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97 lines
4.5 KiB
97 lines
4.5 KiB
.. SPDX-License-Identifier: GPL-2.0 |
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===================== |
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AMD Memory Encryption |
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===================== |
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Secure Memory Encryption (SME) and Secure Encrypted Virtualization (SEV) are |
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features found on AMD processors. |
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SME provides the ability to mark individual pages of memory as encrypted using |
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the standard x86 page tables. A page that is marked encrypted will be |
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automatically decrypted when read from DRAM and encrypted when written to |
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DRAM. SME can therefore be used to protect the contents of DRAM from physical |
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attacks on the system. |
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SEV enables running encrypted virtual machines (VMs) in which the code and data |
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of the guest VM are secured so that a decrypted version is available only |
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within the VM itself. SEV guest VMs have the concept of private and shared |
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memory. Private memory is encrypted with the guest-specific key, while shared |
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memory may be encrypted with hypervisor key. When SME is enabled, the hypervisor |
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key is the same key which is used in SME. |
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A page is encrypted when a page table entry has the encryption bit set (see |
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below on how to determine its position). The encryption bit can also be |
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specified in the cr3 register, allowing the PGD table to be encrypted. Each |
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successive level of page tables can also be encrypted by setting the encryption |
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bit in the page table entry that points to the next table. This allows the full |
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page table hierarchy to be encrypted. Note, this means that just because the |
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encryption bit is set in cr3, doesn't imply the full hierarchy is encrypted. |
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Each page table entry in the hierarchy needs to have the encryption bit set to |
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achieve that. So, theoretically, you could have the encryption bit set in cr3 |
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so that the PGD is encrypted, but not set the encryption bit in the PGD entry |
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for a PUD which results in the PUD pointed to by that entry to not be |
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encrypted. |
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When SEV is enabled, instruction pages and guest page tables are always treated |
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as private. All the DMA operations inside the guest must be performed on shared |
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memory. Since the memory encryption bit is controlled by the guest OS when it |
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is operating in 64-bit or 32-bit PAE mode, in all other modes the SEV hardware |
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forces the memory encryption bit to 1. |
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Support for SME and SEV can be determined through the CPUID instruction. The |
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CPUID function 0x8000001f reports information related to SME:: |
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0x8000001f[eax]: |
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Bit[0] indicates support for SME |
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Bit[1] indicates support for SEV |
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0x8000001f[ebx]: |
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Bits[5:0] pagetable bit number used to activate memory |
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encryption |
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Bits[11:6] reduction in physical address space, in bits, when |
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memory encryption is enabled (this only affects |
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system physical addresses, not guest physical |
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addresses) |
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If support for SME is present, MSR 0xc00100010 (MSR_AMD64_SYSCFG) can be used to |
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determine if SME is enabled and/or to enable memory encryption:: |
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0xc0010010: |
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Bit[23] 0 = memory encryption features are disabled |
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1 = memory encryption features are enabled |
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If SEV is supported, MSR 0xc0010131 (MSR_AMD64_SEV) can be used to determine if |
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SEV is active:: |
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0xc0010131: |
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Bit[0] 0 = memory encryption is not active |
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1 = memory encryption is active |
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Linux relies on BIOS to set this bit if BIOS has determined that the reduction |
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in the physical address space as a result of enabling memory encryption (see |
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CPUID information above) will not conflict with the address space resource |
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requirements for the system. If this bit is not set upon Linux startup then |
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Linux itself will not set it and memory encryption will not be possible. |
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The state of SME in the Linux kernel can be documented as follows: |
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- Supported: |
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The CPU supports SME (determined through CPUID instruction). |
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- Enabled: |
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Supported and bit 23 of MSR_AMD64_SYSCFG is set. |
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- Active: |
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Supported, Enabled and the Linux kernel is actively applying |
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the encryption bit to page table entries (the SME mask in the |
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kernel is non-zero). |
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SME can also be enabled and activated in the BIOS. If SME is enabled and |
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activated in the BIOS, then all memory accesses will be encrypted and it will |
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not be necessary to activate the Linux memory encryption support. If the BIOS |
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merely enables SME (sets bit 23 of the MSR_AMD64_SYSCFG), then Linux can activate |
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memory encryption by default (CONFIG_AMD_MEM_ENCRYPT_ACTIVE_BY_DEFAULT=y) or |
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by supplying mem_encrypt=on on the kernel command line. However, if BIOS does |
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not enable SME, then Linux will not be able to activate memory encryption, even |
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if configured to do so by default or the mem_encrypt=on command line parameter |
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is specified.
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