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

[Kai Huang]

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?

> 
> 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?

Thanks,
-Kai