Re: [Xen-devel] Xenstore domains and XS_RESTRICT

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 18 Jan 2017, Juergen Gross wrote:
> On 18/01/17 12:03, Wei Liu wrote:
> > On Mon, Jan 16, 2017 at 05:47:15PM +0100, Juergen Gross wrote:
> >> On 07/12/16 08:44, Juergen Gross wrote:
> >>> Hi,
> >>>
> >>> today the XS_RESTRICT wire command of Xenstore is supported by
> >>> oxenstored only to drop the privilege of a connection to that of the
> >>> domid given as a parameter to the command.
> >>>
> >>> Using this mechanism with Xenstore running in a stubdom will lead to
> >>> problems as instead of only a dom0 process dropping its privileges
> >>> the privileges of dom0 will be dropped (all dom0 Xenstore requests
> >>> share the same connection).
> >>>
> >>> In order to solve the problem I suggest the following change to the
> >>> Xenstore wire protocol:
> >>>
> >>>  struct xsd_sockmsg
> >>>  {
> >>> -    uint32_t type;  /* XS_??? */
> >>> +    uint16_t type;  /* XS_??? */
> >>> +    uint16_t domid; /* Use privileges of this domain */
> >>>      uint32_t req_id;/* Request identifier, echoed in daemon's response.  
> >>> */
> >>>      uint32_t tx_id; /* Transaction id (0 if not related to a
> >>> transaction). */
> >>>      uint32_t len;   /* Length of data following this. */
> >>>
> >>>      /* Generally followed by nul-terminated string(s). */
> >>>  };
> >>>
> >>> domid will normally be zero having the same effect as today.
> >>>
> >>> Using XS_RESTRICT via a socket connection will run as today by dropping
> >>> the privileges of that connection.
> >>>
> >>> Using XS_RESTRICT via the kernel (Xenstore domain case) will save the
> >>> domid given as parameter in the connection specific private kernel
> >>> structure. All future Xenstore commands of the connection will have
> >>> this domid set in xsd_sockmsg. The kernel will never forward the
> >>> XS_RESTRICT command to Xenstore.
> >>>
> >>> A domid other than 0 in xsd_sockmsg will be handled by Xenstore to use
> >>> the privileges of that domain. Specifying a domid in xsd_sockmsg is
> >>> allowed for privileged domain only, of course. XS_RESTRICT via a
> >>> non-socket connection will be rejected in all cases.
> >>>
> >>> The needed modifications for Xenstore and the kernel are rather small.
> >>> As there is currently no Xenstore domain available supporting
> >>> XS_RESTRICT there are no compatibility issues to expect.
> >>>
> >>> Thoughts?
> >>
> >> As I don't get any further constructive responses even after asking for
> >> them: would patches removing all XS_RESTRICT support be accepted?
> >>
> > 
> > We don't need to actually remove it, do we? If XS_RESTRICT is not supported 
> > by
> > xenstored, the client would get meaningful error code. A patch to
> > deprecate that command should be good enough, right?
> 
> Uuh, no.
> 
> oxenstored does support XS_RESTRICT. The longer it stays the better the
> chances someone is using it.
> 
> > And sorry for the late reply, I'm still mulling over your proposal, I
> > will try to respond as soon as possible.
> 
> I thought a little bit further: the idea of XS_RESTRICT is to avoid qemu
> being capable to overwrite any Xenstore entries of other domains
> including dom0.
> 
> I fail to see how this should work with qemu-based backends (qdisk,
> pvusb), as those rely on paths in Xenstore writable by dom0 only.

It does not work. However, QEMU based backends can be run on a separate
QEMU. Patches were submitted by IanJ and me to run 2 QEMUs per domain,
one to provide emulation, the other to provide the backends. Not sure
what happen to them, but they were more then prototypes.


> We already have a mechanism to de-privilege the device model of a HVM
> domain without hurting the backends: ioemu-stubdom. So I believe we
> should try to make qmeu upstream usable in stubdom instead of
> introducing mechanisms limited in usability ("if you want a secure
> device model you can't use features x, y and z.").

Yes, but ioemu-stubdoms have drawbacks that make them not viable in many
scenarios. There are reasons why they are not enabled by default.
XS_RESTRICT should not replace, but complement ioemu-stubdoms. If we
remove XS_RESTRICT, what's the plan to make QEMU in Dom0 secure by
default?

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