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1 | Secure Memory Encryption (SME) and Secure Encrypted Virtualization (SEV) are |
2 | features found on AMD processors. | |
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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 | ||
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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 | ||
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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 | |
33e63acc | 23 | encryption bit is set in cr3, doesn't imply the full hierarchy is encrypted. |
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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 | ||
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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: | |
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38 | |
39 | 0x8000001f[eax]: | |
40 | Bit[0] indicates support for SME | |
33e63acc | 41 | Bit[1] indicates support for SEV |
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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 | ||
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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 | ||
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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. |