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

[Bhupinder Thakur <bhupinder.thakur@xxxxxxxxxx>]

Hi,

I have done the changes for emulating pl011 in Xen. Currently, I have
verified the emulation code by manually reading/writing data to
/dev/ttyAMA0 which is the device file for pl011 device. The data is
flowing fine between xenconsoled and the guest domain.

As a next step, I wanted to use /dev/ttyAMA0 as a console.

For that I tried adding console=ttyAMA0 instead of console=hvc0 in the
"extra" directive in the domU configuration file. However, I do not
see the output on the console once I attached the console using "xl
console <domain_name>". I tried using "xl console -t serial
<domain_name>" also but that shows the tty1 console and not the
ttyAMA0 one.

Note that currently for testing, I have patched the code in the
xenconsoled to read/write pl011 ring buffers and write them to the
same pty terminal as used by hvc0. Finally, once I have verified, I
will create a separate pty for pl011 and add a new console type
"pl011".

Regards,
Bhupinder


On 18 January 2017 at 00:57, Stefano Stabellini <sstabellini@xxxxxxxxxx> wrote:
> 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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