Revision as of 01:29, 17 December 2017

Secure Memory Encryption (SME) is an x86 instruction set extension introduced by AMD for page-granular memory encryption support using a single ephemeral key. A subset of SME, Transparent SME (SME), is a more limited form of SME used to transparently encrypt the full physical memory. Secure Encrypted Virtualization (SEV) extends SME to AMD-V, allowing individual VMs to run SME using their own secure keys.

Overview

SME was proposed by AMD in their white paper in April 2016. SME adds the ability to mark individual pages of memory as encrypted through the page tables. Any marked page will automatically be encrypted on write and decrypted back when read by software.

Secure Memory Encryption

Secure Memory Encryption (SME) provides the ability for software to market certain pages to be encrypted. Marked pages are automatically decrypted and encrypted upon software read and write. All pages are encrypted using a single 128-bit ephemeral AES key which is created randomly using a hardware random generator at each boot and is not accessible by software. A new key is generated by the processor on every boot.

Transparent SME

Transparent SME (TSME) as the name implies is a stricter subset of SME that requires no software intervention. Under TSME, all memory pages are encrypted regardless of the C-bit value. TSME is designed for legacy OS and hypervisor software that cannot be modified. Note that when TSME is enabled, standard SME as well as SEV are no longer available.

Secure Encrypted Virtualization

Secure Encrypted Virtualization (SEV) is an extension of SME that effectively enables a per-virtual machine SME. In other words, SEV enables running encrypted virtual machines in which the code and data of the VM are private to the VM and may only be decrypted within the VM itself.

Mechanism

SME is typically enabled by BIOS or other firmware at boot time. This is done by setting the appropriate MSR bit to 1. Once activate, software can simply set the encryption C-bit (enCrypted) on the desired page. It's worth noting that the location of the C-bit is actually implementation-specific and must determined by making the appropriate CPUID call. Pages with a C-bit set to 1 go through the encryption engine and are stored encrypted in memory. Likewise, pages with a C-bit set to 0 go directly to memory. This means unencrypted pages do not incur any added latency because of this feature. It's worth noting that encryption I/O pages are not allowed and must have a C-bit of 0.

If SEV is supported and enabled, on each SEV-enabled VM, SME mode is enabled with that VM-specified encryption key. Under SEV, the ASID field in the page table is used as the key index that identifies which encryption key is used to encrypt and decrypt the memory traffic associated with that VM. SEV-enabled VMs can control their own C-bit for memory pages they want encrypted. This allows those VMs to determine which pages are private (C-bit = 1) or shared (C-bit = 0). The location of that C-bit is the same location as defined under SME. It's worth noting that this control is limited to data page. In other words, memory accesses such as guest page tables and instruction fetches are always private, regardless of the value of the C-bit (i.e., in those cases the C-bit is a don't care). This was done in order to ensure that non-guest entities such as the hypervisor itself cannot inject their own code into the SEV-enabled guest VM. If the C-bit is an address bit, this bit is masked from the guest physical address when it is translated through the nested page tables. The hypervisor itself does not need to be aware of which pages the guest VM marked as private. For example, a guest accessing a particular virtual address will get translated to a specific physical address with the C-bit set to 1 indicating the page should be encrypted. A specific virtual address is then used for the translation and the C-bit value from the guest physical address is saved and used on the final physical address after the nested table translation took place.

SEV may be used in conjunction with SME. Under this scenario, each SEV-enabled VM controls its own encryption via the C-bit and the host page tables control the encryption for shared memory.

Key management

Note that key management is actually managed by a separate processor AMD calls the AMD Secure Processor or AMD-SP for short which is present on SEV-enabled chips. Therefore, while a hypervisor can manage the virtual machine keys through the AMD-SP, the actual software is unaware of the key values and cannot access them.