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

Laszlo Ersek

Hello Michael,

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

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

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

The remaining changes did not affect testing but are included for robustness:
* Pass EFI_OPTIONAL_PTR for the DebugDisposition type in the EfiConvertPointer ()
calls for mVariableRuntimeHobCacheBuffer, mVariableRuntimeNvCacheBuffer, and
mVariableRuntimeVolatileCacheBuffer in VariableAddressChangeEvent () in
VariableSmmRuntimeDxe.c as these buffers will not be allocated if the runtime
cache is disabled.
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 flushed)
before writing to VariableRuntimeHobCache.Store.

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

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

* Added a new patch to set gEfiMdeModulePkgTokenSpaceGuid.PcdEnableVariableRuntimeCache
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 functions
* Removed GUIDs added to VariableStandaloneMm.inf that are not required.
* 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 VARIABLE_INFO_ENTRY buffer
* Updated the commit message to clarify the message "structure can be updated
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 parameter
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 in
each variable driver in the SMM variable solution (VariableSmmRuntimeDxe
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 VariableNonVolatile.c
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 usage.
* 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 EDKII_PI_SMM_COMMUNICATION_REGION_TABLE to acquire a
CommBuffer from the EFI System Table and use the same runtime CommBuffer
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 restrictions
on Runtime Services callers.

* Added a new patch to disable gEfiMdeModulePkgTokenSpaceGuid.PcdEnableVariableRuntimeCache
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
3. 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 VariableSmmRuntimeDxe 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

* 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 MdeModulePkg.dec:

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 serve
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 data
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 security
threat presented by serving runtime services callers from a ring 0 memory
buffer of EfiRuntimeServicesData type. The conclusion was that this change
does not fundamentally alter the attack surface. The UEFI variable Runtime
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 one
AP in a loop to monitor the communication data, when the BSP gets an
(authenticated) variable. When the communication buffer is updated and the
status is set to EFI_SUCCESS, the AP may modify the communication buffer
contents such the tampered data is returned to the BSP caller. Or an
interrupt handler on the BSP may alter the communication buffer contents
before the data is returned to the caller. In summary, this was not found to
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 correctness.
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 VariableSmm.

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 synchronizes
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 +
MdeModulePkg/Universal/Variable/RuntimeDxe/VariableRuntimeDxe.inf | 6 +
MdeModulePkg/Universal/Variable/RuntimeDxe/VariableSmm.inf | 6 +
MdeModulePkg/Universal/Variable/RuntimeDxe/VariableSmmRuntimeDxe.inf | 18 +-
MdeModulePkg/Universal/Variable/RuntimeDxe/VariableStandaloneMm.inf | 6 +
MdeModulePkg/Include/Guid/SmmVariableCommon.h | 29 +-
MdeModulePkg/Universal/Variable/RuntimeDxe/Variable.h | 151 +--
MdeModulePkg/Universal/Variable/RuntimeDxe/VariableNonVolatile.h | 67 +
MdeModulePkg/Universal/Variable/RuntimeDxe/VariableParsing.h | 347 +++++
MdeModulePkg/Universal/Variable/RuntimeDxe/VariableRuntimeCache.h | 51 +
MdeModulePkg/Application/VariableInfo/VariableInfo.c | 37 +-
MdeModulePkg/Universal/Variable/RuntimeDxe/Variable.c | 1373 ++++----------------
MdeModulePkg/Universal/Variable/RuntimeDxe/VariableExLib.c | 24 +-
MdeModulePkg/Universal/Variable/RuntimeDxe/VariableNonVolatile.c | 334 +++++
MdeModulePkg/Universal/Variable/RuntimeDxe/VariableParsing.c | 786 +++++++++++
MdeModulePkg/Universal/Variable/RuntimeDxe/VariableRuntimeCache.c | 153 +++
MdeModulePkg/Universal/Variable/RuntimeDxe/VariableSmm.c | 195 ++-
MdeModulePkg/Universal/Variable/RuntimeDxe/VariableSmmRuntimeDxe.c | 655 +++++++++-
21 files changed, 2924 insertions(+), 1329 deletions(-)
create mode 100644 MdeModulePkg/Universal/Variable/RuntimeDxe/VariableNonVolatile.h
create mode 100644 MdeModulePkg/Universal/Variable/RuntimeDxe/VariableParsing.h
create mode 100644 MdeModulePkg/Universal/Variable/RuntimeDxe/VariableRuntimeCache.h
create mode 100644 MdeModulePkg/Universal/Variable/RuntimeDxe/VariableNonVolatile.c
create mode 100644 MdeModulePkg/Universal/Variable/RuntimeDxe/VariableParsing.c
create mode 100644 MdeModulePkg/Universal/Variable/RuntimeDxe/VariableRuntimeCache.c

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