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69 lines
2.6 KiB
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69 lines
2.6 KiB
ReStructuredText
==============
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UEFI variables
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==============
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Guest UEFI variable management
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==============================
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The traditional approach for UEFI Variable storage in qemu guests is
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to work as close as possible to physical hardware. That means
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providing pflash as storage and leaving the management of variables
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and flash to the guest.
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Secure boot support comes with the requirement that the UEFI variable
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storage must be protected against direct access by the OS. All update
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requests must pass the sanity checks. (Parts of) the firmware must
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run with a higher privilege level than the OS so this can be enforced
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by the firmware. On x86 this has been implemented using System
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Management Mode (SMM) in qemu and kvm, which again is the same
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approach taken by physical hardware. Only privileged code running in
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SMM mode is allowed to access flash storage.
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Communication with the firmware code running in SMM mode works by
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serializing the requests to a shared buffer, then trapping into SMM
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mode via SMI. The SMM code processes the request, stores the reply in
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the same buffer and returns.
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Host UEFI variable service
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==========================
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Instead of running the privileged code inside the guest we can run it
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on the host. The serialization protocol can be reused. The
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communication with the host uses a virtual device, which essentially
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configures the shared buffer location and size, and traps to the host
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to process the requests.
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The ``uefi-vars`` device implements the UEFI virtual device. It comes
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in ``uefi-vars-x64`` and ``uefi-vars-sysbus`` flavours. The device
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reimplements the handlers needed, specifically
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``EfiSmmVariableProtocol`` and ``VarCheckPolicyLibMmiHandler``. It
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also consumes events (``EfiEndOfDxeEventGroup``,
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``EfiEventReadyToBoot`` and ``EfiEventExitBootServices``).
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The advantage of the approach is that we do not need a special
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privilege level for the firmware to protect itself, i.e. it does not
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depend on SMM emulation on x64, which allows the removal of a bunch of
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complex code for SMM emulation from the linux kernel
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(CONFIG_KVM_SMM=n). It also allows support for secure boot on arm
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without implementing secure world (el3) emulation in kvm.
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Of course there are also downsides. The added device increases the
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attack surface of the host, and we are adding some code duplication
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because we have to reimplement some edk2 functionality in qemu.
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usage on x86_64
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---------------
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.. code::
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qemu-system-x86_64 \
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-device uefi-vars-x64,jsonfile=/path/to/vars.json
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usage on aarch64
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----------------
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.. code::
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qemu-system-aarch64 -M virt \
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-device uefi-vars-sysbus,jsonfile=/path/to/vars.json
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