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

["Wu, Hao A" <hao.a.wu@intel.com>]

> -----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-firmware-
> 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.in
> f
> b/MdeModulePkg/Universal/FaultTolerantWriteDxe/FaultTolerantWriteSmm.in
> f
> index 85d109e8d9..606cc2266b 100644
> ---
> a/MdeModulePkg/Universal/FaultTolerantWriteDxe/FaultTolerantWriteSmm.in
> f
> +++
> b/MdeModulePkg/Universal/FaultTolerantWriteDxe/FaultTolerantWriteSmm
> +++ .inf
> @@ -55,6 +55,7 @@
>    PcdLib
>    ReportStatusCodeLib
>    SmmMemLib
> +  BaseLib
> 
>  [Guids]
>    #
> --
> 2.12.0.windows.1