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AMD Secure Encrypted Virtualization (SEV) patch for Linux kernel

Brijesh Singh of AMD submitted a 28-part patch to the Linux-(kernel,efi,kvm,…) mailing lists, with a patch for the the Linux kernel to support AMD’s Secure Encrypted Virtualization (SEV), which relies on the recent AMD Secure Memory Encryption (SME) patch by Tom Lendacky of AMD. I’m excerpting the intro text from part 1/28:

[RFC PATCH v1 00/28] x86: Secure Encrypted Virtualization (AMD)

This RFC series provides support for AMD’s new Secure Encrypted Virtualization (SEV) feature. This RFC is build upon Secure Memory Encryption (SME) RFC.

SEV is an extension to the AMD-V architecture which supports running multiple VMs under the control of a hypervisor. When enabled, SEV hardware tags all code and data with its VM ASID which indicates which VM the data originated from or is intended for. This tag is kept with the data at all times when inside the SOC, and prevents that data from being used by anyone other than the owner. While the tag protects VM data inside the SOC, AES with 128 bit encryption protects data outside the SOC. When data leaves or enters the SOC, it is encrypted/decrypted respectively by hardware with a key based on the associated tag.

SEV guest VMs have the concept of private and shared memory.  Private memory is encrypted with the  guest-specific key, while shared memory may be encrypted with hypervisor key.  Certain types of memory (namely instruction pages and guest page tables) are always treated as private memory by the hardware. For data memory, SEV guest VMs can choose which pages they would like to be private. The choice is done using the standard CPU page tables using the C-bit, and is fully controlled by the guest. Due to security reasons all the DMA operations inside the  guest must be performed on shared pages (C-bit clear).  Note that since C-bit is only controllable by the guest OS when it is operating in 64-bit or 32-bit PAE mode, in all other modes the SEV hardware forces the C-bit to a 1.

SEV is designed to protect guest VMs from a benign but vulnerable (i.e. not fully malicious) hypervisor. In particular, it reduces the attack surface of guest VMs and can prevent certain types of VM-escape bugs (e.g. hypervisor read-anywhere) from being used to steal guest data.

The RFC series also includes a crypto driver (psp.ko) which communicates with SEV firmware that runs within the AMD secure processor provides a secure key management interfaces. The hypervisor uses this interface to enable SEV for secure guest and perform common hypervisor activities such as launching, running, snapshotting , migrating and debugging a  guest. A new ioctl (KVM_SEV_ISSUE_CMD) is introduced which will enable Qemu to send commands to the SEV firmware during guest life cycle.

The RFC series also includes patches required in guest OS to enable SEV feature. A guest OS can check SEV support by calling KVM_FEATURE cpuid  instruction.

The following links provide additional details:

AMD Memory Encryption whitepaper:

http://amd-dev.wpengine.netdna-cdn.com/wordpress/media/2013/12/AMD_Memory_Encryption_Whitepaper_v7-Public.pdf

AMD64 Architecture Programmer’s Manual:
http://support.amd.com/TechDocs/24593.pdf
SME is section 7.10
SEV is section 15.34

Secure Encrypted Virutualization Key Management:
http://support.amd.com/TechDocs/55766_SEV-KM%20API_Spec.pdf

See the full patch for more information.
https://lkml.org/lkml/2016/8/22/960

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