Re: [Xen-devel] Xenstore domains and XS_RESTRICT

[Wei Liu <wei.liu2@xxxxxxxxxx>]

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?

And sorry for the late reply, I'm still mulling over your proposal, I
will try to respond as soon as possible.

Wei.

> 
> Juergen
> 

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