[PATCH v2 2/2] SecurityPkg/OpalPWSupportLib: [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
OpalPasswordSupportLib and insert AsmLfence API to mitigate the bounds
check bypass issue.

For SMI handler SmmOpalPasswordHandler():

Under "case SMM_FUNCTION_SET_OPAL_PASSWORD:",
'&DeviceBuffer->OpalDevicePath' can points to a potential cross boundary
access of the 'CommBuffer' (controlled external inputs) during speculative
execution. This cross boundary access pointer is later passed as parameter
'DevicePath' into function OpalSavePasswordToSmm().

Within function OpalSavePasswordToSmm(), 'DevicePathLen' is an access to
the content in 'DevicePath' and can be inferred by code:
"CompareMem (&List->OpalDevicePath, DevicePath, DevicePathLen)". One can
observe which part of the content within either '&List->OpalDevicePath' or
'DevicePath' was brought into cache to possibly reveal the value of
'DevicePathLen'.

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: Star Zeng <star.zeng@intel.com>
Cc: Chao Zhang <chao.b.zhang@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>
---
 SecurityPkg/Library/OpalPasswordSupportLib/OpalPasswordSupportLib.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/SecurityPkg/Library/OpalPasswordSupportLib/OpalPasswordSupportLib.c b/SecurityPkg/Library/OpalPasswordSupportLib/OpalPasswordSupportLib.c
index e377e9ca79..1c3bfffb86 100644
--- a/SecurityPkg/Library/OpalPasswordSupportLib/OpalPasswordSupportLib.c
+++ b/SecurityPkg/Library/OpalPasswordSupportLib/OpalPasswordSupportLib.c
@@ -1,7 +1,7 @@
 /** @file
   Implementation of Opal password support library.
 
-Copyright (c) 2016, Intel Corporation. All rights reserved.<BR>
+Copyright (c) 2016 - 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
@@ -706,6 +706,11 @@ SmmOpalPasswordHandler (
         Status = EFI_INVALID_PARAMETER;
         goto EXIT;
       }
+      //
+      // The AsmLfence() call here is to ensure the above range checks for the
+      // CommBuffer have been completed before calling into OpalSavePasswordToSmm().
+      //
+      AsmLfence ();
 
       Status = OpalSavePasswordToSmm (&DeviceBuffer->OpalDevicePath, DeviceBuffer->Password, DeviceBuffer->PasswordLength);
       break;
-- 
2.12.0.windows.1