mirror of https://gitee.com/openkylin/linux.git
91 lines
4.3 KiB
Plaintext
91 lines
4.3 KiB
Plaintext
Secure Memory Encryption (SME) and Secure Encrypted Virtualization (SEV) are
|
|
features 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.
|
|
|
|
SEV enables running encrypted virtual machines (VMs) in which the code and data
|
|
of the guest VM are secured so that a decrypted version is available only
|
|
within the VM itself. SEV guest VMs have the concept of private and shared
|
|
memory. Private memory is encrypted with the guest-specific key, while shared
|
|
memory may be encrypted with hypervisor key. When SME is enabled, the hypervisor
|
|
key is the same key which is used in SME.
|
|
|
|
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 encrypted.
|
|
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.
|
|
|
|
When SEV is enabled, instruction pages and guest page tables are always treated
|
|
as private. All the DMA operations inside the guest must be performed on shared
|
|
memory. Since the memory encryption bit is controlled by the guest OS when it
|
|
is operating in 64-bit or 32-bit PAE mode, in all other modes the SEV hardware
|
|
forces the memory encryption bit to 1.
|
|
|
|
Support for SME and SEV can be determined through the CPUID instruction. The
|
|
CPUID function 0x8000001f reports information related to SME:
|
|
|
|
0x8000001f[eax]:
|
|
Bit[0] indicates support for SME
|
|
Bit[1] indicates support for SEV
|
|
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
|
|
|
|
If SEV is supported, MSR 0xc0010131 (MSR_AMD64_SEV) can be used to determine if
|
|
SEV is active:
|
|
|
|
0xc0010131:
|
|
Bit[0] 0 = memory encryption is not active
|
|
1 = memory encryption is active
|
|
|
|
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.
|