Re: [PATCH v8 1/8] mm/memfd: Introduce userspace inaccessible memfd

[Fuad Tabba <tabba@google.com>]

Hi,

On Tue, Oct 18, 2022 at 1:34 AM Sean Christopherson <seanjc@google.com> wrote:
>
> On Fri, Sep 30, 2022, Fuad Tabba wrote:
> > > > > > pKVM would also need a way to make an fd accessible again
> > > > > > when shared back, which I think isn't possible with this patch.
> > > > >
> > > > > But does pKVM really want to mmap/munmap a new region at the page-level,
> > > > > that can cause VMA fragmentation if the conversion is frequent as I see.
> > > > > Even with a KVM ioctl for mapping as mentioned below, I think there will
> > > > > be the same issue.
> > > >
> > > > pKVM doesn't really need to unmap the memory. What is really important
> > > > is that the memory is not GUP'able.
> > >
> > > Well, not entirely unguppable, just unguppable without a magic FOLL_* flag,
> > > otherwise KVM wouldn't be able to get the PFN to map into guest memory.
> > >
> > > The problem is that gup() and "mapped" are tied together.  So yes, pKVM doesn't
> > > strictly need to unmap memory _in the untrusted host_, but since mapped==guppable,
> > > the end result is the same.
> > >
> > > Emphasis above because pKVM still needs unmap the memory _somehwere_.  IIUC, the
> > > current approach is to do that only in the stage-2 page tables, i.e. only in the
> > > context of the hypervisor.  Which is also the source of the gup() problems; the
> > > untrusted kernel is blissfully unaware that the memory is inaccessible.
> > >
> > > Any approach that moves some of that information into the untrusted kernel so that
> > > the kernel can protect itself will incur fragmentation in the VMAs.  Well, unless
> > > all of guest memory becomes unguppable, but that's likely not a viable option.
> >
> > Actually, for pKVM, there is no need for the guest memory to be GUP'able at
> > all if we use the new inaccessible_get_pfn().
>
> Ya, I was referring to pKVM without UPM / inaccessible memory.
>
> Jumping back to blocking gup(), what about using the same tricks as secretmem to
> block gup()?  E.g. compare vm_ops to block regular gup() and a_ops to block fast
> gup() on struct page?  With a Kconfig that's selected by pKVM (which would also
> need its own Kconfig), e.g. CONFIG_INACCESSIBLE_MAPPABLE_MEM, there would be zero
> performance overhead for non-pKVM kernels, i.e. hooking gup() shouldn't be
> controversial.
>
> I suspect the fast gup() path could even be optimized to avoid the page_mapping()
> lookup by adding a PG_inaccessible flag that's defined iff the TBD Kconfig is
> selected.  I'm guessing pKVM isn't expected to be deployed on massivve NUMA systems
> anytime soon, so there should be plenty of page flags to go around.
>
> Blocking gup() instead of trying to play refcount games when converting back to
> private would eliminate the need to put heavy restrictions on mapping, as the goal
> of those were purely to simplify the KVM implementation, e.g. the "one mapping per
> memslot" thing would go away entirely.

My implementation of mmap for inaccessible_fops was setting VM_PFNMAP.
That said, I realized that that might be adding an unnecessary
restriction, and now have changed it to do it the secretmem way.
That's straightforward and works well.

> > This of course goes back to what I'd mentioned before in v7; it seems that
> > representing the memslot memory as a file descriptor should be orthogonal to
> > whether the memory is shared or private, rather than a private_fd for private
> > memory and the userspace_addr for shared memory.
>
> I also explored the idea of backing any guest memory with an fd, but came to
> the conclusion that private memory needs a separate handle[1], at least on x86.
>
> For SNP and TDX, even though the GPA is the same (ignoring the fact that SNP and
> TDX steal GPA bits to differentiate private vs. shared), the two types need to be
> treated as separate mappings[2].  Post-boot, converting is lossy in both directions,
> so even conceptually they are two disctint pages that just happen to share (some)
> GPA bits.
>
> To allow conversions, i.e. changing which mapping to use, without memslot updates,
> KVM needs to let userspace provide both mappings in a single memslot.  So while
> fd-based memory is an orthogonal concept, e.g. we could add fd-based shared memory,
> KVM would still need a dedicated private handle.
>
> For pKVM, the fd doesn't strictly need to be mutually exclusive with the existing
> userspace_addr, but since the private_fd is going to be added for x86, I think it
> makes sense to use that instead of adding generic fd-based memory for pKVM's use
> case (which is arguably still "private" memory but with special semantics).
>
> [1] https://lore.kernel.org/all/YulTH7bL4MwT5v5K@google.com
> [2] https://lore.kernel.org/all/869622df-5bf6-0fbb-cac4-34c6ae7df119@kernel.org

As long as the API does not impose this limit, which would imply pKVM
is misusing it, then I agree. I think that's why renaming it to
something like "restricted" might be clearer.

Thanks,
/fuad