Software Guard eXtensions (SGX) — QEMU documentation

# Software Guard eXtensions (SGX)

## Overview

Intel Software Guard eXtensions (SGX) is a set of instructions and mechanisms for memory accesses in order to provide security accesses for sensitive applications and data. SGX allows an application to use it’s pariticular address space as an enclave, which is a protected area provides confidentiality and integrity even in the presence of privileged malware. Accesses to the enclave memory area from any software not resident in the enclave are prevented, including those from privileged software.

## Virtual SGX

SGX feature is exposed to guest via SGX CPUID. Looking at SGX CPUID, we can report the same CPUID info to guest as on host for most of SGX CPUID. With reporting the same CPUID guest is able to use full capacity of SGX, and KVM doesn’t need to emulate those info.

The guest’s EPC base and size are determined by QEMU, and KVM needs QEMU to notify such info to it before it can initialize SGX for guest.

### Virtual EPC

By default, QEMU does not assign EPC to a VM, i.e. fully enabling SGX in a VM requires explicit allocation of EPC to the VM. Similar to other specialized memory types, e.g. hugetlbfs, EPC is exposed as a memory backend.

SGX EPC is enumerated through CPUID, i.e. EPC “devices” need to be realized prior to realizing the vCPUs themselves, which occurs long before generic devices are parsed and realized. This limitation means that EPC does not require -maxmem as EPC is not treated as {cold,hot}plugged memory.

QEMU does not artificially restrict the number of EPC sections exposed to a guest, e.g. QEMU will happily allow you to create 64 1M EPC sections. Be aware that some kernels may not recognize all EPC sections, e.g. the Linux SGX driver is hardwired to support only 8 EPC sections.

The following QEMU snippet creates two EPC sections, with 64M pre-allocated to the VM and an additional 28M mapped but not allocated:

    -object memory-backend-epc,id=mem1,size=64M,prealloc=on \
    -object memory-backend-epc,id=mem2,size=28M \
    -M sgx-epc.0.memdev=mem1,sgx-epc.1.memdev=mem2

Note:

The size and location of the virtual EPC are far less restricted compared to physical EPC. Because physical EPC is protected via range registers, the size of the physical EPC must be a power of two (though software sees a subset of the full EPC, e.g. 92M or 128M) and the EPC must be naturally aligned. KVM SGX’s virtual EPC is purely a software construct and only requires the size and location to be page aligned. QEMU enforces the EPC size is a multiple of 4k and will ensure the base of the EPC is 4k aligned. To simplify the implementation, EPC is always located above 4g in the guest physical address space.

### Migration

QEMU/KVM doesn’t prevent live migrating SGX VMs, although from hardware’s perspective, SGX doesn’t support live migration, since both EPC and the SGX key hierarchy are bound to the physical platform. However live migration can be supported in the sense if guest software stack can support recreating enclaves when it suffers sudden lose of EPC; and if guest enclaves can detect SGX keys being changed, and handle gracefully. For instance, when ERESUME fails with #PF.SGX, guest software can gracefully detect it and recreate enclaves; and when enclave fails to unseal sensitive information from outside, it can detect such error and sensitive information can be provisioned to it again.

### CPUID

Due to its myriad dependencies, SGX is currently not listed as supported in any of QEMU’s built-in CPU configuration. To expose SGX (and SGX Launch Control) to a guest, you must either use `-cpu host` to pass-through the host CPU model, or explicitly enable SGX when using a built-in CPU model, e.g. via `-cpu <model>,+sgx` or `-cpu <model>,+sgx,+sgxlc`.

All SGX sub-features enumerated through CPUID, e.g. SGX2, MISCSELECT, ATTRIBUTES, etc… can be restricted via CPUID flags. Be aware that enforcing restriction of MISCSELECT, ATTRIBUTES and XFRM requires intercepting ECREATE, i.e. may marginally reduce SGX performance in the guest. All SGX sub-features controlled via -cpu are prefixed with “sgx”, e.g.:

    $ qemu-system-x86_64 -cpu help | xargs printf "%s\n" | grep sgx
    sgx
    sgx-debug
    sgx-encls-c
    sgx-enclv
    sgx-exinfo
    sgx-kss
    sgx-mode64
    sgx-provisionkey
    sgx-tokenkey
    sgx1
    sgx2
    sgxlc

The following QEMU snippet passes through the host CPU but restricts access to the provision and EINIT token keys:

    -cpu host,-sgx-provisionkey,-sgx-tokenkey

SGX sub-features cannot be emulated, i.e. sub-features that are not present in hardware cannot be forced on via ‘-cpu’.

### Virtualize SGX Launch Control

QEMU SGX support for Launch Control (LC) is passive, in the sense that it does not actively change the LC configuration. QEMU SGX provides the user the ability to set/clear the CPUID flag (and by extension the associated IA32_FEATURE_CONTROL MSR bit in fw_cfg) and saves/restores the LE Hash MSRs when getting/putting guest state, but QEMU does not add new controls to directly modify the LC configuration. Similar to hardware behavior, locking the LC configuration to a non-Intel value is left to guest firmware. Unlike host bios setting for SGX launch control(LC), there is no special bios setting for SGX guest by our design. If host is in locked mode, we can still allow creating VM with SGX.

### Feature Control

QEMU SGX updates the `etc/msr_feature_control` fw_cfg entry to set the SGX (bit 18) and SGX LC (bit 17) flags based on their respective CPUID support, i.e. existing guest firmware will automatically set SGX and SGX LC accordingly, assuming said firmware supports fw_cfg.msr_feature_control.

## Launching a guest

To launch a SGX guest:

    qemu-system-x86_64 \
     -cpu host,+sgx-provisionkey \
     -object memory-backend-epc,id=mem1,size=64M,prealloc=on \
     -M sgx-epc.0.memdev=mem1,sgx-epc.0.node=0

Utilizing SGX in the guest requires a kernel/OS with SGX support. The support can be determined in guest by:

    $ grep sgx /proc/cpuinfo

and SGX epc info by:

    $ dmesg | grep sgx
    [    0.182807] sgx: EPC section 0x140000000-0x143ffffff
    [    0.183695] sgx: [Firmware Bug]: Unable to map EPC section to online node. Fallback to the NUMA node 0.

To launch a SGX numa guest:

    qemu-system-x86_64 \
     -cpu host,+sgx-provisionkey \
     -object memory-backend-ram,size=2G,host-nodes=0,policy=bind,id=node0 \
     -object memory-backend-epc,id=mem0,size=64M,prealloc=on,host-nodes=0,policy=bind \
     -numa node,nodeid=0,cpus=0-1,memdev=node0 \
     -object memory-backend-ram,size=2G,host-nodes=1,policy=bind,id=node1 \
     -object memory-backend-epc,id=mem1,size=28M,prealloc=on,host-nodes=1,policy=bind \
     -numa node,nodeid=1,cpus=2-3,memdev=node1 \
     -M sgx-epc.0.memdev=mem0,sgx-epc.0.node=0,sgx-epc.1.memdev=mem1,sgx-epc.1.node=1

and SGX epc numa info by:

    $ dmesg | grep sgx
    [    0.369937] sgx: EPC section 0x180000000-0x183ffffff
    [    0.370259] sgx: EPC section 0x184000000-0x185bfffff

    $ dmesg | grep SRAT
    [    0.009981] ACPI: SRAT: Node 0 PXM 0 [mem 0x180000000-0x183ffffff]
    [    0.009982] ACPI: SRAT: Node 1 PXM 1 [mem 0x184000000-0x185bfffff]

## References

- SGX Homepage

- SGX SDK

- SGX specification: Intel SDM Volume 3