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Secure Memory Encryption (SME) is a feature found on AMD processors.
SME provides the ability to mark individual pages of memory as encrypted using
the standard x86 page tables. A page that is marked encrypted will be
automatically decrypted when read from DRAM and encrypted when written to
DRAM. SME can therefore be used to protect the contents of DRAM from physical
attacks on the system.
A page is encrypted when a page table entry has the encryption bit set (see
below on how to determine its position). The encryption bit can also be
specified in the cr3 register, allowing the PGD table to be encrypted. Each
successive level of page tables can also be encrypted by setting the encryption
bit in the page table entry that points to the next table. This allows the full
page table hierarchy to be encrypted. Note, this means that just because the
encryption bit is set in cr3, doesn't imply the full hierarchy is encyrpted.
Each page table entry in the hierarchy needs to have the encryption bit set to
achieve that. So, theoretically, you could have the encryption bit set in cr3
so that the PGD is encrypted, but not set the encryption bit in the PGD entry
for a PUD which results in the PUD pointed to by that entry to not be
encrypted.
Support for SME can be determined through the CPUID instruction. The CPUID
function 0x8000001f reports information related to SME:
0x8000001f[eax]:
Bit[0] indicates support for SME
0x8000001f[ebx]:
Bits[5:0] pagetable bit number used to activate memory
encryption
Bits[11:6] reduction in physical address space, in bits, when
memory encryption is enabled (this only affects
system physical addresses, not guest physical
addresses)
If support for SME is present, MSR 0xc00100010 (MSR_K8_SYSCFG) can be used to
determine if SME is enabled and/or to enable memory encryption:
0xc0010010:
Bit[23] 0 = memory encryption features are disabled
1 = memory encryption features are enabled
Linux relies on BIOS to set this bit if BIOS has determined that the reduction
in the physical address space as a result of enabling memory encryption (see
CPUID information above) will not conflict with the address space resource
requirements for the system. If this bit is not set upon Linux startup then
Linux itself will not set it and memory encryption will not be possible.
The state of SME in the Linux kernel can be documented as follows:
- Supported:
The CPU supports SME (determined through CPUID instruction).
- Enabled:
Supported and bit 23 of MSR_K8_SYSCFG is set.
- Active:
Supported, Enabled and the Linux kernel is actively applying
the encryption bit to page table entries (the SME mask in the
kernel is non-zero).
SME can also be enabled and activated in the BIOS. If SME is enabled and
activated in the BIOS, then all memory accesses will be encrypted and it will
not be necessary to activate the Linux memory encryption support. If the BIOS
merely enables SME (sets bit 23 of the MSR_K8_SYSCFG), then Linux can activate
memory encryption by default (CONFIG_AMD_MEM_ENCRYPT_ACTIVE_BY_DEFAULT=y) or
by supplying mem_encrypt=on on the kernel command line. However, if BIOS does
not enable SME, then Linux will not be able to activate memory encryption, even
if configured to do so by default or the mem_encrypt=on command line parameter
is specified.
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