[v2] UEFI Variable SMI Reduction

[edk2-devel] [PATCH V2 0/9] UEFI Variable SMI Reduction
Posted by Kubacki, Michael A 4 years, 6 months ago
REF:https://bugzilla.tianocore.org/show_bug.cgi?id=2220

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.

Overview
---------
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
environment.

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

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: https://edk2.groups.io/g/devel/message/46939

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. This 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
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.
2. Boot from S5 to Ubuntu 18.04.1 LTS with SMM variable enabled.
3. Boot from S5 to EFI shell with DXE variables enabled.
4. Dump UEFI variable store at shell with dmpstore to verify contents.
5. Dump NvStorage FV from SPI flash after boot to verify contents written.
6. Dump UEFI variable statistics with VariableInfo at shell.
7. Boot with emulated variables enabled.
8. Cycles of adding and deleting a UEFI variable to verify cache results.
9. Set OsIndications to stop at FW UI to verify cache load of non-volatile
   contents.

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
   store.
 * 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. Wait for RuntimeCacheReadLock to be free
 2. Acquire RuntimeCacheReadLock
 3. If RuntimeCachePendingUpdate flag (rare) is set then:
     3.a. Trigger FlushRuntimeCachePendingUpdate SMI
     3.b. Verify RuntimeCachePendingUpdate flag is cleared
 4. Perform read from RuntimeCache
 5. Release RuntimeCacheReadLock

Proposed FlushRuntimeCachePendingUpdate SMI
--------------------------------------------
 1. If RuntimeCachePendingUpdate flag is not set:
     1.a. Return
 2. Copy the data at RuntimeCachePendingOffset of RuntimeCachePendingLength to
    RuntimeCache
 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 <dandan.bi@intel.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao A Wu <hao.a.wu@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Signed-off-by: Michael Kubacki <michael.a.kubacki@intel.com>

Michael Kubacki (9):
  MdeModulePkg/Variable: Consolidate common parsing functions
  MdeModulePkg/Variable: Parameterize GetNextVariableEx() store list
  MdeModulePkg/Variable: Parameterize VARIABLE_INFO_ENTRY buffer
  MdeModulePkg/Variable: Add local 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
  MdeModulePkg/VariableSmm: Remove unused SMI handler functions

 MdeModulePkg/Universal/Variable/RuntimeDxe/VariableRuntimeDxe.inf    |   6 +
 MdeModulePkg/Universal/Variable/RuntimeDxe/VariableSmm.inf           |   6 +
 MdeModulePkg/Universal/Variable/RuntimeDxe/VariableSmmRuntimeDxe.inf |  31 +-
 MdeModulePkg/Universal/Variable/RuntimeDxe/VariableStandaloneMm.inf  |  11 +
 MdeModulePkg/Include/Guid/SmmVariableCommon.h                        |  33 +-
 MdeModulePkg/Universal/Variable/RuntimeDxe/Variable.h                | 158 +---
 MdeModulePkg/Universal/Variable/RuntimeDxe/VariableNonVolatile.h     |  25 +
 MdeModulePkg/Universal/Variable/RuntimeDxe/VariableParsing.h         | 325 ++++++++
 MdeModulePkg/Universal/Variable/RuntimeDxe/VariableRuntimeCache.h    |  47 ++
 MdeModulePkg/Application/VariableInfo/VariableInfo.c                 |  37 +-
 MdeModulePkg/Universal/Variable/RuntimeDxe/Variable.c                | 826 ++------------------
 MdeModulePkg/Universal/Variable/RuntimeDxe/VariableExLib.c           |  11 +-
 MdeModulePkg/Universal/Variable/RuntimeDxe/VariableNonVolatile.c     |  28 +
 MdeModulePkg/Universal/Variable/RuntimeDxe/VariableParsing.c         | 769 ++++++++++++++++++
 MdeModulePkg/Universal/Variable/RuntimeDxe/VariableRuntimeCache.c    | 153 ++++
 MdeModulePkg/Universal/Variable/RuntimeDxe/VariableSmm.c             | 212 +++--
 MdeModulePkg/Universal/Variable/RuntimeDxe/VariableSmmRuntimeDxe.c   | 726 +++++++++++++----
 17 files changed, 2231 insertions(+), 1173 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

-- 
2.16.2.windows.1


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