fwts: RuntimeServicesSupported variable

[Ard Biesheuvel]

On Wed, 14 Oct 2020 at 20:41, Heinrich Schuchardt <xypron.glpk at gmx.de> wrote:
>
> On 14.10.20 19:58, Ard Biesheuvel wrote:
> > On Wed, 14 Oct 2020 at 19:45, Heinrich Schuchardt <xypron.glpk at gmx.de> wrote:
> >>
> >> On 14.10.20 19:31, Heinrich Schuchardt wrote:
> >>> Dear all,
> >>>
> >>> the fwts fails on U-Boot due to testing for a non-existent
> >>> RuntimeServicesSupported variable.
> >>>
> >>> If you look at the UEFI specification 2.8 (Errata B) [1] you will
> >>> discover in the change log:
> >>>
> >>> 2.8 A2049
> >>> RuntimeServicesSupported EFI variable should be a config table
> >>> February 2020
> >>>
> >>> Please, read the configuration table to determine if a runtime service
> >>> is available on UEFI 2.8 systems.
> >>>
> >>> On lower UEFI firmware version neither the variable nor the table exists.
> >>>
> >>> Best regards
> >>>
> >>> Heinrich
> >>>
> >>> [1] UEFI Specification Version 2.8 (Errata B) (released June 2020),
> >>> https://uefi.org/sites/default/files/resources/UEFI%20Spec%202.8B%20May%202020.pdf
> >>>
> >>
> >> Hello Ard,
> >>
> >> what is your idea how the EFI_RT_PROPERTIES_TABLE shall be exposed to
> >> the efi_test driver?
> >>
> >> Will the EFI runtime wrapper simply return EFI_UNSUPPORTED if the
> >> function is not marked as supported in the table? Or will the
> >> configuration table itself be make available?
> >>
> >
> > The UEFI spec permits that runtime services return EFI_UNSUPPORTED at
> > runtime, but requires that they are marked as such in the
> > EFI_RT_PROPERTIES_TABLE.
> >
> > So assuming that the purpose of efi_test is compliance with the spec,
> > it should only allow EFI_UNSUPPORTED as a return value for each of the
> > tested runtime services if it is omitted from
> > efi.runtime_supported_mask.
> >
> > Since the efi_test ioctl returns both an error code and the actual EFI
> > status code, we should only fail the call on a EFI_UNSUPPORTED status
> > code if the RTPROP mask does not allow that.
> >
> > E.g.,
> >
> > --- a/drivers/firmware/efi/test/efi_test.c
> > +++ b/drivers/firmware/efi/test/efi_test.c
> > @@ -265,7 +265,12 @@ static long efi_runtime_set_variable(unsigned long arg)
> >                 goto out;
> >         }
> >
> > -       rv = status == EFI_SUCCESS ? 0 : -EINVAL;
> > +       if (status == EFI_SUCCESS ||
> > +           (status == EFI_UNSUPPORTED &&
> > +            !efi_rt_services_supported(EFI_RT_SUPPORTED_SET_VARIABLE)))
> > +               rv = 0;
> > +       else
> > +               rv = -EINVAL;
> >
> >  out:
> >         kfree(data);
> >
> >
> > Do you think that could work?
> >
>
> The current fwts implementation assumes that EFI_UNSUPPORTED leads to
> ioctl() returning -1. This value should not be changed. It would be
> preferable to use another error code than -EINVAL, e.g. -EDOM if there
> is a mismatch with the EFI_RT_PROPERTIES_TABLE configuration table. Then
> a future verision of fwts can evaluate errno to discover the problem.
>
> Do I read you correctly: the EFI runtime wrapper does not fend of calls
> to runtime services marked as disallowed in EFI_RT_PROPERTIES_TABLE?
> Directly returning an error code might help to avoid crashes on
> non-compliant firmware.
>

It is not the kernel's job to work around non-compliant firmware. The
EFI spec is crystal clear that every runtime service needs to be
implemented, but is permitted to return EFI_UNSUPPORTED after
ExitBootServices(). This means EFI_RT_PROPERTIES_TABLE does not tell
you calling certain runtime services is disallowed, it tells you that
there is no point in even trying. That is why users such as efi-pstore
now take this information into account in their probe path (and
efivarfs will only mount read/write if SetVariable() is not marked as
unsupported).