What: security/secrets/coco
Date: February 2022
Contact: Dov Murik <[email protected]>
Description:
Exposes confidential computing (coco) EFI secrets to
userspace via securityfs.
EFI can declare memory area used by confidential computing
platforms (such as AMD SEV and SEV-ES) for secret injection by
the Guest Owner during VM's launch. The secrets are encrypted
by the Guest Owner and decrypted inside the trusted enclave,
and therefore are not readable by the untrusted host.
The efi_secret module exposes the secrets to userspace. Each
secret appears as a file under <securityfs>/secrets/coco,
where the filename is the GUID of the entry in the secrets
table. This module is loaded automatically by the EFI driver
if the EFI secret area is populated.
Two operations are supported for the files: read and unlink.
Reading the file returns the content of secret entry.
Unlinking the file overwrites the secret data with zeroes and
removes the entry from the filesystem. A secret cannot be read
after it has been unlinked.
For example, listing the available secrets::
# modprobe efi_secret
# ls -l /sys/kernel/security/secrets/coco
-r--r----- 1 root root 0 Jun 28 11:54 736870e5-84f0-4973-92ec-06879ce3da0b
-r--r----- 1 root root 0 Jun 28 11:54 83c83f7f-1356-4975-8b7e-d3a0b54312c6
-r--r----- 1 root root 0 Jun 28 11:54 9553f55d-3da2-43ee-ab5d-ff17f78864d2
-r--r----- 1 root root 0 Jun 28 11:54 e6f5a162-d67f-4750-a67c-5d065f2a9910
Reading the secret data by reading a file::
# cat /sys/kernel/security/secrets/coco/e6f5a162-d67f-4750-a67c-5d065f2a9910
the-content-of-the-secret-data
Wiping a secret by unlinking a file::
# rm /sys/kernel/security/secrets/coco/e6f5a162-d67f-4750-a67c-5d065f2a9910
# ls -l /sys/kernel/security/secrets/coco
-r--r----- 1 root root 0 Jun 28 11:54 736870e5-84f0-4973-92ec-06879ce3da0b
-r--r----- 1 root root 0 Jun 28 11:54 83c83f7f-1356-4975-8b7e-d3a0b54312c6
-r--r----- 1 root root 0 Jun 28 11:54 9553f55d-3da2-43ee-ab5d-ff17f78864d2
Note: The binary format of the secrets table injected by the
Guest Owner is described in
drivers/virt/coco/efi_secret/efi_secret.c under "Structure of
the EFI secret area".
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