[RFC v2 12/13] keys/mktme: Save MKTME data if kernel cmdline parameter allows

[Huang, Kai]

On Mon, 2018-12-03 at 23:39 -0800, Alison Schofield wrote:
> MKTME (Multi-Key Total Memory Encryption) key payloads may include
> data encryption keys, tweak keys, and additional entropy bits. These
> are used to program the MKTME encryption hardware. By default, the
> kernel destroys this payload data once the hardware is programmed.
> 
> However, in order to fully support CPU Hotplug, saving the key data
> becomes important. The MKTME Key Service cannot allow a new physical
> package to come online unless it can program the new packages Key Table
> to match the Key Tables of all existing physical packages.
> 
> With CPU generated keys (a.k.a. random keys or ephemeral keys) the
> saving of user key data is not an issue. The kernel and MKTME hardware
> can generate strong encryption keys without recalling any user supplied
> data.
> 
> With USER directed keys (a.k.a. user type) saving the key programming
> data (data and tweak key) becomes an issue. The data and tweak keys
> are required to program those keys on a new physical package.
> 
> In preparation for adding CPU hotplug support:
> 
>    Add an 'mktme_vault' where key data is stored.
> 
>    Add 'mktme_savekeys' kernel command line parameter that directs
>    what key data can be stored. If it is not set, kernel does not
>    store users data key or tweak key.
> 
>    Add 'mktme_bitmap_user_type' to track when USER type keys are in
>    use. If no USER type keys are currently in use, a physical package
>    may be brought online, despite the absence of 'mktme_savekeys'.

Overall, I am not sure whether saving key is good idea, since it breaks coldboot attack IMHO. We
need to tradeoff between supporting CPU hotplug and security. I am not sure whether supporting CPU
hotplug is that important, since for some other features such as SGX, we don't support CPU hotplug
anyway.

Alternatively, we can choose to use per-socket keyID, but not to program keyID globally across all
sockets, so you don't have to save key while still supporting CPU hotplug.

Thanks,
-Kai