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

[Andy Lutomirski]

On Mon, Jun 17, 2019 at 5:05 PM Kai Huang <kai.huang@xxxxxxxxxxxxxxx> wrote:
>
> On Mon, 2019-06-17 at 12:12 -0700, Andy Lutomirski wrote:
> > On Mon, Jun 17, 2019 at 11:37 AM Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
> > >
> > > Tom Lendacky, could you take a look down in the message to the talk of
> > > SEV?  I want to make sure I'm not misrepresenting what it does today.
> > > ...
> > >
> > >
> > > > > I actually don't care all that much which one we end up with.  It's not
> > > > > like the extra syscall in the second options means much.
> > > >
> > > > The benefit of the second one is that, if sys_encrypt is absent, it
> > > > just works.  In the first model, programs need a fallback because
> > > > they'll segfault of mprotect_encrypt() gets ENOSYS.
> > >
> > > Well, by the time they get here, they would have already had to allocate
> > > and set up the encryption key.  I don't think this would really be the
> > > "normal" malloc() path, for instance.
> > >
> > > > >  How do we
> > > > > eventually stack it on top of persistent memory filesystems or Device
> > > > > DAX?
> > > >
> > > > How do we stack anonymous memory on top of persistent memory or Device
> > > > DAX?  I'm confused.
> > >
> > > If our interface to MKTME is:
> > >
> > >         fd = open("/dev/mktme");
> > >         ptr = mmap(fd);
> > >
> > > Then it's hard to combine with an interface which is:
> > >
> > >         fd = open("/dev/dax123");
> > >         ptr = mmap(fd);
> > >
> > > Where if we have something like mprotect() (or madvise() or something
> > > else taking pointer), we can just do:
> > >
> > >         fd = open("/dev/anything987");
> > >         ptr = mmap(fd);
> > >         sys_encrypt(ptr);
> >
> > I'm having a hard time imagining that ever working -- wouldn't it blow
> > up if someone did:
> >
> > fd = open("/dev/anything987");
> > ptr1 = mmap(fd);
> > ptr2 = mmap(fd);
> > sys_encrypt(ptr1);
> >
> > So I think it really has to be:
> > fd = open("/dev/anything987");
> > ioctl(fd, ENCRYPT_ME);
> > mmap(fd);
>
> This requires "/dev/anything987" to support ENCRYPT_ME ioctl, right?
>
> So to support NVDIMM (DAX), we need to add ENCRYPT_ME ioctl to DAX?

Yes and yes, or we do it with layers -- see below.

I don't see how we can credibly avoid this.  If we try to do MKTME
behind the DAX driver's back, aren't we going to end up with cache
coherence problems?

>
> >
> > But I really expect that the encryption of a DAX device will actually
> > be a block device setting and won't look like this at all.  It'll be
> > more like dm-crypt except without device mapper.
>
> Are you suggesting not to support MKTME for DAX, or adding MKTME support to dm-crypt?

I'm proposing exposing it by an interface that looks somewhat like
dm-crypt.  Either we could have a way to create a device layered on
top of the DAX devices that exposes a decrypted view or we add a way
to tell the DAX device to kindly use MKTME with such-and-such key.

If there is demand for a way to have an fscrypt-like thing on top of
DAX where different files use different keys, I suppose that could be
done too, but it will need filesystem or VFS help.