MdeModulePkg/FaultTolerantWrite:[CVE-2017-5753]Fix bounds check bypass

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

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>