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TDX guest driver exposes IOCTL interfaces to service TDX guest user-specific requests. Currently, it is only used to allow the user to get the TDREPORT to support TDX attestation. Details about the TDX attestation process are documented in Documentation/x86/tdx.rst, and the IOCTL details are documented in Documentation/virt/coco/tdx-guest.rst. Operations like getting TDREPORT involves sending a blob of data as input and getting another blob of data as output. It was considered to use a sysfs interface for this, but it doesn't fit well into the standard sysfs model for configuring values. It would be possible to do read/write on files, but it would need multiple file descriptors, which would be somewhat messy. IOCTLs seem to be the best fitting and simplest model for this use case. The AMD sev-guest driver also uses the IOCTL interface to support attestation. [Bagas Sanjaya: Ack is for documentation portion] Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com> Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com> Reviewed-by: Tony Luck <tony.luck@intel.com> Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com> Acked-by: Kai Huang <kai.huang@intel.com> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Acked-by: Wander Lairson Costa <wander@redhat.com> Link: https://lore.kernel.org/all/20221116223820.819090-3-sathyanarayanan.kuppuswamy%40linux.intel.com
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261 lines
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.. SPDX-License-Identifier: GPL-2.0
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=====================================
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Intel Trust Domain Extensions (TDX)
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=====================================
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Intel's Trust Domain Extensions (TDX) protect confidential guest VMs from
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the host and physical attacks by isolating the guest register state and by
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encrypting the guest memory. In TDX, a special module running in a special
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mode sits between the host and the guest and manages the guest/host
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separation.
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Since the host cannot directly access guest registers or memory, much
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normal functionality of a hypervisor must be moved into the guest. This is
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implemented using a Virtualization Exception (#VE) that is handled by the
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guest kernel. A #VE is handled entirely inside the guest kernel, but some
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require the hypervisor to be consulted.
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TDX includes new hypercall-like mechanisms for communicating from the
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guest to the hypervisor or the TDX module.
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New TDX Exceptions
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==================
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TDX guests behave differently from bare-metal and traditional VMX guests.
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In TDX guests, otherwise normal instructions or memory accesses can cause
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#VE or #GP exceptions.
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Instructions marked with an '*' conditionally cause exceptions. The
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details for these instructions are discussed below.
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Instruction-based #VE
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---------------------
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- Port I/O (INS, OUTS, IN, OUT)
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- HLT
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- MONITOR, MWAIT
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- WBINVD, INVD
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- VMCALL
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- RDMSR*,WRMSR*
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- CPUID*
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Instruction-based #GP
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---------------------
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- All VMX instructions: INVEPT, INVVPID, VMCLEAR, VMFUNC, VMLAUNCH,
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VMPTRLD, VMPTRST, VMREAD, VMRESUME, VMWRITE, VMXOFF, VMXON
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- ENCLS, ENCLU
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- GETSEC
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- RSM
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- ENQCMD
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- RDMSR*,WRMSR*
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RDMSR/WRMSR Behavior
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--------------------
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MSR access behavior falls into three categories:
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- #GP generated
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- #VE generated
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- "Just works"
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In general, the #GP MSRs should not be used in guests. Their use likely
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indicates a bug in the guest. The guest may try to handle the #GP with a
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hypercall but it is unlikely to succeed.
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The #VE MSRs are typically able to be handled by the hypervisor. Guests
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can make a hypercall to the hypervisor to handle the #VE.
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The "just works" MSRs do not need any special guest handling. They might
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be implemented by directly passing through the MSR to the hardware or by
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trapping and handling in the TDX module. Other than possibly being slow,
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these MSRs appear to function just as they would on bare metal.
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CPUID Behavior
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--------------
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For some CPUID leaves and sub-leaves, the virtualized bit fields of CPUID
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return values (in guest EAX/EBX/ECX/EDX) are configurable by the
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hypervisor. For such cases, the Intel TDX module architecture defines two
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virtualization types:
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- Bit fields for which the hypervisor controls the value seen by the guest
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TD.
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- Bit fields for which the hypervisor configures the value such that the
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guest TD either sees their native value or a value of 0. For these bit
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fields, the hypervisor can mask off the native values, but it can not
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turn *on* values.
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A #VE is generated for CPUID leaves and sub-leaves that the TDX module does
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not know how to handle. The guest kernel may ask the hypervisor for the
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value with a hypercall.
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#VE on Memory Accesses
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======================
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There are essentially two classes of TDX memory: private and shared.
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Private memory receives full TDX protections. Its content is protected
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against access from the hypervisor. Shared memory is expected to be
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shared between guest and hypervisor and does not receive full TDX
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protections.
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A TD guest is in control of whether its memory accesses are treated as
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private or shared. It selects the behavior with a bit in its page table
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entries. This helps ensure that a guest does not place sensitive
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information in shared memory, exposing it to the untrusted hypervisor.
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#VE on Shared Memory
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--------------------
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Access to shared mappings can cause a #VE. The hypervisor ultimately
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controls whether a shared memory access causes a #VE, so the guest must be
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careful to only reference shared pages it can safely handle a #VE. For
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instance, the guest should be careful not to access shared memory in the
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#VE handler before it reads the #VE info structure (TDG.VP.VEINFO.GET).
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Shared mapping content is entirely controlled by the hypervisor. The guest
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should only use shared mappings for communicating with the hypervisor.
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Shared mappings must never be used for sensitive memory content like kernel
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stacks. A good rule of thumb is that hypervisor-shared memory should be
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treated the same as memory mapped to userspace. Both the hypervisor and
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userspace are completely untrusted.
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MMIO for virtual devices is implemented as shared memory. The guest must
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be careful not to access device MMIO regions unless it is also prepared to
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handle a #VE.
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#VE on Private Pages
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--------------------
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An access to private mappings can also cause a #VE. Since all kernel
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memory is also private memory, the kernel might theoretically need to
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handle a #VE on arbitrary kernel memory accesses. This is not feasible, so
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TDX guests ensure that all guest memory has been "accepted" before memory
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is used by the kernel.
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A modest amount of memory (typically 512M) is pre-accepted by the firmware
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before the kernel runs to ensure that the kernel can start up without
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being subjected to a #VE.
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The hypervisor is permitted to unilaterally move accepted pages to a
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"blocked" state. However, if it does this, page access will not generate a
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#VE. It will, instead, cause a "TD Exit" where the hypervisor is required
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to handle the exception.
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Linux #VE handler
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=================
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Just like page faults or #GP's, #VE exceptions can be either handled or be
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fatal. Typically, an unhandled userspace #VE results in a SIGSEGV.
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An unhandled kernel #VE results in an oops.
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Handling nested exceptions on x86 is typically nasty business. A #VE
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could be interrupted by an NMI which triggers another #VE and hilarity
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ensues. The TDX #VE architecture anticipated this scenario and includes a
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feature to make it slightly less nasty.
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During #VE handling, the TDX module ensures that all interrupts (including
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NMIs) are blocked. The block remains in place until the guest makes a
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TDG.VP.VEINFO.GET TDCALL. This allows the guest to control when interrupts
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or a new #VE can be delivered.
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However, the guest kernel must still be careful to avoid potential
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#VE-triggering actions (discussed above) while this block is in place.
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While the block is in place, any #VE is elevated to a double fault (#DF)
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which is not recoverable.
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MMIO handling
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=============
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In non-TDX VMs, MMIO is usually implemented by giving a guest access to a
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mapping which will cause a VMEXIT on access, and then the hypervisor
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emulates the access. That is not possible in TDX guests because VMEXIT
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will expose the register state to the host. TDX guests don't trust the host
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and can't have their state exposed to the host.
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In TDX, MMIO regions typically trigger a #VE exception in the guest. The
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guest #VE handler then emulates the MMIO instruction inside the guest and
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converts it into a controlled TDCALL to the host, rather than exposing
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guest state to the host.
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MMIO addresses on x86 are just special physical addresses. They can
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theoretically be accessed with any instruction that accesses memory.
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However, the kernel instruction decoding method is limited. It is only
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designed to decode instructions like those generated by io.h macros.
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MMIO access via other means (like structure overlays) may result in an
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oops.
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Shared Memory Conversions
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=========================
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All TDX guest memory starts out as private at boot. This memory can not
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be accessed by the hypervisor. However, some kernel users like device
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drivers might have a need to share data with the hypervisor. To do this,
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memory must be converted between shared and private. This can be
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accomplished using some existing memory encryption helpers:
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* set_memory_decrypted() converts a range of pages to shared.
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* set_memory_encrypted() converts memory back to private.
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Device drivers are the primary user of shared memory, but there's no need
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to touch every driver. DMA buffers and ioremap() do the conversions
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automatically.
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TDX uses SWIOTLB for most DMA allocations. The SWIOTLB buffer is
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converted to shared on boot.
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For coherent DMA allocation, the DMA buffer gets converted on the
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allocation. Check force_dma_unencrypted() for details.
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Attestation
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===========
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Attestation is used to verify the TDX guest trustworthiness to other
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entities before provisioning secrets to the guest. For example, a key
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server may want to use attestation to verify that the guest is the
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desired one before releasing the encryption keys to mount the encrypted
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rootfs or a secondary drive.
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The TDX module records the state of the TDX guest in various stages of
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the guest boot process using the build time measurement register (MRTD)
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and runtime measurement registers (RTMR). Measurements related to the
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guest initial configuration and firmware image are recorded in the MRTD
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register. Measurements related to initial state, kernel image, firmware
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image, command line options, initrd, ACPI tables, etc are recorded in
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RTMR registers. For more details, as an example, please refer to TDX
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Virtual Firmware design specification, section titled "TD Measurement".
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At TDX guest runtime, the attestation process is used to attest to these
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measurements.
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The attestation process consists of two steps: TDREPORT generation and
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Quote generation.
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TDX guest uses TDCALL[TDG.MR.REPORT] to get the TDREPORT (TDREPORT_STRUCT)
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from the TDX module. TDREPORT is a fixed-size data structure generated by
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the TDX module which contains guest-specific information (such as build
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and boot measurements), platform security version, and the MAC to protect
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the integrity of the TDREPORT. A user-provided 64-Byte REPORTDATA is used
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as input and included in the TDREPORT. Typically it can be some nonce
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provided by attestation service so the TDREPORT can be verified uniquely.
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More details about the TDREPORT can be found in Intel TDX Module
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specification, section titled "TDG.MR.REPORT Leaf".
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After getting the TDREPORT, the second step of the attestation process
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is to send it to the Quoting Enclave (QE) to generate the Quote. TDREPORT
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by design can only be verified on the local platform as the MAC key is
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bound to the platform. To support remote verification of the TDREPORT,
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TDX leverages Intel SGX Quoting Enclave to verify the TDREPORT locally
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and convert it to a remotely verifiable Quote. Method of sending TDREPORT
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to QE is implementation specific. Attestation software can choose
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whatever communication channel available (i.e. vsock or TCP/IP) to
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send the TDREPORT to QE and receive the Quote.
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References
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==========
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TDX reference material is collected here:
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https://www.intel.com/content/www/us/en/developer/articles/technical/intel-trust-domain-extensions.html
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