Re: [PATCH Part2 v5 39/45] KVM: SVM: Introduce ops for the post gfn map and unmap

[Marc Orr]

On Wed, Sep 14, 2022 at 5:32 PM Marc Orr <marcorr@xxxxxxxxxx> wrote:
>
> On Wed, Sep 14, 2022 at 5:15 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> >
> > On Wed, Sep 14, 2022, Marc Orr wrote:
> > > On Wed, Sep 14, 2022 at 9:05 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> > > >
> > > > On Thu, Sep 08, 2022, Michael Roth wrote:
> > > > > On Fri, Oct 15, 2021 at 05:16:28PM +0000, Sean Christopherson wrote:
> > > > > So in the context of this interim solution, we're trying to look for a
> > > > > solution that's simple enough that it can be used reliably, without
> > > > > introducing too much additional complexity into KVM. There is one
> > > > > approach that seems to fit that bill, that Brijesh attempted in an
> > > > > earlier version of this series (I'm not sure what exactly was the
> > > > > catalyst to changing the approach, as I wasn't really in the loop at
> > > > > the time, but AIUI there weren't any showstoppers there, but please
> > > > > correct me if I'm missing anything):
> > > > >
> > > > >  - if the host is writing to a page that it thinks is supposed to be
> > > > >    shared, and the guest switches it to private, we get an RMP fault
> > > > >    (actually, we will get a !PRESENT fault, since as of v5 we now
> > > > >    remove the mapping from the directmap as part of conversion)
> > > > >  - in the host #PF handler, if we see that the page is marked private
> > > > >    in the RMP table, simply switch it back to shared
> > > > >  - if this was a bug on the part of the host, then the guest will see
> > > >
> > > > As discussed off-list, attempting to fix up RMP violations in the host #PF handler
> > > > is not a viable approach.  There was also extensive discussion on-list a while back:
> > > >
> > > > https://lore.kernel.org/all/8a244d34-2b10-4cf8-894a-1bf12b59cf92@xxxxxxxxxxxxxxxx
> > >
> > > I mentioned this during Mike's talk at the micro-conference: For pages
> > > mapped in by the kernel can we disallow them to be converted to
> > > private?
> >
> > In theory, yes.  Do we want to do something like this?  No.  kmap() does something
> > vaguely similar for 32-bit PAE/PSE kernels, but that's a lot of complexity and
> > overhead to take on.  And this issue goes far beyond a kmap(); when the kernel gup()s
> > a page, the kernel expects the pfn to be available, no exceptions (pun intended).
> >
> > > Note, userspace accesses are already handled by UPM.
> >
> > I'm confused by the UPM comment.  Isn't the gist of this thread about the ability
> > to merge SNP _without_ UPM?  Or am I out in left field?
>
> I think that was the overall gist: yes. But it's not what I was trying
> to comment on :-).
>
> HOWEVER, thinking about this more: I was confused when I wrote out my
> last reply. I had thought that the issue that Michael brought up
> applied even with UPM. That is, I was thinking it was still possibly
> for a guest to maliciously convert a page to private mapped in by the
> kernel and assumed to be shared.
>
> But I now realize that is not what will actually happen. To be
> concrete, let's assume the GHCB page. What will happen is:
> - KVM has GHCB page mapped in. GHCB is always assumed to be shared. So
> far so good.
> - Malicious guest converts GHCB page to private (e.g., via Page State
> Change request)
> - Guest exits to KVM
> - KVM exits to userspace VMM
> - Userspace VM allocates page in private FD.
>
> Now, what happens here depends on how UPM works. If we allow double
> allocation then our host kernel is safe. However, now we have the
> "double allocation problem".
>
> If on the other hand, we deallocate the page in the shared FD, the
> host kernel can segfault. And now we actually do have essentially the
> same problem Michael was describing that we have without UPM. Because
> we'll end up in fault.c in the kernel context and likely panic the
> host.

Thinking about this even more... Even if we deallocate in the
userspace VMM's shared FD, the kernel has its own page tables --
right? So maybe we are actually 100% OK under UPM then regardless of
the userspace VMM's policy around managing the private and shared FDs.