may speculate as to what will be executed.
> 
> If the speculation is incorrect, the speculatively executed 
> instructions might leave hints such as which memory locations have been brought into cache.
> Malicious actors can use the bounds check bypass method (code gadgets 
> with controlled external inputs) to infer data values that have been 
> used in speculative operations to reveal secrets which should not 
> otherwise be accessed.
> 
> This commit will focus on the SMI handler(s) registered within the 
> FaultTolerantWriteDxe driver and insert AsmLfence API to mitigate the 
> bounds check bypass issue.
> 
> For SMI handler SmmFaultTolerantWriteHandler():
> 
> Under "case FTW_FUNCTION_WRITE:", 'SmmFtwWriteHeader->Length' can be a 
> potential cross boundary access of the 'CommBuffer' (controlled 
> external
> inputs) during speculative execution. This cross boundary access is 
> later passed as parameter 'Length' into function FtwWrite().
> 
> Within function FtwWrite(), the value of 'Length' can be inferred by code:
> "CopyMem (MyBuffer + Offset, Buffer, Length);". One can observe which 
> part of the content within 'Buffer' was brought into cache to possibly 
> reveal the value of 'Length'.
> 
> Hence, this commit adds a AsmLfence() after the boundary/range checks 
> of 'CommBuffer' to prevent the speculative execution.
> 
> A more detailed explanation of the purpose of commit is under the 
> 'Bounds check bypass mitigation' section of the below link:
> https://software.intel.com/security-software-guidance/insights/host-fi
> rmware- speculative-execution-side-channel-mitigation
> 
> And the document at:
> https://software.intel.com/security-software-guidance/api-
> app/sites/default/files/337879-analyzing-potential-bounds-Check-bypass
> -
> vulnerabilities.pdf
> 
> Cc: Jiewen Yao <jiewen.yao@intel.com>
> Cc: Star Zeng <star.zeng@intel.com>
> Contributed-under: TianoCore Contribution Agreement 1.1
> Signed-off-by: Hao Wu <hao.a.wu@intel.com>
> ---
>  MdeModulePkg/Universal/FaultTolerantWriteDxe/FaultTolerantWriteSmm.c
> | 7 +++++++
>  
> MdeModulePkg/Universal/FaultTolerantWriteDxe/FaultTolerantWriteSmm.inf
> | 1 +
>  2 files changed, 8 insertions(+)
> 
> diff --git
> a/MdeModulePkg/Universal/FaultTolerantWriteDxe/FaultTolerantWriteSmm.c
> b/MdeModulePkg/Universal/FaultTolerantWriteDxe/FaultTolerantWriteSmm.c
> index 632313f076..27fcab19b6 100644
> ---
> a/MdeModulePkg/Universal/FaultTolerantWriteDxe/FaultTolerantWriteSmm.c
> +++
> b/MdeModulePkg/Universal/FaultTolerantWriteDxe/FaultTolerantWriteSmm
> +++ .c
> @@ -57,6 +57,7 @@ WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, 
> EITHER EXPRESS OR IMPLIED.
>  #include <PiSmm.h>
>  #include <Library/SmmServicesTableLib.h>  #include 
> <Library/SmmMemLib.h>
> +#include <Library/BaseLib.h>
>  #include <Protocol/SmmSwapAddressRange.h>  #include 
> "FaultTolerantWrite.h"
>  #include "FaultTolerantWriteSmmCommon.h"
> @@ -417,6 +418,12 @@ SmmFaultTolerantWriteHandler (
>                   &SmmFvbHandle
>                   );
>        if (!EFI_ERROR (Status)) {
> +        //
> +        // The AsmLfence() call here is to ensure the previous range/content
> +        // checks for the CommBuffer have been completed before calling into
> +        // FtwWrite().
> +        //
> +        AsmLfence ();
>          Status = FtwWrite(
>                     &mFtwDevice->FtwInstance,
>                     SmmFtwWriteHeader->Lba, diff --git 
> a/MdeModulePkg/Universal/FaultTolerantWriteDxe/FaultTolerantWriteSmm.i
> n
> f
> b/MdeModulePkg/Universal/FaultTolerantWriteDxe/FaultTolerantWriteSmm.i
> n
> f
> index 85d109e8d9..606cc2266b 100644
> ---
> a/MdeModulePkg/Universal/FaultTolerantWriteDxe/FaultTolerantWriteSmm.i
> n
> f
> +++
> b/MdeModulePkg/Universal/FaultTolerantWriteDxe/FaultTolerantWriteSmm
> +++ .inf
> @@ -55,6 +55,7 @@
>    PcdLib
>    ReportStatusCodeLib
>    SmmMemLib
> +  BaseLib
> 
>  [Guids]
>    #
> --
> 2.12.0.windows.1