Re: [PATCH V7 00/10] UEFI Variable SMI Reduction

Kubacki, Michael A

Hi Laszlo,

I did not make any changes to the OvmfPkg patch and I forgot to carry forward the R-b.
I'll keep that in mind in the future.

For V7, I request the MdeModulePkg maintainers please add the R-b for the patches
not changed. If this is not acceptable, I will be happy to send out a patch series
with all of the R-b present.


-----Original Message-----
From: Laszlo Ersek <>
Sent: Friday, November 1, 2019 3:19 PM
To: Kubacki, Michael A <>;
Cc: Bi, Dandan <>; Ard Biesheuvel
<>; Dong, Eric <>; Gao, Liming
<>; Kinney, Michael D <>;
Ni, Ray <>; Wang, Jian J <>; Wu, Hao
A <>; Yao, Jiewen <>
Subject: Re: [PATCH V7 00/10] UEFI Variable SMI Reduction

Hello Michael,

On 11/01/19 18:34, Michael Kubacki wrote:

V7 Changes:
[PATCH V6 07/10] MdeModulePkg/Variable: Add RT GetVariable() cache
* Remove VariableRuntimeCache.c and VariableRuntimeCache.h from
VariableSmmRuntimeDxe.inf since they are not needed to build the

V6 Changes:
[PATCH V5 07/10] MdeModulePkg/Variable: Add RT GetVariable() cache
support The most significant change is:
* Free mVariableRuntimeHobCacheBuffer in CheckForRuntimeCacheSync
() in
VariableSmmRuntimeDxe.c with FreePages () instead of FreePool ().
This issue was not found in earlier testing because on the initial
set of platforms tested, the variable HOB flush was finished prior to
the variable HOB runtime cache buffer being allocated so the
FreePages () call was not executed.

The remaining changes did not affect testing but are included for
* Pass EFI_OPTIONAL_PTR for the DebugDisposition type in the
EfiConvertPointer ()
calls for mVariableRuntimeHobCacheBuffer,
mVariableRuntimeNvCacheBuffer, and
mVariableRuntimeVolatileCacheBuffer in VariableAddressChangeEvent ()
VariableSmmRuntimeDxe.c as these buffers will not be allocated if the
cache is disabled.
* In the
case in
SmmVariableHandler () in VariableSmm.c, explicitly verify that
VariableRuntimeHobCache.Store is not NULL in addition to checking that
VariableGlobal.HobVariableBase is not set to zero (variable HOB is
before writing to VariableRuntimeHobCache.Store.

V5 Changes:
[PATCH V4 07/10] MdeModulePkg/Variable: Add RT GetVariable() cache
* Increased validation of the runtime buffers passed in the SMM comm
structure to the
function in
SmmVariableHandler () in VariableSmm.c.
* Most notably, each runtime buffer given is checked to ensure its
range does not overlap with SMRAM ranges via
VariableSmmIsBufferOutsideSmmValid ().

V4 Changes:
[PATCH V3 7/9] MdeModulePkg/Variable: Add RT GetVariable() cache
* Set
gEfiMdeModulePkgTokenSpaceGuid.PcdEnableVariableRuntimeCache to
by default in MdeModulePkg.dec.

* Added a new patch to set
to TRUE at the end of the patch series. This allows someone to bisect an
issue at
patch #7 or patch #8 in the series with no change in variable caching
behavior. The
runtime cache variable logic would be applied explicitly in V4 patch #10.
I gave my R-b for the OvmfPkg patch in the v4 posting:

(alternative link:

In the v5 posting -- assuming you had not changed that specific OvmfPkg
patch, relative to v4 -- you should have picked up my R-b, and carried it
forward ever since (to the present v7). Basically, do a git-rebase, select the
"reword" action for the patch, then cut&paste my R-b from my
v4 review email to the bottom of the commit message. Then every further
posting will contain it.

If there *have* been changes to the patch, relative to v4, then it's indeed
right to drop (or simply not pick up) my R-b. FWIW, the changelog above
does not suggest the particular patch has seen any changes since v4.


V3 Changes:
[PATCH V2 1/9] MdeModulePkg/Variable: Consolidate common parsing
* Removed GUIDs added to VariableStandaloneMm.inf that are not
* Added more details to the commit message describing the criteria of
moving the chosen functions to VariableParsing.c.

[PATCH V2 2/9] MdeModulePkg/Variable: Parameterize
GetNextVariableEx() store list
* RenamedGetNextVariableEx () to
VariableServiceGetNextVariableInternal ()
* Updated comments in VariableServiceGetNextVariableInternal () to
refer to
"FindVariablEx ()" instead of "FindVariable ()" since FindVariableEx ()
is not used in the function.

[PATCH V2 3/9] MdeModulePkg/Variable: Parameterize
* Updated the commit message to clarify the message "structure can be
outside the fixed global variable".

[edk2-devel] [PATCH V2 4/9] MdeModulePkg/Variable: Add local auth
status in VariableParsing
* Remove the function InitVariableParsing ()
* Remove the mAuthFormat global variable and instead add a BOOLEAN
to all functions that require variable authentication information to
indicate if authenticated variables are used.
* This allows the authenticated variable status to be tracked in one place
each variable driver in the SMM variable solution
and VariableSmm).

[edk2-devel] [PATCH V2 5/9] MdeModulePkg/Variable: Add a file for NV
variable functions
* Added the following non-volatile related functions to
from Variable.c:
* InitRealNonVolatileVariableStore ()
* InitEmuNonVolatileVariableStore ()
* InitNonVolatileVariableStore ()

[edk2-devel] [PATCH V2 7/9] MdeModulePkg/Variable: Add RT
GetVariable() cache support
* Added a FeaturePCD to control enabling the runtime variable cache -
* Removed usage of the TimerLib and the wait to acquire
mVariableRuntimeCacheReadLock. Can rely on the UEFI specification
restrictions on Runtime Services callers.
* Removed the EFIAPI keyword from internal functions.
* Removed PCDs in VariableSmmRuntimeDxe.inf not required.
* Removed the HobVariableBackupBase variable no longer required.
* Renamed SynchronizeRuntimeVariableCacheEx () to better reflect
* Renamed functions in VariableRuntimeCache.c to better reflect usage.
* Introduced a local variable in
FlushPendingRuntimeVariableCacheUpdates ()
to reduce duplication of
* Corrected the macro used in SmmVariableHandler () to
* Remove usage of the
CommBuffer from the EFI System Table and use the same runtime
allocated for variable SMM communicate calls.

[PATCH V2 8/9] MdeModulePkg/Variable: Add RT GetNextVariableName()
cache support
* Removed usage of the TimerLib and the wait to acquire
mVariableRuntimeCacheReadLock. Can rely on the UEFI specification
on Runtime Services callers.

* Added a new patch to disable
for all OvmfPkg package builds as requested by maintainer Laszlo Ersek.

V2 Changes:

Patch #1 in V1 both moved functions to VariableParsing.c and modified
some functionality in those functions. In V2, the functions are first
moved and then functionality is modified in subsequent patches. This
resulted in the following new patches in the V2 patch series:

1. MdeModulePkg/Variable: Parameterize GetNextVariableEx() store list
2. MdeModulePkg/Variable: Parameterize VARIABLE_INFO_ENTRY buffer
MdeModulePkg/Variable: Add local auth status in VariableParsing 4.
MdeModulePkg/Variable: Add a file for NV variable functions

Apart from this refactor in the patches, no functionally impacting
changes were made.

This patch series reduces SMM usage when using
with VariableSmm. It does so by eliminating SMM usage for runtime
service GetVariable () and GetNextVariableName () invocations. Most
UEFI variable usage in typical systems after the variable store is
initialized (e.g. manufacturing boots) is due to GetVariable ( ) and
GetNextVariableName () not SetVariable (). GetVariable () calls can
regularly exceed 100 per boot while SetVariable () calls typically
remain less than 10 per boot. By focusing on the common case, the
majority of overhead associated with SMM can be avoided while still
using existing and proven code for operations such as variable
authentication that require an isolated execution environment.

* Advantage: Reduces overall system SMM usage
* Disadvantage: Requires more Runtime data memory usage

The runtime cache behavior described for this patch series is enabled
by default but can be disabled with a new FeaturePCD added to

The reaminder of this blurb describes the changes and behavior when
the PCD is set to TRUE.

Initial Performance Observations
* With these proposed changes, an Intel Atom based SoC saw GetVariable
( )
time for an existing variable reduce from ~220us to ~5us.

Major Changes
1. Two UEFI variable caches will be maintained.
a. "Runtime Cache" - Maintained in VariableSmmRuntimeDxe. Used to
runtime service GetVariable () and GetNextVariableName () callers.
b. "SMM cache" - Maintained in VariableSmm to service SMM
GetVariable ()
and GetNextVariableName () callers.
i. A cache in SMRAM is retained so SMM modules do not operate on
outside SMRAM.
2. A new UEFI variable read and write flow will be used as described below.

At any given time, the two caches would be coherent. On a variable
write, the runtime cache is only updated after validation in SMM and,
in the case of a non-volatile UEFI variable, the variable must also be
successfully written to non-volatile storage.

Prior RFC Feedback Addressed
RFC sent Sept. 5, 2019:

1. UEFI variable data retrieval from a ring 0 buffer

A common concern with this proposed set of changes is the potential
threat presented by serving runtime services callers from a ring 0 memory
buffer of EfiRuntimeServicesData type. The conclusion was that this
does not fundamentally alter the attack surface. The UEFI variable
Services are invoked from ring 0 and the data already travels through ring
0 buffers (such as the SMM communicate buffer) to reach the caller. Even
today if ring 0 is assumed to be malicious, the malicious code may keep
AP in a loop to monitor the communication data, when the BSP gets an
(authenticated) variable. When the communication buffer is updated and
status is set to EFI_SUCCESS, the AP may modify the communication
contents such the tampered data is returned to the BSP caller. Or an
interrupt handler on the BSP may alter the communication buffer
before the data is returned to the caller. In summary, this was not found
introduce any attack not possible today.

2. VarCheckLib impact

VarCheckLib plays a role in SetVariable () calls. This patch series only
changes GetVariable () behavior. Therefore, VarCheckLib is expected to
have no impact due to these changes.

Testing Performed
This code was tested with the master branch of edk2 on an Intel Kaby
Lake U and Intel Whiskey Lake U reference validation platform. The set
of tests performed included:

1. Boot from S5 to Windows 10 OS with SMM variables enabled and
the variable runtime cache enabled.
2. Boot from S5 to Ubuntu 18.04.1 LTS with SMM variable enabled
and the variable runtime cache enabled.
3. Boot from S5 to Windows 10 OS with SMM variables enabled and
the variable runtime cache disabled.
4. Boot from S5 to Ubuntu 18.04.1 LTS with SMM variable enabled
and the variable runtime cache disabled.
5. Boot from S5 to EFI shell with DXE variables enabled.
(commit 2de1f611be broke OvmfPkgIa32X64 boot regardless of
this patch series; testing without this commit was successful) 6.
Dump UEFI variable store at shell with dmpstore to verify contents.
7. Dump NvStorage FV from SPI flash after boot to verify contents written.
8. Dump UEFI variable statistics with VariableInfo at shell.
9. Boot with emulated variables enabled.
10. Cycles of adding and deleting a UEFI variable to verify cache
11. Set OsIndications to stop at FW UI to verify cache load of non-volatile

Why Keep SMM on Variable Writes
* SMM provides a ubiquitous isolated execution environment in x86 for
authenticated UEFI variables.
* BIOS region SPI flash write restrictions to SMM in platforms today can
be retained.

Today's UEFI Variable Cache (for reference)
* Maintained in SMRAM via VariableSmm.
* A "write-through" cache of variable data in the form of a UEFI variable
* Non-volatile and volatile variables are maintained in separate buffers
(variable stores).

Runtime & SMM Cache Coherency
The non-volatile cache should always accurately reflect non-volatile
storage contents (done today) and the "SMM cache" and "Runtime cache"
should always be coherent on access. The runtime cache is updated by

Updating both caches from within a SMM SetVariable () operation is
fairly straightforward but a race condition can occur if an SMI occurs
during the execution of runtime code reading from the runtime cache.
To handle this case, a runtime cache read lock is introduced that
explicitly moves pending updates from SMM to the runtime cache if an
SMM update occurs while the runtime cache is locked. Note that it is
not expected a Runtime services call will interrupt SMM processing since all
CPU cores rendezvous in SMM.

New Key Elements for Coherence
Runtime DXE (VariableSmmRuntimeDxe)
1. RuntimeCacheReadLock - A global lock used to lock read access to the
runtime cache.
2. RuntimeCachePendingUpdate - A global flag used to notify runtime code
of a
pending cache update in SMM.

SMM (VariableSmm)
1. FlushRuntimeCachePendingUpdate SMI - A SW SMI handler that
the runtime cache buffer with the SMM
cache buffer.

Proposed Runtime DXE Read Flow
1. Acquire RuntimeCacheReadLock
2. If RuntimeCachePendingUpdate flag (rare) is set then:
2.a. Trigger FlushRuntimeCachePendingUpdate SMI
2.b. Verify RuntimeCachePendingUpdate flag is cleared 3. Perform
read from RuntimeCache 4. Release RuntimeCacheReadLock

Proposed FlushRuntimeCachePendingUpdate SMI
1. If RuntimeCachePendingUpdate flag is not set:
1.a. Return
2. Copy the data at RuntimeCachePendingOffset of
RuntimeCachePendingLength to
3. Clear the RuntimeCachePendingUpdate flag

Proposed SMM Write Flow
1. Perform variable authentication and non-volatile write. If either fail,
return an error to the caller.
2. If RuntimeCacheReadLock is set then:
2.a. Set RuntimeCachePendingUpdate flag
2.b. Update RuntimeCachePendingOffset and
RuntimeCachePendingLength to
cover the a superset of the pending chunk (for simplicity, the
entire variable store is currently synchronized).
3. Else:
3.a. Update RuntimeCache
4. Update SmmCache
- Note: RT read cannot occur during SMI processing since all cores are
locked in SMM.

Cc: Dandan Bi <>
Cc: Ard Biesheuvel <>
Cc: Eric Dong <>
Cc: Laszlo Ersek <>
Cc: Liming Gao <>
Cc: Michael D Kinney <>
Cc: Ray Ni <>
Cc: Jian J Wang <>
Cc: Hao A Wu <>
Cc: Jiewen Yao <>
Signed-off-by: Michael Kubacki <>

Michael Kubacki (10):
MdeModulePkg/Variable: Consolidate common parsing functions
MdeModulePkg/Variable: Parameterize GetNextVariableInternal () stores
MdeModulePkg/Variable: Parameterize VARIABLE_INFO_ENTRY buffer
MdeModulePkg/Variable: Parameterize auth status in VariableParsing
MdeModulePkg/Variable: Add a file for NV variable functions
MdeModulePkg VariableInfo: Always consider RT DXE and SMM stats
MdeModulePkg/Variable: Add RT GetVariable() cache support
MdeModulePkg/Variable: Add RT GetNextVariableName() cache support
OvmfPkg: Disable variable runtime cache
MdeModulePkg: Enable variable runtime cache by default

MdeModulePkg/MdeModulePkg.dec | 12 +
OvmfPkg/OvmfPkgIa32.dsc | 1 +
OvmfPkg/OvmfPkgIa32X64.dsc | 1 +
OvmfPkg/OvmfPkgX64.dsc | 1 +
| 6 +
MdeModulePkg/Universal/Variable/RuntimeDxe/VariableSmm.inf |
6 +
nf | 18 +-
| 6 +
MdeModulePkg/Include/Guid/SmmVariableCommon.h | 29
MdeModulePkg/Universal/Variable/RuntimeDxe/Variable.h | 151
| 67 +
MdeModulePkg/Universal/Variable/RuntimeDxe/VariableParsing.h |
347 +++++
| 51 +
MdeModulePkg/Application/VariableInfo/VariableInfo.c | 37 +-
MdeModulePkg/Universal/Variable/RuntimeDxe/Variable.c | 1373
MdeModulePkg/Universal/Variable/RuntimeDxe/VariableExLib.c |
24 +-
| 334 +++++
MdeModulePkg/Universal/Variable/RuntimeDxe/VariableParsing.c |
786 +++++++++++
| 153 +++
MdeModulePkg/Universal/Variable/RuntimeDxe/VariableSmm.c |
195 ++-
c | 655 +++++++++-
21 files changed, 2924 insertions(+), 1329 deletions(-) create mode
create mode 100644
create mode 100644
create mode 100644
create mode 100644
create mode 100644

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