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

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

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

-----Original Message-----
From: edk2-devel [mailto:edk2-devel-bounces@lists.01.org] On Behalf Of Hao Wu
Sent: Friday, November 16, 2018 12:13 PM
To: edk2-devel@lists.01.org
Cc: Wu, Hao A <hao.a.wu@intel.com>; Laszlo Ersek <lersek@redhat.com>; Yao, Jiewen <jiewen.yao@intel.com>; Zeng, Star <star.zeng@intel.com>
Subject: [edk2] [PATCH v2 1/2] MdeModulePkg/SmmCorePerfLib: [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 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

_______________________________________________
edk2-devel mailing list
edk2-devel@lists.01.org
https://lists.01.org/mailman/listinfo/edk2-devel