Re: [Qemu-devel] [Qemu-ppc] [PATCH v2] spapr-rtas: add ibm, get-vpd RTAS interface

[David Gibson]

On Tue, Feb 26, 2019 at 04:11:40PM -0300, Murilo Opsfelder Araujo wrote:
> On Tue, Feb 26, 2019 at 02:08:30PM -0300, Murilo Opsfelder Araujo wrote:
> > Hi, Maxiwell.
> >
> > On Tue, Feb 26, 2019 at 11:21:26AM -0300, Maxiwell S. Garcia wrote:
> > > On Tue, Feb 26, 2019 at 02:21:03PM +1100, David Gibson wrote:
> > > > On Mon, Feb 25, 2019 at 08:20:09PM -0300, Murilo Opsfelder Araujo wrote:
> > > > > Hi, Maxiwell.
> > > > >
> > > > > On Mon, Feb 25, 2019 at 01:23:25PM -0300, Maxiwell S. Garcia wrote:
> > > > > > This adds a handler for ibm,get-vpd RTAS calls, allowing pseries
> > > > > > guest to collect host information. It is disabled by default to
> > > > > > avoid unwanted information leakage. To enable it, use:
> > > > > > ‘-M pseries,vpd-export=on’
> > > > >
> > > > > The patch for setting host-serial and host-model already landed 
> > > > > Gibson's
> > > > > ppc-for-4.0 branch:
> > > > >
> > > > >   commit 9e584f45868f6945c1282c938278038cba0e4af2
> > > > >   Author: Prasad J Pandit <address@hidden>
> > > > >   Date:   Mon Feb 18 23:43:49 2019 +0530
> > > > >
> > > > >       ppc: add host-serial and host-model machine attributes 
> > > > > (CVE-2019-8934)
> > > > >
> > > > >
> > > > > QEMU should only return host-serial and host-model from the host if 
> > > > > the
> > > > > following combination of parameters are provided:
> > > > >
> > > > >   -M host-serial=passthrough,host-model=passthrough,vpd-export=on
> > > > >
> > > > > If host-serial or host-model are set with a user-string, ibm,get-vpd 
> > > > > should
> > > > > honor these values and return them, not exposing host information by 
> > > > > accident.
> > > > >
> > > > > I'm not even sure if we need vpd-export=<bool> setting. Its logic 
> > > > > could be
> > > > > derived from the presence of host-serial=passthrough and 
> > > > > host-model=passthrough
> > > > > options.
> > > > >
> > > > > What do you think?
> > > >
> > > > That's an excellent point - I hadn't thought through the fact that
> > > > this is the same information exposed by those properties.  I do indeed
> > > > think that exposing the same information set in those properties - and
> > > > thereby avoiding the new machine option - would be a better plan.
> > > >
> > >
> > > I saw that the patch was applied. So I will work in another patch
> > > to use these properties and remove the export-vpd option.
> > >
> > > Another thing that I thought the fact that 'host-serial' and 'host-model'
> > > nodes in Device Tree are not in accordance with LoPAPR document. What
> > > you think in use only get-vpd to get these information and remove
> > > nodes from device tree?
> >
> > Both "system-id" and "model" properties are described in the section 
> > "3.3.2.1
> > Root Node Properties" of the "Device Tree Bindings: Linux on Power 
> > Architecture
> > Reference" document:
> >
> >   https://members.openpowerfoundation.org/wg/SYSSW/document/1455
> 
> I replied too early. As Maxiwell explained to me (thanks Max!), guest can end 
> up
> having the following entries under /proc/device-tree/ (among other entries):
> 
>   $ cat host-serial
>   12A3B4C
> 
>   $ cat host-model
>   1234-56A
> 
>   $ cat system-id
>   c7b62da9-3d0c-44f9-8edb-2318271f3c1a
> 
>   $ cat model
>   IBM pSeries (emulated by qemu)
> 
> Where:
> 
>   - host-serial/host-model: depend on "-M host-serial" and "-M host-model"
>     machine options.
> 
>   - system-id: created when "-uuid <val>" exists in qemu command line options.
> 
>   - model: hard-coded.
> 
> With this patch, RTAS ibm,get-vpd call will return "system-id" and "model" 
> from
> the host, not from the guest. I found this confusing.

Hrm.  I found the bit in LoPAPR describing how ibm,get-vpd operates,
but not something describing the actual data that appears within it.
Where's that described?  It'd be nice to check if those values are
supposed to be describing the host or the guest.


> Perhaps we can trick ibm,get-vpd call to return "host-serial" and "host-model"
> values from the guest, which will hold safe values after commit
> 9e584f45868f6945c1282c938278038cba0e4af2 "ppc: add host-serial and host-model
> machine attributes (CVE-2019-8934)".

Well, if it is supposed to be host values, then yes, absolutely, we
should be using the "host" values that the user supplies (if any).

> What do you think?
> 
> As to removing "host-serial" and "host-model", I am not entirely sure for what
> they are used. There should be a reason for them to exist.
> 

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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