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Tue, 11 Feb 2025 09:24:08 +0000 (UTC) Received: from sirius.home.kraxel.org (unknown [10.44.32.57]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id A1C8B1800873; Tue, 11 Feb 2025 09:24:07 +0000 (UTC) Received: by sirius.home.kraxel.org (Postfix, from userid 1000) id 106AF1800629; Tue, 11 Feb 2025 10:23:26 +0100 (CET) From: Gerd Hoffmann To: qemu-devel@nongnu.org Cc: Gerd Hoffmann , graf@amazon.com, Eric Blake , Peter Maydell , Paolo Bonzini , =?utf-8?q?Daniel_P=2E_Berrang=C3=A9?= , Thomas Huth , =?utf-8?q?Marc-Andr=C3=A9_Lureau?= , qemu-arm@nongnu.org, Michael Roth , Markus Armbruster , =?utf-8?q?Philippe_Mathieu-Daud?= =?utf-8?q?=C3=A9?= , Ard Biesheuvel Subject: [PATCH v3 22/23] docs: add uefi variable service documentation Date: Tue, 11 Feb 2025 10:23:20 +0100 Message-ID: <20250211092324.965440-23-kraxel@redhat.com> In-Reply-To: <20250211092324.965440-1-kraxel@redhat.com> References: <20250211092324.965440-1-kraxel@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 Received-SPF: pass client-ip=170.10.133.124; envelope-from=kraxel@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -34 X-Spam_score: -3.5 X-Spam_bar: --- X-Spam_report: (-3.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1.388, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Signed-off-by: Gerd Hoffmann --- docs/devel/index-internals.rst | 1 + docs/devel/uefi-vars.rst | 66 ++++++++++++++++++++++++++++++++++ hw/uefi/LIMITATIONS.md | 7 ++++ 3 files changed, 74 insertions(+) create mode 100644 docs/devel/uefi-vars.rst create mode 100644 hw/uefi/LIMITATIONS.md diff --git a/docs/devel/index-internals.rst b/docs/devel/index-internals.rst index bca597c65895..7a0678cbdd3a 100644 --- a/docs/devel/index-internals.rst +++ b/docs/devel/index-internals.rst @@ -20,6 +20,7 @@ Details about QEMU's various subsystems including how to add features to them. s390-cpu-topology s390-dasd-ipl tracing + uefi-vars vfio-iommufd writing-monitor-commands virtio-backends diff --git a/docs/devel/uefi-vars.rst b/docs/devel/uefi-vars.rst new file mode 100644 index 000000000000..3e7bd98b5208 --- /dev/null +++ b/docs/devel/uefi-vars.rst @@ -0,0 +1,66 @@ +============== +UEFI variables +============== + +Guest UEFI variable management +============================== + +The traditional approach for UEFI Variable storage in qemu guests is +to work as close as possible to physical hardware. That means +providing pflash as storage and leaving the management of variables +and flash to the guest. + +Secure boot support comes with the requirement that the UEFI variable +storage must be protected against direct access by the OS. All update +requests must pass the sanity checks. (Parts of) the firmware must +run with a higher privilege level than the OS so this can be enforced +by the firmware. On x86 this has been implemented using System +Management Mode (SMM) in qemu and kvm, which again is the same +approach taken by physical hardware. Only privileged code running in +SMM mode is allowed to access flash storage. + +Communication with the firmware code running in SMM mode works by +serializing the requests to a shared buffer, then trapping into SMM +mode via SMI. The SMM code processes the request, stores the reply in +the same buffer and returns. + +Host UEFI variable service +========================== + +Instead of running the privileged code inside the guest we can run it +on the host. The serialization protocol can be reused. The +communication with the host uses a virtual device, which essentially +configures the shared buffer location and size, and traps to the host +to process the requests. + +The ``uefi-vars`` device implements the UEFI virtual device. It comes +in ``uefi-vars-isa`` and ``uefi-vars-sysbus`` flavours. The device +reimplements the handlers needed, specifically +``EfiSmmVariableProtocol`` and ``VarCheckPolicyLibMmiHandler``. It +also consumes events (``EfiEndOfDxeEventGroup``, +``EfiEventReadyToBoot`` and ``EfiEventExitBootServices``). + +The advantage of the approach is that we do not need a special +privilege level for the firmware to protect itself, i.e. it does not +depend on SMM emulation on x64, which allows the removal of a bunch of +complex code for SMM emulation from the linux kernel +(CONFIG_KVM_SMM=n). It also allows support for secure boot on arm +without implementing secure world (el3) emulation in kvm. + +Of course there are also downsides. The added device increases the +attack surface of the host, and we are adding some code duplication +because we have to reimplement some edk2 functionality in qemu. + +usage on x86_64 (isa) +--------------------- + +.. code:: + + qemu-system-x86_64 -device uefi-vars-isa,jsonfile=/path/to/vars.json + +usage on aarch64 (sysbus) +------------------------- + +.. code:: + + qemu-system-aarch64 -M virt,x-uefi-vars=on diff --git a/hw/uefi/LIMITATIONS.md b/hw/uefi/LIMITATIONS.md new file mode 100644 index 000000000000..29308bd587aa --- /dev/null +++ b/hw/uefi/LIMITATIONS.md @@ -0,0 +1,7 @@ +known issues and limitations +---------------------------- + +* works only on little endian hosts + - accessing structs in guest ram is done without endian conversion. +* works only for 64-bit guests + - UINTN is mapped to uint64_t, for 32-bit guests that would be uint32_t