[PATCH v4 14/14] qemu: arm64: Add documentation for capsule update

[Sughosh Ganu <sughosh.ganu@linaro.org>]

On Tue, 2 Mar 2021 at 21:27, Heinrich Schuchardt <xypron.glpk@gmx.de> wrote:

> On 02.03.21 15:48, Sughosh Ganu wrote:
> > hi Heinrich,
> >
> > On Tue, 2 Mar 2021 at 16:45, Heinrich Schuchardt <xypron.glpk@gmx.de
> > <mailto:xypron.glpk@gmx.de>> wrote:
> >
> >     On 30.12.20 14:57, Sughosh Ganu wrote:
> >     > Add documentation highlighting the steps for using the uefi capsule
> >     > update feature for updating the u-boot firmware image.
> >     >
> >     > Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org
> >     <mailto:sughosh.ganu@linaro.org>>
> >     > ---
> >     >
> >     > Changes since V3: None
> >     >
> >     >  doc/board/emulation/index.rst               |   1 +
> >     >  doc/board/emulation/qemu_capsule_update.rst | 210
> >     ++++++++++++++++++++
> >     >  2 files changed, 211 insertions(+)
> >     >  create mode 100644 doc/board/emulation/qemu_capsule_update.rst
> >     >
> >     > diff --git a/doc/board/emulation/index.rst
> >     b/doc/board/emulation/index.rst
> >     > index 1adefee155..a09ead1c35 100644
> >     > --- a/doc/board/emulation/index.rst
> >     > +++ b/doc/board/emulation/index.rst
> >     > @@ -10,3 +10,4 @@ Emulation
> >     >     qemu-mips
> >     >     qemu-riscv
> >     >     qemu-x86
> >     > +   qemu_capsule_update
> >     > diff --git a/doc/board/emulation/qemu_capsule_update.rst
> >     b/doc/board/emulation/qemu_capsule_update.rst
> >     > new file mode 100644
> >     > index 0000000000..9fec75f8f1
> >     > --- /dev/null
> >     > +++ b/doc/board/emulation/qemu_capsule_update.rst
> >     > @@ -0,0 +1,210 @@
> >     > +.. SPDX-License-Identifier: GPL-2.0+
> >     > +.. Copyright (C) 2020, Linaro Limited
> >     > +
> >     > +Enabling UEFI Capsule Update feature
> >     > +------------------------------------
> >     > +
> >     > +Support has been added for the UEFI capsule update feature which
> >     > +enables updating the U-Boot image using the UEFI firmware
> management
> >     > +protocol (fmp). The capsules are not passed to the firmware
> through
> >     > +the UpdateCapsule runtime service. Instead, capsule-on-disk
> >     > +functionality is used for fetching the capsule from the EFI System
> >     > +Partition (ESP) by placing the capsule file under the
> >     > +\EFI\UpdateCapsule directory.
> >     > +
> >     > +Currently, support has been added on the QEMU ARM64 virt platform
> for
> >     > +updating the U-Boot binary as a raw image when the platform is
> booted
> >     > +in non-secure mode, i.e. with CONFIG_TFABOOT disabled. For this
> >     > +configuration, the QEMU platform needs to be booted with
> >     > +'secure=off'. The U-Boot binary placed on the first bank of the
> NOR
> >     > +flash at offset 0x0. The U-Boot environment is placed on the
> second
> >     > +NOR flash bank at offset 0x4000000.
> >     > +
> >     > +The capsule update feature is enabled with the following
> >     configuration
> >     > +settings::
> >     > +
> >     > +    CONFIG_MTD=y
> >     > +    CONFIG_FLASH_CFI_MTD=y
> >     > +    CONFIG_CMD_MTDPARTS=y
> >     > +    CONFIG_CMD_DFU=y
> >     > +    CONFIG_DFU_MTD=y
> >     > +    CONFIG_PCI_INIT_R=y
> >     > +    CONFIG_EFI_CAPSULE_ON_DISK=y
> >     > +    CONFIG_EFI_CAPSULE_FIRMWARE_MANAGEMENT=y
> >     > +    CONFIG_EFI_CAPSULE_FIRMWARE=y
> >     > +    CONFIG_EFI_CAPSULE_FIRMWARE_RAW=y
> >     > +    CONFIG_EFI_CAPSULE_FMP_HEADER=y
> >     > +
> >     > +In addition, the following config needs to be disabled(QEMU ARM
> >     specific)::
> >     > +
> >     > +    CONFIG_TFABOOT
> >     > +
> >     > +The capsule file can be generated by using the GenerateCapsule.py
> >     > +script in EDKII::
> >     > +
> >     > +    $ ./BaseTools/BinWrappers/PosixLike/GenerateCapsule -e -o \
> >     > +    <capsule_file_name> --fw-version <val> --lsv <val> --guid \
> >     > +    e2bb9c06-70e9-4b14-97a3-5a7913176e3f --verbose
> >     --update-image-index \
> >     > +    <val> --verbose <u-boot.bin>
> >     > +
> >     > +The above is a wrapper script(GenerateCapsule) which eventually
> calls
> >     > +the actual GenerateCapsule.py script.
> >     > +
> >     > +As per the UEFI specification, the capsule file needs to be
> placed on
> >     > +the EFI System Partition, under the \EFI\UpdateCapsule directory.
> The
> >     > +EFI System Partition can be a virtio-blk-device.
> >     > +
> >     > +Before initiating the firmware update, the efi variables BootNext,
> >     > +BootXXXX and OsIndications need to be set. The BootXXXX variable
> >     needs
> >     > +to be pointing to the EFI System Partition which contains the
> capsule
> >     > +file. The BootNext, BootXXXX and OsIndications variables can be
> set
> >     > +using the following commands::
> >     > +
> >     > +    => efidebug boot add 0 Boot0000 virtio 0:1 <capsule_file_name>
> >     > +    => efidebug boot next 0
> >     > +    => setenv -e -nv -bs -rt -v OsIndications =0x04
> >     > +    => saveenv
> >     > +
> >     > +Finally, the capsule update can be initiated with the following
> >     > +command::
> >     > +
> >     > +    => efidebug capsule disk-update
> >     > +
> >     > +The updated U-Boot image will be booted on subsequent boot.
> >     > +
> >     > +Enabling Capsule Authentication
> >     > +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> >     > +
> >     > +The UEFI specification defines a way of authenticating the
> capsule to
> >     > +be updated by verifying the capsule signature. The capsule
> signature
> >     > +is computed and prepended to the capsule payload at the time of
> >     > +capsule generation. This signature is then verified by using the
> >     > +public key stored as part of the X509 certificate. This
> >     certificate is
> >     > +in the form of an efi signature list (esl) file, which is
> embedded as
> >     > +part of the platform's device tree blob using the mkeficapsule
> >     > +utility.
> >     > +
> >     > +On the QEMU virt platforms, the device-tree is generated on the
> fly
> >     > +based on the devices configured. This device tree is then passed
> >     on to
> >     > +the various software components booting on the platform, including
> >     > +U-Boot. Therefore, on the QEMU virt platform, the signatute is
> >     > +embedded on an overlay. This overlay is then applied at runtime
> >     to the
> >     > +base platform device-tree. Steps needed for embedding the esl
> file in
> >     > +the overlay are highlighted below.
> >     > +
> >     > +The capsule authentication feature can be enabled through the
> >     > +following config, in addition to the configs listed above for
> capsule
> >     > +update::
> >     > +
> >     > +    CONFIG_EFI_CAPSULE_AUTHENTICATE=y
> >     > +
> >     > +The public and private keys used for the signing process are
> >     generated
> >     > +and used by the steps highlighted below::
> >     > +
> >     > +    1. Install utility commands on your host
> >     > +       * OPENSSL
> >     > +       * efitools
> >     > +
> >     > +    2. Create signing keys and certificate files on your host
> >     > +
> >     > +        $ openssl req -x509 -sha256 -newkey rsa:2048 -subj
> /CN=CRT/ \
> >     > +            -keyout CRT.key -out CRT.crt -nodes -days 365
> >     > +        $ cert-to-efi-sig-list CRT.crt CRT.esl
> >     > +
> >     > +        $ openssl x509 -in CRT.crt -out CRT.cer -outform DER
> >     > +        $ openssl x509 -inform DER -in CRT.cer -outform PEM -out
> >     CRT.pub.pem
> >     > +
> >     > +        $ openssl pkcs12 -export -out CRT.pfx -inkey CRT.key -in
> >     CRT.crt
> >     > +        $ openssl pkcs12 -in CRT.pfx -nodes -out CRT.pem
> >     > +
> >     > +The capsule file can be generated by using the GenerateCapsule.py
> >     > +script in EDKII::
> >     > +
> >     > +    $ ./BaseTools/BinWrappers/PosixLike/GenerateCapsule -e -o \
> >     > +      <capsule_file_name> --monotonic-count <val> --fw-version \
> >     > +      <val> --lsv <val> --guid \
> >     > +      e2bb9c06-70e9-4b14-97a3-5a7913176e3f --verbose \
> >     > +      --update-image-index <val> --signer-private-cert \
> >     > +      /path/to/CRT.pem --trusted-public-cert \
> >     > +      /path/to/CRT.pub.pem --other-public-cert
> /path/to/CRT.pub.pem \
> >     > +      <u-boot.bin>
> >     > +
> >     > +Place the capsule generated in the above step on the EFI System
> >     > +Partition under the EFI/UpdateCapsule directory
> >     > +
> >     > +For embedding the public key certificate, the following steps
> need to
> >     > +be followed::
> >     > +
> >     > +    1. Generate a skeleton overlay dts file, with a single
> fragment
> >     > +       node and an empty __overlay__ node
> >     > +
> >     > +       A typical skeleton overlay file will look like this
> >     > +
> >     > +       /dts-v1/;
> >     > +       /plugin/;
> >     > +
> >     > +       / {
> >     > +               fragment at 0 {
> >     > +                       target-path = "/";
> >     > +                       __overlay__ {
> >     > +                       };
> >     > +               };
> >     > +       };
> >     > +
> >     > +
> >     > +    2. Convert the dts to a corresponding dtb with the following
> >
> >     Shouldn't this be dtbo?
> >
> >     > +       command
> >     > +        ./scripts/dtc/dtc -@ -I dts -O dtb -o <ov_dtb_file_name> \
> >     > +        <dts_file>
> >     > +
> >     > +    3. Run the dtb file generated above through the mkeficapsule
> tool
> >     > +       in U-Boot
> >     > +        ./tools/mkeficapsule -O <pub_key.esl> -D <ov_dtb>
> >     > +
> >     > +Running the above command results in the creation of a 'signature'
> >     > +node in the dtb, under which the public key is stored as a
> >     > +'capsule-key' property. The '-O' option is to be used since the
> >     > +public key certificate(esl) file is being embedded in an overlay.
> >     > +
> >     > +The dtb file embedded with the certificate is now to be placed on
> an
> >     > +EFI System Partition. This would then be loaded and "merged" with
> the
> >     > +base platform flattened device-tree(dtb) at runtime.
> >     > +
> >     > +Build U-Boot with the following steps(QEMU ARM64)::
> >     > +
> >     > +    $ make qemu_arm64_defconfig
> >     > +    $ make menuconfig
> >     > +        Disable CONFIG_TFABOOT
> >     > +        Enable CONFIG_EFI_CAPSULE_AUTHENTICATE
> >     > +        Enable all configs needed for capsule update(listed above)
> >     > +    $ make all
> >     > +
> >     > +Boot the platform and perform the following steps on the U-Boot
> >     > +command line::
> >     > +
> >     > +    1. Enable capsule authentication by setting the following env
> >     > +       variable
> >     > +
> >     > +        => setenv capsule_authentication_enabled 1
> >     > +        => saveenv
> >     > +
> >     > +    2. Load the overlay dtb to memory and merge it with the base
> fdt
> >     > +
> >     > +        => fatload virtio 0:1 <$fdtovaddr> EFI/<ov_dtb_file>
> >     > +        => fdt addr $fdtcontroladdr
> >     > +        => fdt resize <size_of_ov_dtb_file>
> >     > +        => fdt apply <$fdtovaddr>
> >
> >     Having the public key on the disk means that any public key can be
> >     placed here and we get zero security.
> >
> >
> > But that does not mean the authentication will succeed unless the
> > private key is compromised. Deleting or tampering the public key on the
> > disk can result in a denial of service attack, as the capsule
> > authentication would fail, but that is true even when the public key is
> > embedded in u-boot -- the public key or the u-boot image can be tampered
> > with, resulting in a board brick. For countering this kind of denial of
> > service attack, the public key needs to be placed on a secure storage
> > device, which cannot be modified or removed from the normal world.
> > Moreover, how is this different to the placement of the signature
> > database used for the uefi secure boot as part of the uefi authenticated
> > variables on a storage device that can be accessed from the normal world.
>
> The public key is what you use to verify that a capsule was signed by an
> authorized party. Who controls the public keys used for capsule checking
> can crack the device:
>
> I just have to create a public/private key pair to sign my malware and
> place both the public key and the malware capsule on the disk.
>

But when tf-a verifies this this BL33 during boot, it would fail
authentication and would not boot that BL33 image. So it is the same as
denial of service, isn't it.

-sughosh


>
> We should not allow public keys for capsules to be on disk.
>
> TF-A checks BL33 (U-Boot). If the public key is part of BL33 then only
> capsules signed with this trusted key can be installed.
>
> An attacker can still change U-Boot in a way that TF-A will not load it
> leading to a denial of service. But he cannot launch malware via capsules.
>
> Best regards
>
> Heinrich
> >
> >
> >     We need to build the public key into U-Boot.
> >
> >     Could you, please, investigate how we can adjust the build process
> >     accordingly.
> >
> >     Best regards
> >
> >     Heinrich
> >
> >     > +
> >     > +    3. Set the following environment and UEFI boot variables
> >     > +
> >     > +        => setenv -e -nv -bs -rt -v OsIndications =0x04
> >     > +        => efidebug boot add 0 Boot0000 virtio 0:1
> >     <capsule_file_name>
> >     > +        => efidebug boot next 0
> >     > +        => saveenv
> >     > +
> >     > +    4. Finally, the capsule update can be initiated with the
> >     following
> >     > +       command
> >     > +
> >     > +        => efidebug capsule disk-update
> >     > +
> >     > +On subsequent reboot, the platform should boot the updated U-Boot
> >     binary.
> >     >
> >
>
>