Re: fwts: RuntimeServicesSupported variable

[Ard Biesheuvel <ardb@kernel.org>]

On Mon, 19 Oct 2020 at 12:00, Heinrich Schuchardt <xypron.glpk@gmx.de> wrote:
>
> On 19.10.20 11:31, Ard Biesheuvel wrote:
> > On Wed, 14 Oct 2020 at 20:41, Heinrich Schuchardt <xypron.glpk@gmx.de> wrote:
> >>
> >> On 14.10.20 19:58, Ard Biesheuvel wrote:
> >>> On Wed, 14 Oct 2020 at 19:45, Heinrich Schuchardt <xypron.glpk@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).
> >
>
> How about the return code?
>

As I attempted to explain, I think EFI_UNSUPPORTED should not be
reported as an error if RT_PROP_TABLE permits it. The caller has
access to the raw efi_status_t that was returned, so it can
distinguish between the two cases.