.. SPDX-License-Identifier: GPL-2.0
Confidential Computing VMs
==========================
Hyper-V can create and run Linux guests that are Confidential Computing
(CoCo) VMs. Such VMs cooperate with the physical processor to better protect
the confidentiality and integrity of data in the VM's memory, even in the
face of a hypervisor/VMM that has been compromised and may behave maliciously.
CoCo VMs on Hyper-V share the generic CoCo VM threat model and security
objectives described in Documentation/security/snp-tdx-threat-model.rst. Note
that Hyper-V specific code in Linux refers to CoCo VMs as "isolated VMs" or
"isolation VMs".
A Linux CoCo VM on Hyper-V requires the cooperation and interaction of the
following:
* Physical hardware with a processor that supports CoCo VMs
* The hardware runs a version of Windows/Hyper-V with support for CoCo VMs
* The VM runs a version of Linux that supports being a CoCo VM
The physical hardware requirements are as follows:
* AMD processor with SEV-SNP. Hyper-V does not run guest VMs with AMD SME,
SEV, or SEV-ES encryption, and such encryption is not sufficient for a CoCo
VM on Hyper-V.
* Intel processor with TDX
To create a CoCo VM, the "Isolated VM" attribute must be specified to Hyper-V
when the VM is created. A VM cannot be changed from a CoCo VM to a normal VM,
or vice versa, after it is created.
Operational Modes
-----------------
Hyper-V CoCo VMs can run in two modes. The mode is selected when the VM is
created and cannot be changed during the life of the VM.
* Fully-enlightened mode. In this mode, the guest operating system is
enlightened to understand and manage all aspects of running as a CoCo VM.
* Paravisor mode. In this mode, a paravisor layer between the guest and the
host provides some operations needed to run as a CoCo VM. The guest operating
system can have fewer CoCo enlightenments than is required in the
fully-enlightened case.
Conceptually, fully-enlightened mode and paravisor mode may be treated as
points on a spectrum spanning the degree of guest enlightenment needed to run
as a CoCo VM. Fully-enlightened mode is one end of the spectrum. A full
implementation of paravisor mode is the other end of the spectrum, where all
aspects of running as a CoCo VM are handled by the paravisor, and a normal
guest OS with no knowledge of memory encryption or other aspects of CoCo VMs
can run successfully. However, the Hyper-V implementation of paravisor mode
does not go this far, and is somewhere in the middle of the spectrum. Some
aspects of CoCo VMs are handled by the Hyper-V paravisor while the guest OS
must be enlightened for other aspects. Unfortunately, there is no
standardized enumeration of feature/functions that might be provided in the
paravisor, and there is no standardized mechanism for a guest OS to query the
paravisor for the feature/functions it provides. The understanding of what
the paravisor provides is hard-coded in the guest OS.
Paravisor mode has similarities to the `Coconut project`_, which aims to provide
a limited paravisor to provide services to the guest such as a virtual TPM.
However, the Hyper-V paravisor generally handles more aspects of CoCo VMs
than is currently envisioned for Coconut, and so is further toward the "no
guest enlightenments required" end of the spectrum.
.. _Coconut project: https://github.com/coconut-svsm/svsm
In the CoCo VM threat model, the paravisor is in the guest security domain
and must be trusted by the guest OS. By implication, the hypervisor/VMM must
protect itself against a potentially malicious paravisor just like it
protects against a potentially malicious guest.
The hardware architectural approach to fully-enlightened vs. paravisor mode
varies depending on the underlying processor.
* With AMD SEV-SNP processors, in fully-enlightened mode the guest OS runs in
VMPL 0 and has full control of the guest context. In paravisor mode, the
guest OS runs in VMPL 2 and the paravisor runs in VMPL 0. The paravisor
running in VMPL 0 has privileges that the guest OS in VMPL 2 does not have.
Certain operations require the guest to invoke the paravisor. Furthermore, in
paravisor mode the guest OS operates in "virtual Top Of Memory" (vTOM) mode
as defined by the SEV-SNP architecture. This mode simplifies guest management
of memory encryption when a paravisor is used.
* With Intel TDX processor, in fully-enlightened mode the guest OS runs in an
L1 VM. In paravisor mode, TD partitioning is used. The paravisor runs in the
L1 VM, and the guest OS runs in a nested L2 VM.
Hyper-V exposes a synthetic MSR to guests that describes the CoCo mode. This
MSR indicates if the underlying processor uses AMD SEV-SNP or Intel TDX, and
whether a paravisor is being used. It is straightforward to build a single
kernel image that can boot and run properly on either architecture, and in
either mode.
Paravisor Effects
-----------------
Running in paravisor mode affects the following areas of generic Linux kernel
CoCo VM functionality:
* Initial guest memory setup. When a new VM is created in paravisor mode, the
paravisor runs first and sets up the guest physical memory as encrypted. The
guest Linux does normal memory initialization, except for explicitly marking
appropriate ranges as decrypted (shared). In paravisor mode, Linux does not
perform the early boot memory setup steps that are particularly tricky with
AMD SEV-SNP in fully-enlightened mode.
* #VC/#VE exception handling. In paravisor mode, Hyper-V configures the guest
CoCo VM to route #VC and #VE exceptions to VMPL 0 and the L1 VM,
respectively, and not the guest Linux. Consequently, these exception handlers
do not run in the guest Linux and are not a required enlightenment for a
Linux guest in paravisor mode.
* CPUID flags. Both AMD SEV-SNP and Intel TDX provide a CPUID flag in the
guest indicating that the VM is operating with the respective hardware
support. While these CPUID flags are visible in fully-enlightened CoCo VMs,
the paravisor filters out these flags and the guest Linux does not see them.
Throughout the Linux kernel, explicitly testing these flags has mostly been
eliminated in favor of the cc_platform_has() function, with the goal of
abstracting the differences between SEV-SNP and TDX. But the
cc_platform_has() abstraction also allows the Hyper-V paravisor configuration
to selectively enable aspects of CoCo VM functionality even when the CPUID
flags are not set. The exception is early boot memory setup on SEV-SNP, which
tests the CPUID SEV-SNP flag. But not having the flag in Hyper-V paravisor
mode VM achieves the desired effect or not running SEV-SNP specific early
boot memory setup.
* Device emulation. In paravisor mode, the Hyper-V paravisor provides
emulation of devices such as the IO-APIC and TPM. Because the emulation
happens in the paravisor in the guest context (instead of the hypervisor/VMM
context), MMIO accesses to these devices must be encrypted references instead
of the decrypted references that would be used in a fully-enlightened CoCo
VM. The __ioremap_caller() function has been enhanced to make a callback to
check whether a particular address range should be treated as encrypted
(private). See the "is_private_mmio" callback.
* Encrypt/decrypt memory transitions. In a CoCo VM, transitioning guest
memory between encrypted and decrypted requires coordinating with the
hypervisor/VMM. This is done via callbacks invoked from
__set_memory_enc_pgtable(). In fully-enlightened mode, the normal SEV-SNP and
TDX implementations of these callbacks are used. In paravisor mode, a Hyper-V
specific set of callbacks is used. These callbacks invoke the paravisor so
that the paravisor can coordinate the transitions and inform the hypervisor
as necessary. See hv_vtom_init() where these callback are set up.
* Interrupt injection. In fully enlightened mode, a malicious hypervisor
could inject interrupts into the guest OS at times that violate x86/x64
architectural rules. For full protection, the guest OS should include
enlightenments that use the interrupt injection management features provided
by CoCo-capable processors. In paravisor mode, the paravisor mediates
interrupt injection into the guest OS, and ensures that the guest OS only
sees interrupts that are "legal". The paravisor uses the interrupt injection
management features provided by the CoCo-capable physical processor, thereby
masking these complexities from the guest OS.
Hyper-V Hypercalls
------------------
When in fully-enlightened mode, hypercalls made by the Linux guest are routed
directly to the hypervisor, just as in a non-CoCo VM. But in paravisor mode,
normal hypercalls trap to the paravisor first, which may in turn invoke the
hypervisor. But the paravisor is idiosyncratic in this regard, and a few
hypercalls made by the Linux guest must always be routed directly to the
hypervisor. These hypercall sites test for a paravisor being present, and use
a special invocation sequence. See hv_post_message(), for example.
Guest communication with Hyper-V
--------------------------------
Separate from the generic Linux kernel handling of memory encryption in Linux
CoCo VMs, Hyper-V has VMBus and VMBus devices that communicate using memory
shared between the Linux guest and the host. This shared memory must be
marked decrypted to enable communication. Furthermore, since the threat model
includes a compromised and potentially malicious host, the guest must guard
against leaking any unintended data to the host through this shared memory.
These Hyper-V and VMBus memory pages are marked as decrypted:
* VMBus monitor pages
* Synthetic interrupt controller (synic) related pages (unless supplied by
the paravisor)
* Per-cpu hypercall input and output pages (unless running with a paravisor)
* VMBus ring buffers. The direct mapping is marked decrypted in
__vmbus_establish_gpadl(). The secondary mapping created in
hv_ringbuffer_init() must also include the "decrypted" attribute.
When the guest writes data to memory that is shared with the host, it must
ensure that only the intended data is written. Padding or unused fields must
be initialized to zeros before copying into the shared memory so that random
kernel data is not inadvertently given to the host.
Similarly, when the guest reads memory that is shared with the host, it must
validate the data before acting on it so that a malicious host cannot induce
the guest to expose unintended data. Doing such validation can be tricky
because the host can modify the shared memory areas even while or after
validation is performed. For messages passed from the host to the guest in a
VMBus ring buffer, the length of the message is validated, and the message is
copied into a temporary (encrypted) buffer for further validation and
processing. The copying adds a small amount of overhead, but is the only way
to protect against a malicious host. See hv_pkt_iter_first().
Many drivers for VMBus devices have been "hardened" by adding code to fully
validate messages received over VMBus, instead of assuming that Hyper-V is
acting cooperatively. Such drivers are marked as "allowed_in_isolated" in the
vmbus_devs[] table. Other drivers for VMBus devices that are not needed in a
CoCo VM have not been hardened, and they are not allowed to load in a CoCo
VM. See vmbus_is_valid_offer() where such devices are excluded.
Two VMBus devices depend on the Hyper-V host to do DMA data transfers:
storvsc for disk I/O and netvsc for network I/O. storvsc uses the normal
Linux kernel DMA APIs, and so bounce buffering through decrypted swiotlb
memory is done implicitly. netvsc has two modes for data transfers. The first
mode goes through send and receive buffer space that is explicitly allocated
by the netvsc driver, and is used for most smaller packets. These send and
receive buffers are marked decrypted by __vmbus_establish_gpadl(). Because
the netvsc driver explicitly copies packets to/from these buffers, the
equivalent of bounce buffering between encrypted and decrypted memory is
already part of the data path. The second mode uses the normal Linux kernel
DMA APIs, and is bounce buffered through swiotlb memory implicitly like in
storvsc.
Finally, the VMBus virtual PCI driver needs special handling in a CoCo VM.
Linux PCI device drivers access PCI config space using standard APIs provided
by the Linux PCI subsystem. On Hyper-V, these functions directly access MMIO
space, and the access traps to Hyper-V for emulation. But in CoCo VMs, memory
encryption prevents Hyper-V from reading the guest instruction stream to
emulate the access. So in a CoCo VM, these functions must make a hypercall
with arguments explicitly describing the access. See
_hv_pcifront_read_config() and _hv_pcifront_write_config() and the
"use_calls" flag indicating to use hypercalls.
load_unaligned_zeropad()
------------------------
When transitioning memory between encrypted and decrypted, the caller of
set_memory_encrypted() or set_memory_decrypted() is responsible for ensuring
the memory isn't in use and isn't referenced while the transition is in
progress. The transition has multiple steps, and includes interaction with
the Hyper-V host. The memory is in an inconsistent state until all steps are
complete. A reference while the state is inconsistent could result in an
exception that can't be cleanly fixed up.
However, the kernel load_unaligned_zeropad() mechanism may make stray
references that can't be prevented by the caller of set_memory_encrypted() or
set_memory_decrypted(), so there's specific code in the #VC or #VE exception
handler to fixup this case. But a CoCo VM running on Hyper-V may be
configured to run with a paravisor, with the #VC or #VE exception routed to
the paravisor. There's no architectural way to forward the exceptions back to
the guest kernel, and in such a case, the load_unaligned_zeropad() fixup code
in the #VC/#VE handlers doesn't run.
To avoid this problem, the Hyper-V specific functions for notifying the
hypervisor of the transition mark pages as "not present" while a transition
is in progress. If load_unaligned_zeropad() causes a stray reference, a
normal page fault is generated instead of #VC or #VE, and the page-fault-
based handlers for load_unaligned_zeropad() fixup the reference. When the
encrypted/decrypted transition is complete, the pages are marked as "present"
again. See hv_vtom_clear_present() and hv_vtom_set_host_visibility().
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