Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.
1 Secure Memory Encryption (SME) and Secure Encrypted Virtualization (SEV) are 2 features found on AMD processors. 3 4 SME provides the ability to mark individual pages of memory as encrypted using 5 the standard x86 page tables. A page that is marked encrypted will be 6 automatically decrypted when read from DRAM and encrypted when written to 7 DRAM. SME can therefore be used to protect the contents of DRAM from physical 8 attacks on the system. 9 10 SEV enables running encrypted virtual machines (VMs) in which the code and data 11 of the guest VM are secured so that a decrypted version is available only 12 within the VM itself. SEV guest VMs have the concept of private and shared 13 memory. Private memory is encrypted with the guest-specific key, while shared 14 memory may be encrypted with hypervisor key. When SME is enabled, the hypervisor 15 key is the same key which is used in SME. 16 17 A page is encrypted when a page table entry has the encryption bit set (see 18 below on how to determine its position). The encryption bit can also be 19 specified in the cr3 register, allowing the PGD table to be encrypted. Each 20 successive level of page tables can also be encrypted by setting the encryption 21 bit in the page table entry that points to the next table. This allows the full 22 page table hierarchy to be encrypted. Note, this means that just because the 23 encryption bit is set in cr3, doesn't imply the full hierarchy is encrypted. 24 Each page table entry in the hierarchy needs to have the encryption bit set to 25 achieve that. So, theoretically, you could have the encryption bit set in cr3 26 so that the PGD is encrypted, but not set the encryption bit in the PGD entry 27 for a PUD which results in the PUD pointed to by that entry to not be 28 encrypted. 29 30 When SEV is enabled, instruction pages and guest page tables are always treated 31 as private. All the DMA operations inside the guest must be performed on shared 32 memory. Since the memory encryption bit is controlled by the guest OS when it 33 is operating in 64-bit or 32-bit PAE mode, in all other modes the SEV hardware 34 forces the memory encryption bit to 1. 35 36 Support for SME and SEV can be determined through the CPUID instruction. The 37 CPUID function 0x8000001f reports information related to SME: 38 39 0x8000001f[eax]: 40 Bit[0] indicates support for SME 41 Bit[1] indicates support for SEV 42 0x8000001f[ebx]: 43 Bits[5:0] pagetable bit number used to activate memory 44 encryption 45 Bits[11:6] reduction in physical address space, in bits, when 46 memory encryption is enabled (this only affects 47 system physical addresses, not guest physical 48 addresses) 49 50 If support for SME is present, MSR 0xc00100010 (MSR_K8_SYSCFG) can be used to 51 determine if SME is enabled and/or to enable memory encryption: 52 53 0xc0010010: 54 Bit[23] 0 = memory encryption features are disabled 55 1 = memory encryption features are enabled 56 57 If SEV is supported, MSR 0xc0010131 (MSR_AMD64_SEV) can be used to determine if 58 SEV is active: 59 60 0xc0010131: 61 Bit[0] 0 = memory encryption is not active 62 1 = memory encryption is active 63 64 Linux relies on BIOS to set this bit if BIOS has determined that the reduction 65 in the physical address space as a result of enabling memory encryption (see 66 CPUID information above) will not conflict with the address space resource 67 requirements for the system. If this bit is not set upon Linux startup then 68 Linux itself will not set it and memory encryption will not be possible. 69 70 The state of SME in the Linux kernel can be documented as follows: 71 - Supported: 72 The CPU supports SME (determined through CPUID instruction). 73 74 - Enabled: 75 Supported and bit 23 of MSR_K8_SYSCFG is set. 76 77 - Active: 78 Supported, Enabled and the Linux kernel is actively applying 79 the encryption bit to page table entries (the SME mask in the 80 kernel is non-zero). 81 82 SME can also be enabled and activated in the BIOS. If SME is enabled and 83 activated in the BIOS, then all memory accesses will be encrypted and it will 84 not be necessary to activate the Linux memory encryption support. If the BIOS 85 merely enables SME (sets bit 23 of the MSR_K8_SYSCFG), then Linux can activate 86 memory encryption by default (CONFIG_AMD_MEM_ENCRYPT_ACTIVE_BY_DEFAULT=y) or 87 by supplying mem_encrypt=on on the kernel command line. However, if BIOS does 88 not enable SME, then Linux will not be able to activate memory encryption, even 89 if configured to do so by default or the mem_encrypt=on command line parameter 90 is specified.