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

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

Please double check whether the AsmLfence calling should be before the line below.

        PrivateData = (VOID *)&SmmFtwWriteHeader->Data[Length];


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