Re: [edk2-devel] [PATCH V1 1/1] MdeModulePkg/Variable: Fix VarErrorFlag RT cache offset calculation

["Kubacki, Michael A" <michael.a.kubacki@intel.com>]

Hi Jian,

I considered that but these are the reasons I settled on the approach in patch V1.

1. With the variable store filled, the length of mVariableModuleGlobal->NonVolatileLastVariableOffset will only marginally be a smaller value than mNvVariableCache->Size (since variable writes grow the store for SPI flash wear leveling). In this case, it will be ~CommonRuntimeVariableSpace which is usually a major portion of the variable store size anyway.
2. Since mVariableModuleGlobal->NonVolatileLastVariableOffset is a global moving value that is more frequently manipulated than the fixed variable store size, depending upon it increases the likelihood it will be set to an invalid value somewhere else.
3. This is a relatively rare case (an error condition) and the memory copy is within DRAM for variable stores that are typically ~128KB - ~512KB.

To reduce the copy size, the Offset parameter can be "(UINTN) VarErrFlag - (UINTN) mNvVariableCache" (just remove the unnecessary addition of (UINTN) mVariableModuleGlobal->VariableGlobal.NonVolatileVariableBase) with size "sizeof (TempFlag)". How about this in a V2?

Thanks,
Michael

> -----Original Message-----
> From: Wang, Jian J <jian.j.wang@intel.com>
> Sent: Monday, January 13, 2020 10:43 PM
> To: devel@edk2.groups.io; Kubacki, Michael A
> <michael.a.kubacki@intel.com>
> Cc: Gao, Liming <liming.gao@intel.com>; Kinney, Michael D
> <michael.d.kinney@intel.com>; Michael Turner
> <michael.turner@microsoft.com>; Wu, Hao A <hao.a.wu@intel.com>
> Subject: RE: [edk2-devel] [PATCH V1 1/1] MdeModulePkg/Variable: Fix
> VarErrorFlag RT cache offset calculation
> 
> Michael,
> 
> I'm not sure sync-ing whole variable cache memory is an efficient operation.
> What about using mVariableModuleGlobal->NonVolatileLastVariableOffset
> as Length parameter?
> 
>        Status =  SynchronizeRuntimeVariableCache (
>                    &mVariableModuleGlobal-
> >VariableGlobal.VariableRuntimeCacheContext.VariableRuntimeNvCache,
>                    0,
>                    mVariableModuleGlobal->NonVolatileLastVariableOffset
>                    );
> 
> Regards,
> Jian
> 
> > -----Original Message-----
> > From: devel@edk2.groups.io <devel@edk2.groups.io> On Behalf Of
> > Kubacki, Michael A
> > Sent: Tuesday, January 14, 2020 7:19 AM
> > To: devel@edk2.groups.io
> > Cc: Gao, Liming <liming.gao@intel.com>; Kinney, Michael D
> > <michael.d.kinney@intel.com>; Michael Turner
> > <michael.turner@microsoft.com>; Wang, Jian J <jian.j.wang@intel.com>;
> > Wu, Hao A <hao.a.wu@intel.com>
> > Subject: [edk2-devel] [PATCH V1 1/1] MdeModulePkg/Variable: Fix
> > VarErrorFlag RT cache offset calculation
> >
> > REF:https://bugzilla.tianocore.org/show_bug.cgi?id=2457
> >
> > This commit fixes an offset calculation that is used to write the
> > VarErrorFlag UEFI variable to the UEFI variable runtime cache.
> >
> > Currently a physical address is used instead of an offset. This commit
> > changes the offset to zero with a length of the entire non-volatile
> > variable store so the entire non-volatile variable store buffer in
> > SMRAM (with the variable update modification) is copied to the runtime
> > variable cache. This follows the same pattern used in other
> > SynchronizeRuntimeVariableCache () calls for consistency.
> >
> > * Observable symptom: An exception in SMM will most likely occur
> >   due to the invalid memory reference when the VarErrorFlag variable
> >   is written. The variable is most commonly written when the UEFI
> >   variable store is full.
> >
> > * The issue only occurs when the variable runtime cache is enabled
> >   by the following PCD being set to TRUE:
> >   gEfiMdeModulePkgTokenSpaceGuid.PcdEnableVariableRuntimeCache
> >
> > Fixes: aab3b9b9a1e5e1f3fa966fb1667fc3e6c47e7706
> >
> > Cc: Liming Gao <liming.gao@intel.com>
> > Cc: Michael D Kinney <michael.d.kinney@intel.com>
> > Cc: Michael Turner <michael.turner@microsoft.com>
> > Cc: Jian J Wang <jian.j.wang@intel.com>
> > Cc: Hao A Wu <hao.a.wu@intel.com>
> > Signed-off-by: Michael Kubacki <michael.a.kubacki@intel.com>
> > ---
> >  MdeModulePkg/Universal/Variable/RuntimeDxe/Variable.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/MdeModulePkg/Universal/Variable/RuntimeDxe/Variable.c
> > b/MdeModulePkg/Universal/Variable/RuntimeDxe/Variable.c
> > index b0ee5e50d0..d23aea4bc7 100644
> > --- a/MdeModulePkg/Universal/Variable/RuntimeDxe/Variable.c
> > +++ b/MdeModulePkg/Universal/Variable/RuntimeDxe/Variable.c
> > @@ -16,7 +16,7 @@
> >    VariableServiceSetVariable() should also check authenticate data to
> > avoid buffer overflow,
> >    integer overflow. It should also check attribute to avoid authentication
> bypass.
> >
> > -Copyright (c) 2006 - 2019, Intel Corporation. All rights
> > reserved.<BR>
> > +Copyright (c) 2006 - 2020, Intel Corporation. All rights
> > +reserved.<BR>
> >  (C) Copyright 2015-2018 Hewlett Packard Enterprise Development LP<BR>
> >  SPDX-License-Identifier: BSD-2-Clause-Patent
> >
> > @@ -335,8 +335,8 @@ RecordVarErrorFlag (
> >        *VarErrFlag = TempFlag;
> >        Status =  SynchronizeRuntimeVariableCache (
> >                    &mVariableModuleGlobal-
> > >VariableGlobal.VariableRuntimeCacheContext.VariableRuntimeNvCache,
> > -                  (UINTN) VarErrFlag - (UINTN) mNvVariableCache + (UINTN)
> > mVariableModuleGlobal->VariableGlobal.NonVolatileVariableBase,
> > -                  sizeof (TempFlag)
> > +                  0,
> > +                  mNvVariableCache->Size
> >                    );
> >        ASSERT_EFI_ERROR (Status);
> >      }
> > --
> > 2.16.2.windows.1
> >
> >
> > 
>