Re: [PATCH v3 1/5] efi/libstub: Use correct event size when measuring data into the TPM

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On Fri, 8 Mar 2024 at 11:43, Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
>
> On Fri, 8 Mar 2024 at 10:38, Ilias Apalodimas
> <ilias.apalodimas@xxxxxxxxxx> wrote:
> >
> > Hi Ard
> >
> > On Fri, 8 Mar 2024 at 10:58, Ard Biesheuvel <ardb+git@xxxxxxxxxx> wrote:
> > >
> > > From: Ard Biesheuvel <ardb@xxxxxxxxxx>
> > >
> > > Our efi_tcg2_tagged_event is not defined in the EFI spec, but it is not
> > > a local invention either: it was taken from the TCG PC Client spec,
> > > where it is called TCG_PCClientTaggedEvent.
> > >
> > > This spec also contains some guidance on how to populate it, which
> > > is not being followed closely at the moment; the event size should cover
> > > the TCG_PCClientTaggedEvent and its payload only, but it currently
> > > covers the preceding efi_tcg2_event too, and this may result in trailing
> > > garbage being measured into the TPM.
> >
> > I think there's a confusion here and the current code we have is correct.
> > The EFI TCG spec [0] says that the tdEFI_TCG2_EVENT size is:
> > "Total size of the event including the Size component, the header and the
> > Event data." which obviously contradicts the definition of the tagged
> > event in the PC client spec.
> > But given the fact that TCG_PCClientTaggedEvent has its own size field
> > I think we should use what we already have.
> >
> >
> > [0] https://trustedcomputinggroup.org/wp-content/uploads/EFI-Protocol-Specification-rev13-160330final.pdf
> > page 33
> >
>
> Fair enough.
>
> It is rather disappointing that the TCG2 specs contradict each other,

Indeed

> but a quick inspection of the EDK2 implementation shows that it
> follows your interpretation.
>
> For example, in Tcg2HashLogExtendEvent()
> [SecurityPkg/Tcg/Tcg2Dxe/Tcg2Dxe.c], we have a check
>
> if (Event->Size < Event->Header.HeaderSize + sizeof (UINT32)) {
>   return EFI_INVALID_PARAMETER;
> }
>
> which ensures that the event size covers at least the EFI_TCG2_EVENT,
> which obviously implies that 'size' is expected to include it.
>

FWIW the same principle applies in the u-boot implementation as well.
As far as this series is concerned, keeping the
TCG_PCClientTaggedEvent will make reading the code & spec in parallel
easier, but I don't have any strong opinions -- I am fine with both

Cheers
/Ilias

> So please disregard this series




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