Re: [Xen-devel] Xen ARM - Exposing a PL011 to the guest

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 17 Jan 2017, Julien Grall wrote:
> Hi,
> 
> Sorry for the late answer, I am just back from holidays and still catching-up
> with my e-mails.
> 
> On 03/01/17 20:08, Stefano Stabellini wrote:
> > On Thu, 29 Dec 2016, Bhupinder Thakur wrote:
> > > On 28 December 2016 at 23:19, Julien Grall <julien.grall@xxxxxxx> wrote:
> > > > On 21/12/16 22:12, Stefano Stabellini wrote:
> > > > > 
> > > > > On Wed, 21 Dec 2016, Julien Grall wrote:
> > > > > > 
> > > > > > On 20/12/2016 20:53, Stefano Stabellini wrote:
> > > > > > > 
> > > > > > > On Tue, 20 Dec 2016, Julien Grall wrote:
> > > > > > > > 
> > > > > > > > On 19/12/2016 21:24, Stefano Stabellini wrote:
> > > > > > > > > 
> > > > > > > > > On Mon, 19 Dec 2016, Christoffer Dall wrote:
> > > > > > > > > > 
> > > > > > > > > > On Fri, Dec 16, 2016 at 05:03:13PM +0000, Julien Grall
> > > > > > > > > > wrote:
> > > > > > > > > 
> > > > > > > > > If we use hvm_params for this, we need two new hvm_params and
> > > > > > > > > Xen
> > > > > > > > > needs
> > > > > > > > > to unmap the pfn from the guest immediately, because we don't
> > > > > > > > > want the
> > > > > > > > > guest to have access to it.
> > > > > > > > 
> > > > > > > > 
> > > > > > > > If you unmap the pfn, the PV backend will not be able to request
> > > > > > > > the
> > > > > > > > page
> > > > > > > > because there will be no translation available.
> > > > > > > > 
> > > > > > > > So what you want to do is preventing the guest to at least write
> > > > > > > > into
> > > > > > > > region
> > > > > > > > (not sure if it is worth to restrict read)
> > > > > > > 
> > > > > > > 
> > > > > > > That's a good idea.
> > > > > > > 
> > > > > > > 
> > > > > > > > and unmap the page via the hypercall
> > > > > > > > XENMEM_decrease_reservation.
> > > > > > > 
> > > > > > > 
> > > > > > > That would be issued by the guest itself, right? To save address
> > > > > > > space?
> > > > > > 
> > > > > > 
> > > > > > Correct. The main use case today is ballooning, but guest could call
> > > > > > it
> > > > > > on any
> > > > > > other RAM baked page.
> > > > > > 
> > > > > > I was thinking about more about the protection needed. Technically
> > > > > > the
> > > > > > data in
> > > > > > the ring are not trusted. So if the guest is messing up with it, it
> > > > > > would
> > > > > > not
> > > > > > be a big issue. Or did I miss anything here?
> > > > > 
> > > > > 
> > > > > I understand that a guest would be smart to call
> > > > > XENMEM_decrease_reservation on the PV console page for pl011, but it
> > > > > cannot be a security measure, because, in fact, it needs to be called
> > > > > by
> > > > > the guest.  Of course, a malicious guest can simply not call
> > > > > XENMEM_decrease_reservation for it.
> > > > 
> > > > 
> > > > Sorry I was not clear. I was not suggested the guest to call
> > > > XENMEM_decrease_reservation on ring for security but a malicious guest
> > > > issuing the hypercall on the ring protected and replacing by another
> > > > page.
> > > > 
> > > > This is the exact same problem as the one I mentioned on the ITS thread.
> > > > The
> > > > page live in guest memory but contains data that will only be touched by
> > > > Xen.
> > > > 
> > > > If you remove those page from stage-2, the translation IPA -> MFN will
> > > > be
> > > > lost unless you store somewhere else. You would have to do it per-page
> > > > as
> > > > the buffer will use contiguous IPA but potentially noncontiguous MFN.
> > > > 
> > > > In the case of ITS the memory is provisioned by the guest. So there are
> > > > not
> > > > much to do there except adding protection in stage-2 such as write
> > > > protection and preventing the guest to unmap it. However for the pl011
> > > > ring,
> > > > as Andrew pointed on IRC, what we need to do is accounting this page to
> > > > the
> > > > domain memory. No mapping is necessary in stage-2.
> > > 
> > > Please clarify what is meant by that no stage-2 mapping is required.
> > > Does it mean that no stage-2 mapping is required for the guest as it
> > > never needs to access this page?
> > 
> > That's right.
> > 
> > 
> > > However, the Xen HYP will need the stage-2 mapping to find out the
> > > pl011 PFN --> physical MFN mapping so that it can map the page to its
> > > own address space. Currently, I am using prepare_ring_for_helper () to
> > > map the pl011 PFN (passed via hvm call) ---> phyiscal MFN ---> Xen HYP
> > > VA.
> > 
> > I am not sure what Julien had in mind exactly. I like the idea of not
> > mapping the page at stage-2, but it is true that many interfaces expect
> > pfns. If Xen is the one to allocate the pl011 PV console page, then Xen
> > knows the mfn and could use it to map the page, instead of the pfn.
> > However, the PV console backend also needs to map the same page, and it
> > currently does that by calling xc_map_foreign_range, which I believe
> > also expect a pfn.
> 
> Do you agree that page such as ioreq and the pl011 PV console are only used
> for communication between Xen and a backend? 

I agree


> We don't want the guest to access
> the content of the pages but still be able to map. I would guess that using
> the gfn was a convenience but has some security impact (see  guess using the
> gfn is mostly for convenience but has some security impact today (see XSA-197
> and XSA-199). AFAIU, we also want to account those pages to a domain.

Right


> For the credit, this is a suggestion made by Andrew on IRC. If I remember
> correctly what he suggested, it would be to have an add-to-physmap version
> restricted for non-domU components (e.g backend, toolstack) which would allow
> to map a given page (maybe via an index?).
>
> > 
> > So maybe it is easier to use write-protection in stage-2 (as for ITS),
> > unless Julien has a better idea?
> > 
> 
> We need write-protection and also prevent the page to be removed.
> 
> But to be fair, I would not bother to try to write-protect it for now if we
> are going towards keeping the pl011 console in the guest memory. I don't think
> letting the guest writing in the console page is a big problem. The backend
> console should already be protected against malicious guest. If not, we are
> already in trouble as it part of the PV protocol :).

True


> Regarding Xen, the console protocol is very easy and I think only bound
> checking is sufficient. So what a guest could do is hijacking its own console.
> Any thoughts?

I think you are correct, it is no different than providing a PV console
to a guest, which we already do.

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