219 строки
8.3 KiB
ReStructuredText
219 строки
8.3 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
|
|
|
|
=====================================
|
|
Intel Trust Domain Extensions (TDX)
|
|
=====================================
|
|
|
|
Intel's Trust Domain Extensions (TDX) protect confidential guest VMs from
|
|
the host and physical attacks by isolating the guest register state and by
|
|
encrypting the guest memory. In TDX, a special module running in a special
|
|
mode sits between the host and the guest and manages the guest/host
|
|
separation.
|
|
|
|
Since the host cannot directly access guest registers or memory, much
|
|
normal functionality of a hypervisor must be moved into the guest. This is
|
|
implemented using a Virtualization Exception (#VE) that is handled by the
|
|
guest kernel. A #VE is handled entirely inside the guest kernel, but some
|
|
require the hypervisor to be consulted.
|
|
|
|
TDX includes new hypercall-like mechanisms for communicating from the
|
|
guest to the hypervisor or the TDX module.
|
|
|
|
New TDX Exceptions
|
|
==================
|
|
|
|
TDX guests behave differently from bare-metal and traditional VMX guests.
|
|
In TDX guests, otherwise normal instructions or memory accesses can cause
|
|
#VE or #GP exceptions.
|
|
|
|
Instructions marked with an '*' conditionally cause exceptions. The
|
|
details for these instructions are discussed below.
|
|
|
|
Instruction-based #VE
|
|
---------------------
|
|
|
|
- Port I/O (INS, OUTS, IN, OUT)
|
|
- HLT
|
|
- MONITOR, MWAIT
|
|
- WBINVD, INVD
|
|
- VMCALL
|
|
- RDMSR*,WRMSR*
|
|
- CPUID*
|
|
|
|
Instruction-based #GP
|
|
---------------------
|
|
|
|
- All VMX instructions: INVEPT, INVVPID, VMCLEAR, VMFUNC, VMLAUNCH,
|
|
VMPTRLD, VMPTRST, VMREAD, VMRESUME, VMWRITE, VMXOFF, VMXON
|
|
- ENCLS, ENCLU
|
|
- GETSEC
|
|
- RSM
|
|
- ENQCMD
|
|
- RDMSR*,WRMSR*
|
|
|
|
RDMSR/WRMSR Behavior
|
|
--------------------
|
|
|
|
MSR access behavior falls into three categories:
|
|
|
|
- #GP generated
|
|
- #VE generated
|
|
- "Just works"
|
|
|
|
In general, the #GP MSRs should not be used in guests. Their use likely
|
|
indicates a bug in the guest. The guest may try to handle the #GP with a
|
|
hypercall but it is unlikely to succeed.
|
|
|
|
The #VE MSRs are typically able to be handled by the hypervisor. Guests
|
|
can make a hypercall to the hypervisor to handle the #VE.
|
|
|
|
The "just works" MSRs do not need any special guest handling. They might
|
|
be implemented by directly passing through the MSR to the hardware or by
|
|
trapping and handling in the TDX module. Other than possibly being slow,
|
|
these MSRs appear to function just as they would on bare metal.
|
|
|
|
CPUID Behavior
|
|
--------------
|
|
|
|
For some CPUID leaves and sub-leaves, the virtualized bit fields of CPUID
|
|
return values (in guest EAX/EBX/ECX/EDX) are configurable by the
|
|
hypervisor. For such cases, the Intel TDX module architecture defines two
|
|
virtualization types:
|
|
|
|
- Bit fields for which the hypervisor controls the value seen by the guest
|
|
TD.
|
|
|
|
- Bit fields for which the hypervisor configures the value such that the
|
|
guest TD either sees their native value or a value of 0. For these bit
|
|
fields, the hypervisor can mask off the native values, but it can not
|
|
turn *on* values.
|
|
|
|
A #VE is generated for CPUID leaves and sub-leaves that the TDX module does
|
|
not know how to handle. The guest kernel may ask the hypervisor for the
|
|
value with a hypercall.
|
|
|
|
#VE on Memory Accesses
|
|
======================
|
|
|
|
There are essentially two classes of TDX memory: private and shared.
|
|
Private memory receives full TDX protections. Its content is protected
|
|
against access from the hypervisor. Shared memory is expected to be
|
|
shared between guest and hypervisor and does not receive full TDX
|
|
protections.
|
|
|
|
A TD guest is in control of whether its memory accesses are treated as
|
|
private or shared. It selects the behavior with a bit in its page table
|
|
entries. This helps ensure that a guest does not place sensitive
|
|
information in shared memory, exposing it to the untrusted hypervisor.
|
|
|
|
#VE on Shared Memory
|
|
--------------------
|
|
|
|
Access to shared mappings can cause a #VE. The hypervisor ultimately
|
|
controls whether a shared memory access causes a #VE, so the guest must be
|
|
careful to only reference shared pages it can safely handle a #VE. For
|
|
instance, the guest should be careful not to access shared memory in the
|
|
#VE handler before it reads the #VE info structure (TDG.VP.VEINFO.GET).
|
|
|
|
Shared mapping content is entirely controlled by the hypervisor. The guest
|
|
should only use shared mappings for communicating with the hypervisor.
|
|
Shared mappings must never be used for sensitive memory content like kernel
|
|
stacks. A good rule of thumb is that hypervisor-shared memory should be
|
|
treated the same as memory mapped to userspace. Both the hypervisor and
|
|
userspace are completely untrusted.
|
|
|
|
MMIO for virtual devices is implemented as shared memory. The guest must
|
|
be careful not to access device MMIO regions unless it is also prepared to
|
|
handle a #VE.
|
|
|
|
#VE on Private Pages
|
|
--------------------
|
|
|
|
An access to private mappings can also cause a #VE. Since all kernel
|
|
memory is also private memory, the kernel might theoretically need to
|
|
handle a #VE on arbitrary kernel memory accesses. This is not feasible, so
|
|
TDX guests ensure that all guest memory has been "accepted" before memory
|
|
is used by the kernel.
|
|
|
|
A modest amount of memory (typically 512M) is pre-accepted by the firmware
|
|
before the kernel runs to ensure that the kernel can start up without
|
|
being subjected to a #VE.
|
|
|
|
The hypervisor is permitted to unilaterally move accepted pages to a
|
|
"blocked" state. However, if it does this, page access will not generate a
|
|
#VE. It will, instead, cause a "TD Exit" where the hypervisor is required
|
|
to handle the exception.
|
|
|
|
Linux #VE handler
|
|
=================
|
|
|
|
Just like page faults or #GP's, #VE exceptions can be either handled or be
|
|
fatal. Typically, an unhandled userspace #VE results in a SIGSEGV.
|
|
An unhandled kernel #VE results in an oops.
|
|
|
|
Handling nested exceptions on x86 is typically nasty business. A #VE
|
|
could be interrupted by an NMI which triggers another #VE and hilarity
|
|
ensues. The TDX #VE architecture anticipated this scenario and includes a
|
|
feature to make it slightly less nasty.
|
|
|
|
During #VE handling, the TDX module ensures that all interrupts (including
|
|
NMIs) are blocked. The block remains in place until the guest makes a
|
|
TDG.VP.VEINFO.GET TDCALL. This allows the guest to control when interrupts
|
|
or a new #VE can be delivered.
|
|
|
|
However, the guest kernel must still be careful to avoid potential
|
|
#VE-triggering actions (discussed above) while this block is in place.
|
|
While the block is in place, any #VE is elevated to a double fault (#DF)
|
|
which is not recoverable.
|
|
|
|
MMIO handling
|
|
=============
|
|
|
|
In non-TDX VMs, MMIO is usually implemented by giving a guest access to a
|
|
mapping which will cause a VMEXIT on access, and then the hypervisor
|
|
emulates the access. That is not possible in TDX guests because VMEXIT
|
|
will expose the register state to the host. TDX guests don't trust the host
|
|
and can't have their state exposed to the host.
|
|
|
|
In TDX, MMIO regions typically trigger a #VE exception in the guest. The
|
|
guest #VE handler then emulates the MMIO instruction inside the guest and
|
|
converts it into a controlled TDCALL to the host, rather than exposing
|
|
guest state to the host.
|
|
|
|
MMIO addresses on x86 are just special physical addresses. They can
|
|
theoretically be accessed with any instruction that accesses memory.
|
|
However, the kernel instruction decoding method is limited. It is only
|
|
designed to decode instructions like those generated by io.h macros.
|
|
|
|
MMIO access via other means (like structure overlays) may result in an
|
|
oops.
|
|
|
|
Shared Memory Conversions
|
|
=========================
|
|
|
|
All TDX guest memory starts out as private at boot. This memory can not
|
|
be accessed by the hypervisor. However, some kernel users like device
|
|
drivers might have a need to share data with the hypervisor. To do this,
|
|
memory must be converted between shared and private. This can be
|
|
accomplished using some existing memory encryption helpers:
|
|
|
|
* set_memory_decrypted() converts a range of pages to shared.
|
|
* set_memory_encrypted() converts memory back to private.
|
|
|
|
Device drivers are the primary user of shared memory, but there's no need
|
|
to touch every driver. DMA buffers and ioremap() do the conversions
|
|
automatically.
|
|
|
|
TDX uses SWIOTLB for most DMA allocations. The SWIOTLB buffer is
|
|
converted to shared on boot.
|
|
|
|
For coherent DMA allocation, the DMA buffer gets converted on the
|
|
allocation. Check force_dma_unencrypted() for details.
|
|
|
|
References
|
|
==========
|
|
|
|
TDX reference material is collected here:
|
|
|
|
https://www.intel.com/content/www/us/en/developer/articles/technical/intel-trust-domain-extensions.html
|