69 lines
3.3 KiB
Plaintext
69 lines
3.3 KiB
Plaintext
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Secure Memory Encryption (SME) is a feature 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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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 encyrpted.
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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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Support for SME can be determined through the CPUID instruction. The CPUID
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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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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_K8_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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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_K8_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_K8_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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