[PATCH v4 14/14] qemu: arm64: Add documentation for capsule update
Heinrich Schuchardt
xypron.glpk at gmx.de
Tue Mar 2 18:06:03 CET 2021
On 02.03.21 17:39, Sughosh Ganu wrote:
>
>
> On Tue, 2 Mar 2021 at 21:27, Heinrich Schuchardt <xypron.glpk at gmx.de
> <mailto:xypron.glpk at 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 at gmx.de <mailto:xypron.glpk at gmx.de>
> > <mailto:xypron.glpk at gmx.de <mailto:xypron.glpk at 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 at linaro.org
> <mailto:sughosh.ganu at linaro.org>
> > <mailto:sughosh.ganu at linaro.org <mailto:sughosh.ganu at 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.
You cannot stop denial of service, but you can stop malware.
That is why I don't want the public key used for capsule verification on
disk.
Best regards
Heinrich
>
> -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.
> > >
> >
>
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