[RFC] efi_loader: improve firmware capsule authentication

[Sughosh Ganu]

Takahiro,

On Fri, 23 Apr 2021 at 12:30, AKASHI Takahiro <takahiro.akashi at linaro.org>
wrote:

> Sughosh,
>
> On Fri, Apr 23, 2021 at 11:55:04AM +0530, Sughosh Ganu wrote:
> > Takahiro,
> >
> > On Fri, 23 Apr 2021 at 11:17, AKASHI Takahiro <
> takahiro.akashi at linaro.org>
> > wrote:
> >
> > > Heinrich,
> > >
> > > I'm currently thinking of improving capsule authentication
> > > that Sughosh has made, particularly around mkeficapsule command:
> > >
> > > 1) Add a signing feature to the command
> > >    This will allow us to create a *signed* capsule file solely
> > >    with mkeficapsule. We will no longer rely on EDK2's script.
> > > 2) Delete "-K" and "-D" option
> > >    Specifically, revert 322c813f4bec ("mkeficapsule: Add support
> > >    for embedding public key in a dtb")
> > >    As I said, this feature doesn't have anything to do with
> > >    creating a capsule file. Above all, we can do the same thing
> > >    with the existing commands (dtc and fdtoverlay).
> > >
> >
> > I would vote against this particular revert that you are suggesting. I
> have
> > already submitted a patchset which is under review[1], which is adding
> > support for embedding the key in the platform's dtb, using the above
> > functionality in mkeficapsule.
>
> That is why I insisted "(2) should be done in 2021.04"
> as we should stop it being merged immediately.
>
> > I don't see any reason why we should be
> > adding this logic in another utility,
>
> ?
> I never tried to add anything about this issue. Just remove.
> FYI, we can get the exact same result with:
> === pubkey.dts ===
> /dts-v1/;
> /plugin/;
>
> &{/} {
>         signature {
>                 capsule-key = /incbin/("CRT.esl");
>         };
> };
> ===
> $ dtc -@ -I dts -O dtb -o pubkey.dtbo pubkey.dts
> $ fdtoverlay -i test.dtb -o test_pubkey.dtb -v pubkey.dtbo
>
> No "C" code needed here. You also re-invented the almost same function
> as fdt_overlay_apply() in mkeficapsule, and yet your function is
> incompatible with dtc/fdtoverlay commands in terms of overlay syntax.
>
> I have already confirmed the capsule file signed by my mkeficapsule
> + above dtb work perfectly with efi_capsule_authenticate()
> in my pytest with sandbox.
>
> And again, the feature has nothing to do with generating a capsule file.
> It is simply to perform fdt overlay which is already supported by standard
> commands.
>
> Those are the reasons why we should revert the patch.
>

I am sure that the method you have shown above would work for embedding the
key into the dtb. But having the logic in mkeficapsule also does not hurt.
I would say that a patch should be reverted in the scenario that it causes
some regression and there is no easy or obvious fix available. This is
adding some logic to a host tool, and not breaking any existing
functionality. Also, this code being part of a host tool, there is no case
of it causing any increase to the u-boot size. If you think that there are
some bugs, or certain things can be improved in the code, I am open to
making changes and fixing stuff. But I am still of the opinion that a
revert in a host tool, and that too when it is not breaking any stuff is
not needed.


> > and cannot use the mkeficapsule
> > utility for embedding the public key in the platform's dtb.
>
> ?
> No need to use mkeficapsule any more.
>

? When did I say that. I said that there is no reason why mkeficapsule
utility cannot be used for embedding the public key in the platform's dtb.

-sughosh


>
> -Takahiro Akashi
>
> > The
> > mkeficapsule utility can be extended to add the authentication
> information
> > that you plan to submit.
> >
> > -sughosh
> >
> > [1] - https://lists.denx.de/pipermail/u-boot/2021-April/447183.html
> >
> >
> > > 3) Add pytest for capsule authentication with sandbox
> > >
> > > Now I have done all of them above although some cleanup work is
> > > still needed. I think that (2) should be done in 2021.04.
> > >
> > > I plan to send patches for 1-3 (and maybe 5 and 7 below) if you agree.
> > >
> > > Other concerns:
> > > 4) Documentation
> > >    Currently, "doc/board/emulation/qemu_capsule_update.rst" is
> > >    the only document about the usage of UEFI capsule on U-Boot.
> > >    Unfortunately, it contains some errors and more importantly,
> > >    most of the content are generic, not qemu-specific.
> > >
> > > 5) Certificate (public key) in dtb
> > >    That's fine, but again "board/emulation/common/qemu_capsule.c"
> > >    is naturally generic. It should be available for other platforms
> > >    with a new Kconfig option.
> > >
> > >    # IMHO, I don't understand why the data in dtb needs be in
> > >    efi-signature-list structure. A single key (cert) would be enough.
> > >
> > > 6) "capsule_authentication_enabled"
> > >    I think that we have agreed with deleting this variable.
> > >    But I don't see any patch.
> > >    Moreover, capsule authentication must be enforced only
> > >    if the attribute, IMAGE_ATTRIBUTE_AUTHENTICATION_REQUIRED,
> > >    is set. But there is no code to check the flag.
> > >
> > > 7) Pytest is broken
> > >    Due to your and Ilias' recent patches, existing pytests for
> > >    secure boot as well as capsule update are now broken.
> > >    I'm not sure why you have not yet noticed.
> > >    Presumably, Travis CI now skips those tests?
> > >
> > > If I have some time in the future, I will address them.
> > > But Sughosh can do as well.
> > >
> > > -Takahiro Akashi
> > >
>