ndctl wait-overwrite <nmem0> [<nmem1>..<nmemN>] [<options>]
The kernel provides a POLL(2) capable sysfs file (security) to indicate the state of overwrite. This command waits for a change in the state of this file across all specified dimms.
A nmemX device name, or a dimm id number. Restrict the operation to the specified dimm(s). The keyword all can be specified to indicate the lack of any restriction, however this is the same as not supplying a --dimm option at all.
- -b, --bus=
A bus id number, or a provider string (e.g. "ACPI.NFIT"). Restrict the operation to the specified bus(es). The keyword all can be specified to indicate the lack of any restriction, however this is the same as not supplying a --bus option at all.
- -v, --verbose
Emit debug messages.
Theory of Operation
The Intel Device Specific Methods (DSM) specification v1.7 and v1.8  introduced the following security management operations: enable passphrase, update passphrase, unlock DIMM, disable security, freeze security, secure (crypto) erase, overwrite, master passphrase enable, master passphrase update, and master passphrase secure erase.
The security management for NVDIMMs is comprised of two parts. The front end uses the Linux key management framework (trusted and encrypted keys ) to store the encrypted passphrases in the kernel-managed keyring. The interface for this is the keyutils utility which uses the key management APIs in the Linux kernel. The back end takes the decrypted payload (which is the DIMM passphrase) and passes it to the DIMM.
Unlike other DSMs which are composed by libndctl and sent to the kernel via an ioctl, the security DSMs are managed through the security sysfs attribute under the dimm device. A key-ID is written to the security attribute and the kernel pulls the associated key material from the user keyring that is maintained by the kernel.
The security process begins with the generation of a master key that is used to seal (encrypt) the passphrase for the DIMM. There can either be one common master key that is used to encrypt every DIMM’s passphrase, or a separate key can be generated for each DIMM. The master key is also referred to as the key-encryption-key (kek). The kek can either be generated by the TPM (Trusted Platform Module) on the system, or alternatively, the System Master Key can also be used as the kek
For testing purposes a user key with randomized payload can also be used as a kek. See  for details. To perform any security operations, it is expected that the kek has been added to the kernel’s user keyring as shown in example below:
# keyctl show Session Keyring 736023423 --alswrv 0 0 keyring: _ses 675104189 --alswrv 0 65534 \_ keyring: _uid.0 680187394 --alswrv 0 0 \_ trusted: nvdimm-master
Before performing any of the security operations, all the regions associated with the DIMM in question need to be disabled. For the overwrite operation, in addition to the regions, the dimm also needs to be disabled.
 http://pmem.io/documents/NVDIMM_DSM_Interface-V1.8.pdf  https://www.kernel.org/doc/Documentation/security/keys/trusted-encrypted.rst
The following sub-sections describe specifics of each security feature.
Unlock is performed by the kernel, however a preparation step must happen before the unlock DSM can be issued by the kernel. It is expected that from the initramfs, a setup command (ndctl load-keys) is executed before the libnvdimm module is loaded by modprobe. This command will inject the kek and the encrypted passphrases into the kernel’s user keyring. During the probe of the libnvdimm driver, it will:
- Check the security state of the device and see if the DIMM is locked
- Request the associated encrypted passphrase from the kernel’s user key ring
- Use the kek to decrypt the passphrase
- Create the unlock DSM, copy the decrypted payload into the DSM
- Issue the DSM to unlock the DIMM
If the DIMM is already unlocked, the kernel will attempt to revalidate the passphrase. If we fail to revalidate the passphrase, the kernel will freeze the security and disallow any further security configuration changes. A kernel module parameter is available to override this behavior.
Setup User Passphrase
To setup the passphrase for a DIMM, it is expected that the kek to be used is present in the kernel’s user keyring. The kek encrypts the DIMM passphrase using the enc32 key format. The plaintext passphrase is never provided by or made visible to the user. It is instead randomly generated by the kernel and userspace does not have access to it. Upon encryption, a binary blob of the passphrase is written to the passphrase blob storage directory (/etc/ndctl/keys). The user is responsible for backing up the passphrase blobs to a secure location.
Update User Passphrase
The update user passphrase operation uses the same DSM command as enable user passphrase. Most of the work is done on the key management side. The user has the option of providing a new kek for the new passphrase, but continuing to use the existing kek is also acceptable. The following operations are performed for update-passphrase:
- Remove the encrypted passphrase from the kernel’s user keyring.
- Rename the passphrase blob to old.
- Load this old passphrase blob into the keyring with an "old" name.
- Create the new passphrase and encrypt with the kek.
- Send DSM with the old and new decrypted passphrases.
- Remove old passphrase and the passphrase blob from the keyring.
Remove User Passphrase
The key-ID for the passphrase to be removed is written to sysfs. The kernel then sends the DSM to disable security, and the passphrase is then removed from the keyring, and the associated passphrase blob is deleted.
Crypto (Secure) Erase
This operation is similar to remove-passphrase. The kernel issues a WBINVD instruction before and after the operation to ensure no data corruption from a stale CPU cache. Use ndctl’s sanitize-dimm command with the --crypto-erase option to perform this operation.
This is invoked using --overwrite option for ndctl sanitize-dimm. The overwrite operation wipes the entire NVDIMM. The operation can take a significant amount of time. NOTE: When the command returns successfully, it just means overwrite has been successfully started, and not that the overwrite is complete. Subsequently, ndctl wait-overwrite can be used to wait for the NVDIMMs that are performing overwrite. Upon successful completion of an overwrite, the WBINVD instruction is issued by the kernel. If both --crypto-erase and --overwrite options are supplied, then crypto-erase is performed before overwrite.
This operation does not require a passphrase. This will cause any security command other than a status query to be locked out until the next boot.
MASTER PASSPHRASE SETUP, UPDATE, and CRYPTO ERASE
These operations are similar to the user passphrase enable and update. The only difference is that a different passphrase is used. The master passphrase has no relation to the master key (kek) which is used for encryption of either passphrase.
Copyright © 2016 - 2022, Intel Corporation. License GPLv2: GNU GPL version 2 http://gnu.org/licenses/gpl.html. This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law.