Re: [RFC PATCH 00/13] XOM for KVM guest userspace

[Andy Lutomirski]

On Fri, Oct 4, 2019 at 1:10 PM Edgecombe, Rick P
<rick.p.edgecombe@xxxxxxxxx> wrote:
>
> On Fri, 2019-10-04 at 07:56 -0700, Andy Lutomirski wrote:
> > On Thu, Oct 3, 2019 at 2:38 PM Rick Edgecombe
> > <rick.p.edgecombe@xxxxxxxxx> wrote:
> > >
> > > This patchset enables the ability for KVM guests to create execute-only (XO)
> > > memory by utilizing EPT based XO permissions. XO memory is currently
> > > supported
> > > on Intel hardware natively for CPU's with PKU, but this enables it on older
> > > platforms, and can support XO for kernel memory as well.
> >
> > The patchset seems to sometimes call this feature "XO" and sometimes
> > call it "NR".  To me, XO implies no-read and no-write, whereas NR
> > implies just no-read.  Can you please clarify *exactly* what the new
> > bit does and be consistent?
> >
> > I suggest that you make it NR, which allows for PROT_EXEC and
> > PROT_EXEC|PROT_WRITE and plain PROT_WRITE.  WX is of dubious value,
> > but I can imagine plain W being genuinely useful for logging and for
> > JITs that could maintain a W and a separate X mapping of some code.
> > In other words, with an NR bit, all 8 logical access modes are
> > possible.  Also, keeping the paging bits more orthogonal seems nice --
> > we already have a bit that controls write access.
>
> Sorry, yes the behavior of this bit needs to be documented a lot better. I will
> definitely do this for the next version.
>
> To clarify, since the EPT permissions in the XO/NR range are executable, and not
> readable or writeable the new bit really means XO, but only when NX is 0 since
> the guest page tables are being checked as well. When NR=1, W=1, and NX=0, the
> memory is still XO.
>
> NR was picked over XO because as you say. The idea is that it can be defined
> that in the case of KVM XO, NR and writable is not a valid combination, like
> writeable but not readable is defined as not valid for the EPT.
>

Ugh, I see, this is an "EPT Misconfiguration".  Oh, well.  I guess
just keep things as they are and document things better, please.
Don't try to emulate.

I don't suppose Intel could be convinced to get rid of that in a
future CPU and allow write-only memory?

BTW, is your patch checking for support in IA32_VMX_EPT_VPID_CAP?  I
didn't notice it, but I didn't look that hard.