[edk2-devel] [PATCH 3/3] OvmfPkg/PlatformPei: Mark TPM MMIO range as unencrypted for SEV

Thu Apr 22 08:31:14 UTC 2021

On 04/22/21 09:34, Laszlo Ersek wrote:

> Anyway: I think the TPM (MMIO) access you see comes from this PEIM:
>
>   OvmfPkg/Tcg/Tcg2Config/Tcg2ConfigPei.inf
>
> The driver uses the following library instance:
>
>   SecurityPkg/Library/Tpm2DeviceLibDTpm/Tpm2DeviceLibDTpm.inf
>
> This library instance is what depends on "PcdTpmBaseAddress".
>
> And it's not just that decrypting the TPM MMIO range in PlatformPei
> "looks awkward", but I don't even see it immediately why PlatformPei
> is guaranteed to be dispatched before Tcg2ConfigPei. The effective
> depex of Tcg2ConfigPei is just "gEfiPeiPcdPpiGuid" (on X64), according
> to the build report file. If Tcg2ConfigPei runs first, whatever we do
> in PlatformPei is too late.
>
> I also don't like that, with this patch, we'd decrypt the TPM range
> even if OVMF weren't built with "-D TPM_ENABLE". Namely, OVMF uses
> "PcdTpmBaseAddress" as fixed (not dynamic), inheriting the nonzero
> default from "SecurityPkg.dec". (In ArmVirtQemu, PcdTpmBaseAddress is
> set dynamically, which is why Tcg2ConfigPei has an ARM-specific depex
> too.)
>
>
> (2) So, can you please try the following, in the
> "OvmfPkg/Tcg/Tcg2Config/Tcg2ConfigPei.inf" module:
>
>> diff --git a/OvmfPkg/Tcg/Tcg2Config/Tcg2ConfigPei.inf b/OvmfPkg/Tcg/Tcg2Config/Tcg2ConfigPei.inf
>> index 6776ec931ce0..0d0572b83599 100644
>> --- a/OvmfPkg/Tcg/Tcg2Config/Tcg2ConfigPei.inf
>> +++ b/OvmfPkg/Tcg/Tcg2Config/Tcg2ConfigPei.inf
>> @@ -20,13 +20,16 @@ [Defines]
>>    ENTRY_POINT                    = Tcg2ConfigPeimEntryPoint
>>
>>  [Sources]
>> +  MemEncrypt.h
>>    Tcg2ConfigPeim.c
>>    Tpm12Support.h
>>
>>  [Sources.IA32, Sources.X64]
>> +  MemEncryptSev.c
>>    Tpm12Support.c
>>
>>  [Sources.ARM, Sources.AARCH64]
>> +  MemEncryptNull.c
>>    Tpm12SupportNull.c
>>
>>  [Packages]
>> @@ -43,6 +46,7 @@ [LibraryClasses]
>>
>>  [LibraryClasses.IA32, LibraryClasses.X64]
>>    BaseLib
>> +  MemEncryptSevLib
>>    Tpm12DeviceLib
>>
>>  [Guids]
>> @@ -56,6 +60,9 @@ [Ppis]
>>  [Pcd]
>>    gEfiSecurityPkgTokenSpaceGuid.PcdTpmInstanceGuid                 ## PRODUCES
>>
>> +[Pcd.IA32, Pcd.X64]
>> +  gEfiSecurityPkgTokenSpaceGuid.PcdTpmBaseAddress         ## SOMETIMES_CONSUMES
>> +
>>  [Depex.IA32, Depex.X64]
>>    TRUE
>>
>
> In the "MemEncrypt.h" file, declare a function called
> InternalTpmDecryptAddressRange(). The function definition in
> "MemEncryptNull.c" should do nothing, while the one in
> "MemEncryptSev.c" should check MemEncryptSevIsEnabled(), and then make
> the above-seen MemEncryptSevClearPageEncMask() call.
>
> The new InternalTpmDecryptAddressRange() function should be called
> from Tcg2ConfigPeimEntryPoint(), before the latter calls
> InternalTpm12Detect(). Regarding error checking... if
> InternalTpmDecryptAddressRange() fails, I think we can log an error
> message, and hang with CpuDeadLoop().
>
> (An alternative approach would be to call MemEncryptSevIsEnabled() and
> MemEncryptSevClearPageEncMask() regardless of architecture, i.e., also
> on ARM / AARCH64. In addition to that, we'd have to implement a Null
> instance of MemEncryptSevLib, and resolve MemEncryptSevLib to the Null
> instance in the ArmVirtPkg DSC files. But I don't like that: the
> library *class* carries SEV in the name, which is inherently
> X64-specific, thus I wouldn't even like the lib *class* to leak into
> ArmVirtPkg.)

Here's another thing. Above, I mention that nothing guarantees that
Tcg2ConfigPei runs before PlatformPei. That raises a problem even if we
use approach (2).

In approach (2), we massage page table entries, and ultimately use

  OvmfPkg/Library/BaseMemEncryptSevLib/PeiMemEncryptSevLib.inf

for that. But that library instance can allocate full pages, in case
page table splitting is needed (from 1GB to 2MB to 4KB).

I can't tell off-hand if such page table splitting will actually occur
when we decrypt the TPM MMIO address range -- but even if it does not,
for whatever reason, I wouldn't like to rely on that particular reason.
And I definitely don't want such page allocations to be satisfied from
the temporary SEC/PEI heap, before we migrate to permanent PEI RAM. See
how "PcdOvmfSecPeiTempRamBase" and "PcdOvmfSecPeiTempRamSize" are set in
the FDF files, and see the OVMF log too:

> Temp Stack : BaseAddress=0x818000 Length=0x8000
> Temp Heap  : BaseAddress=0x810000 Length=0x8000
> Total temporary memory:    65536 bytes.
>   temporary memory stack ever used:       30128 bytes.
>   temporary memory heap used for HobList: 7208 bytes.
>   temporary memory heap occupied by memory pages: 0 bytes.

What I'm saying is that we've probably been missing the following hunk
for a long time now:

> diff --git a/OvmfPkg/Library/BaseMemEncryptSevLib/PeiMemEncryptSevLib.inf b/OvmfPkg/Library/BaseMemEncryptSevLib/PeiMemEncryptSevLib.inf
> index 03a78c32df28..1b3808305415 100644
> --- a/OvmfPkg/Library/BaseMemEncryptSevLib/PeiMemEncryptSevLib.inf
> +++ b/OvmfPkg/Library/BaseMemEncryptSevLib/PeiMemEncryptSevLib.inf
> @@ -55,3 +55,6 @@ [FeaturePcd]
>
>  [FixedPcd]
>    gUefiCpuPkgTokenSpaceGuid.PcdSevEsWorkAreaBase
> +
> +[Depex]
> +  gEfiPeiMemoryDiscoveredPpiGuid

In other words, whatever PEIM uses the PeiMemEncryptSevLib instance,
should be delayed until PlatformPei installs the permanent PEI RAM, by
inheriting a gEfiPeiMemoryDiscoveredPpiGuid depex from
PeiMemEncryptSevLib.

... Unfortunately, this wouldn't work, because PlatformPei itself uses
PeiMemEncryptSevLib [*], so we'd create a circular dependency.

[*] first from commit 13b5d743c87a ("OvmfPkg/PlatformPei: Set memory
    encryption PCD when SEV is enabled", 2017-07-10), which called
    MemEncryptSevIsEnabled(),

    and then from commit 449a6e493418 ("OvmfPkg: Create GHCB pages for
    use during Pei and Dxe phase", 2020-08-17), which even called
    MemEncryptSevClearPageEncMask() -- but note that AmdSevInitialize()
    is called *after* PublishPeiMemory(), in PlatformPei!

So, we can't add this "permanent PEI RAM" dependency to
"PeiMemEncryptSevLib.inf" directly. Instead, as a work-around, we should
add the dependency to "Tcg2ConfigPei".

(5a) So ultimately, please update the "Tcg2ConfigPei.inf" file like
this:

> diff --git a/OvmfPkg/Tcg/Tcg2Config/Tcg2ConfigPei.inf b/OvmfPkg/Tcg/Tcg2Config/Tcg2ConfigPei.inf
> index 6776ec931ce0..6605b9bbaf91 100644
> --- a/OvmfPkg/Tcg/Tcg2Config/Tcg2ConfigPei.inf
> +++ b/OvmfPkg/Tcg/Tcg2Config/Tcg2ConfigPei.inf
> @@ -20,13 +20,16 @@ [Defines]
>    ENTRY_POINT                    = Tcg2ConfigPeimEntryPoint
>
>  [Sources]
> +  MemEncrypt.h
>    Tcg2ConfigPeim.c
>    Tpm12Support.h
>
>  [Sources.IA32, Sources.X64]
> +  MemEncryptSev.c
>    Tpm12Support.c
>
>  [Sources.ARM, Sources.AARCH64]
> +  MemEncryptNull.c
>    Tpm12SupportNull.c
>
>  [Packages]
> @@ -43,6 +46,7 @@ [LibraryClasses]
>
>  [LibraryClasses.IA32, LibraryClasses.X64]
>    BaseLib
> +  MemEncryptSevLib
>    Tpm12DeviceLib
>
>  [Guids]
> @@ -56,8 +60,11 @@ [Ppis]
>  [Pcd]
>    gEfiSecurityPkgTokenSpaceGuid.PcdTpmInstanceGuid                 ## PRODUCES
>
> +[Pcd.IA32, Pcd.X64]
> +  gEfiSecurityPkgTokenSpaceGuid.PcdTpmBaseAddress         ## SOMETIMES_CONSUMES
> +
>  [Depex.IA32, Depex.X64]
> -  TRUE
> +  gEfiPeiMemoryDiscoveredPpiGuid
>
>  [Depex.ARM, Depex.AARCH64]
>    gOvmfTpmDiscoveredPpiGuid

(5b) And in the commit message, please state that:

    We don't want PeiMemEncryptSevLib to allocate any pages, for
    potential page table splitting, from the temporary SEC/PEI heap. But
    we can't make PeiMemEncryptSevLib itself depend on
    gEfiPeiMemoryDiscoveredPpiGuid, because PlatformPei, which installs
    the permanent PEI RAM, consumes PeiMemEncryptSevLib too. Thus,
    restrict the DEPEX of Tcg2ConfigPei directly.

--*--

(

Note that, in OVMF, the other PEIM that (indirectly) uses
PeiMemEncryptSevLib is "UefiCpuPkg/CpuMpPei/CpuMpPei.inf". But, the
effective initialization of that PEIM is already delayed until after the
permanent PEI RAM is installed -- not with a depex, but with a notify
callback.

Also note that the library instance

  OvmfPkg/Library/BaseMemEncryptSevLib/SecMemEncryptSevLib.inf

doesn't support manipulating the page tables at all, and so it doesn't
need to allocate memory for page table splitting either. So that's good.

)

Thanks
Laszlo

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