Re: [PATCH, RFC 45/62] mm: Add the encrypt_mprotect() system call for MKTME

From: Kai Huang
Date: Mon Jun 17 2019 - 22:16:46 EST

On Mon, 2019-06-17 at 18:50 -0700, Andy Lutomirski wrote:
> On Mon, Jun 17, 2019 at 5:48 PM Kai Huang <kai.huang@xxxxxxxxxxxxxxx> wrote:
> >
> >
> > >
> > > > And another silly argument: if we had /dev/mktme, then we could
> > > > possibly get away with avoiding all the keyring stuff entirely.
> > > > Instead, you open /dev/mktme and you get your own key under the hook.
> > > > If you want two keys, you open /dev/mktme twice. If you want some
> > > > other program to be able to see your memory, you pass it the fd.
> > >
> > > We still like the keyring because it's one-stop-shopping as the place
> > > that *owns* the hardware KeyID slots. Those are global resources and
> > > scream for a single global place to allocate and manage them. The
> > > hardware slots also need to be shared between any anonymous and
> > > file-based users, no matter what the APIs for the anonymous side.
> >
> > MKTME driver (who creates /dev/mktme) can also be the one-stop-shopping. I think whether to
> > choose
> > keyring to manage MKTME key should be based on whether we need/should take advantage of existing
> > key
> > retention service functionalities. For example, with key retention service we can
> > revoke/invalidate/set expiry for a key (not sure whether MKTME needs those although), and we
> > have
> > several keyrings -- thread specific keyring, process specific keyring, user specific keyring,
> > etc,
> > thus we can control who can/cannot find the key, etc. I think managing MKTME key in MKTME driver
> > doesn't have those advantages.
> >
> Trying to evaluate this with the current proposed code is a bit odd, I
> think. Suppose you create a thread-specific key and then fork(). The
> child can presumably still use the key regardless of whether the child
> can nominally access the key in the keyring because the PTEs are still
> there.

Right. This is a little bit odd, although virtualization (Qemu, which is the main use case of MKTME
at least so far) doesn't use fork().

> More fundamentally, in some sense, the current code has no semantics.
> Associating a key with memory and "encrypting" it doesn't actually do
> anything unless you are attacking the memory bus but you haven't
> compromised the kernel. There's no protection against a guest that
> can corrupt its EPT tables, there's no protection against kernel bugs
> (*especially* if the duplicate direct map design stays), and there
> isn't even any fd or other object around by which you can only access
> the data if you can see the key.

I am not saying managing MKTME key/keyID in key retention service is definitely better, but it seems
all those you mentioned are not related to whether to choose key retention service to manage MKTME
key/keyID? Or you are saying it doesn't matter we manage key/keyID in key retention service or in
MKTME driver, since MKTME barely have any security benefits (besides physical attack)?

> I'm also wondering whether the kernel will always be able to be a
> one-stop shop for key allocation -- if the MKTME hardware gains
> interesting new uses down the road, who knows how key allocation will
> work?

I by now don't have any use case which requires to manage key/keyID specifically for its own use,
rather than letting kernel to manage keyID allocation. Please inspire us if you have any potential.