Re: [PATCH 3/3] efi_loader: fix efi_tcg2_hash_log_extend_event() parameter check

[Ilias Apalodimas <ilias.apalodimas@linaro.org>]

Hi Heinrich,

On Fri, Sep 03, 2021 at 08:22:30AM +0200, Heinrich Schuchardt wrote:
> On 9/3/21 3:55 AM, Masahisa Kojima wrote:
> > TCG EFI Protocol Specification defines that PCRIndex parameter
> > passed from caller must be 0 to 23.
> > TPM2_MAX_PCRS is currently used to check the range of PCRIndex,
> > but TPM2_MAX_PCRS is tpm2 device dependent and may have larger value.
> > This commit newly adds EFI_TCG2_MAX_PCR_INDEX macro, it is used to
> > check the range of PCRIndex parameter.
> 
> In the U-Boot code we have TPM2_MAX_PCRS hard coded as 32. Can the value
> be less?

This is defined in [1] 
[1] https://trustedcomputinggroup.org/wp-content/uploads/TSS_Overview_Common_v0p9_r10_12june2021.pdf

> 
> > 
> > Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
> > ---
> >   include/efi_tcg2.h        | 2 ++
> >   lib/efi_loader/efi_tcg2.c | 2 +-
> >   2 files changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/efi_tcg2.h b/include/efi_tcg2.h
> > index 45788d55d5..b647361d44 100644
> > --- a/include/efi_tcg2.h
> > +++ b/include/efi_tcg2.h
> > @@ -28,6 +28,8 @@
> >   #define EFI_TCG2_EXTEND_ONLY 0x0000000000000001
> >   #define PE_COFF_IMAGE 0x0000000000000010
> > 
> > +#define EFI_TCG2_MAX_PCR_INDEX 23
> > +
> >   /* Algorithm Registry */
> >   #define EFI_TCG2_BOOT_HASH_ALG_SHA1    0x00000001
> >   #define EFI_TCG2_BOOT_HASH_ALG_SHA256  0x00000002
> > diff --git a/lib/efi_loader/efi_tcg2.c b/lib/efi_loader/efi_tcg2.c
> > index c4e9f61fd6..b268a02976 100644
> > --- a/lib/efi_loader/efi_tcg2.c
> > +++ b/lib/efi_loader/efi_tcg2.c
> > @@ -958,7 +958,7 @@ efi_tcg2_hash_log_extend_event(struct efi_tcg2_protocol *this, u64 flags,
> >   		goto out;
> >   	}
> > 
> > -	if (efi_tcg_event->header.pcr_index > TPM2_MAX_PCRS) {
> 
> If TPM2_MAX_PCRS were device dependent and could be less then 23 we
> would need a check against both constants.
> 
> Could you, please, clarify the issue. If TPM2_MAX_PCRS is always greater
> then 23, please, mention this in the commit message and perhaps add a
> comment in the code here.

You don't need that (I think). If the spec says you have to check against
23, then that's what Kojima-san fixes here. 
If the device supports less than 23, then the current code as-is will
return EFI_DEVICE_ERROR, in case the device has less than 23 PCRs. 

We could ofc just check against the value we get from GetCapability, which
would be correct as well?

> 
> Best regards
> 
> Heinrich
> 
> > +	if (efi_tcg_event->header.pcr_index > EFI_TCG2_MAX_PCR_INDEX) {
> >   		ret = EFI_INVALID_PARAMETER;
> >   		goto out;
> >   	}
> > 

[1] https://trustedcomputinggroup.org/wp-content/uploads/TSS_Overview_Common_v0p9_r10_12june2021.pdf

Regards
/Ilias