[PATCH v2 1/2] MdeModulePkg/SmmCorePerfLib: [CVE-2017-5753] Fix bounds check bypass

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

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
SmmCorePerformanceLib and insert AsmLfence API to mitigate the bounds
check bypass issue.

For SMI handler SmmPerformanceHandlerEx():

Under "case SMM_PERF_FUNCTION_GET_GAUGE_DATA :",
'SmmPerfCommData->LogEntryKey' can be a potential cross boundary access of
the 'CommBuffer' (controlled external inputs) during speculative
execution. This cross boundary access is then assign to parameter
'LogEntryKey'. And the value of 'LogEntryKey' can be inferred by code:

  CopyMem (
    (UINT8 *) &GaugeDataEx[Index],
    (UINT8 *) &GaugeEntryExArray[LogEntryKey++],
    sizeof (GAUGE_DATA_ENTRY_EX)
    );

One can observe which part of the content within 'GaugeEntryExArray' was
brought into cache to possibly reveal the value of 'LogEntryKey'.

Hence, this commit adds a AsmLfence() after the boundary/range checks of
'CommBuffer' to prevent the speculative execution.

And there is 1 similar case for SMI handler SmmPerformanceHandler() as
well. This commit also handles it.

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: Star Zeng <star.zeng@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
---
 MdeModulePkg/Library/SmmCorePerformanceLib/SmmCorePerformanceLib.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/MdeModulePkg/Library/SmmCorePerformanceLib/SmmCorePerformanceLib.c b/MdeModulePkg/Library/SmmCorePerformanceLib/SmmCorePerformanceLib.c
index cd1f1a5d5f..63c1eea3a2 100644
--- a/MdeModulePkg/Library/SmmCorePerformanceLib/SmmCorePerformanceLib.c
+++ b/MdeModulePkg/Library/SmmCorePerformanceLib/SmmCorePerformanceLib.c
@@ -16,7 +16,7 @@
 
  SmmPerformanceHandlerEx(), SmmPerformanceHandler() will receive untrusted input and do basic validation.
 
-Copyright (c) 2011 - 2017, Intel Corporation. All rights reserved.<BR>
+Copyright (c) 2011 - 2018, Intel Corporation. All rights reserved.<BR>
 This program and the accompanying materials
 are licensed and made available under the terms and conditions of the BSD License
 which accompanies this distribution.  The full text of the license may be found at
@@ -538,6 +538,13 @@ SmmPerformanceHandlerEx (
          break;
        }
 
+       //
+       // The AsmLfence() call here is to ensure the previous range/content
+       // checks for the CommBuffer have been completed before calling
+       // CopyMem().
+       //
+       AsmLfence ();
+
        GaugeEntryExArray = (GAUGE_DATA_ENTRY_EX *) (mGaugeData + 1);
 
        for (Index = 0; Index < NumberOfEntries; Index++) {
@@ -650,6 +657,13 @@ SmmPerformanceHandler (
          break;
        }
 
+       //
+       // The AsmLfence() call here is to ensure the previous range/content
+       // checks for the CommBuffer have been completed before calling
+       // CopyMem().
+       //
+       AsmLfence ();
+
        GaugeEntryExArray = (GAUGE_DATA_ENTRY_EX *) (mGaugeData + 1);
 
        for (Index = 0; Index < NumberOfEntries; Index++) {
-- 
2.12.0.windows.1