Re: [PATCH v3 2/5] xen: make evtchn_alloc_unbound public

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 15 Mar 2022, Daniel P. Smith wrote:
> On 1/28/22 16:33, Stefano Stabellini wrote:
> > From: Luca Miccio <lucmiccio@xxxxxxxxx>
> > 
> > The xenstore event channel will be allocated for dom0less domains. It is
> > necessary to have access to the evtchn_alloc_unbound function to do
> > that, so make evtchn_alloc_unbound public.
> > 
> > Add a skip_xsm parameter to allow disabling the XSM check in
> > evtchn_alloc_unbound (xsm_evtchn_unbound wouldn't work for a call
> > originated from Xen before running any domains.)
> > 
> > Signed-off-by: Luca Miccio <lucmiccio@xxxxxxxxx>
> > Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>
> > CC: Julien Grall <julien@xxxxxxx>
> > CC: Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
> > CC: Bertrand Marquis <bertrand.marquis@xxxxxxx>
> > CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > CC: George Dunlap <george.dunlap@xxxxxxxxxx>
> > CC: Jan Beulich <jbeulich@xxxxxxxx>
> > CC: Wei Liu <wl@xxxxxxx>
> > ---
> > Changes v3:
> > - expose evtchn_alloc_unbound, assing a skip_xsm parameter
> > ---
> >  xen/common/event_channel.c | 13 ++++++++-----
> >  xen/include/xen/event.h    |  3 +++
> >  2 files changed, 11 insertions(+), 5 deletions(-)
> > 
> > diff --git a/xen/common/event_channel.c b/xen/common/event_channel.c
> > index da88ad141a..be57d00a15 100644
> > --- a/xen/common/event_channel.c
> > +++ b/xen/common/event_channel.c
> > @@ -284,7 +284,7 @@ void evtchn_free(struct domain *d, struct evtchn *chn)
> >      xsm_evtchn_close_post(chn);
> >  }
> >  
> > -static int evtchn_alloc_unbound(evtchn_alloc_unbound_t *alloc)
> > +int evtchn_alloc_unbound(evtchn_alloc_unbound_t *alloc, bool skip_xsm)
> >  {
> >      struct evtchn *chn;
> >      struct domain *d;
> > @@ -301,9 +301,12 @@ static int evtchn_alloc_unbound(evtchn_alloc_unbound_t 
> > *alloc)
> >          ERROR_EXIT_DOM(port, d);
> >      chn = evtchn_from_port(d, port);
> >  
> > -    rc = xsm_evtchn_unbound(XSM_TARGET, d, chn, alloc->remote_dom);
> > -    if ( rc )
> > -        goto out;
> > +    if ( !skip_xsm )
> > +    {
> > +        rc = xsm_evtchn_unbound(XSM_TARGET, d, chn, alloc->remote_dom);
> > +        if ( rc )
> > +            goto out;
> > +    }
> 
> Please do not subvert the security framework because it causes an
> inconvenience. As Jan recommended, work within the XSM check to allow
> your access so that we may ensure it is done safely. If you need any
> help making modifications to XSM, please do not hesitate to reach out as
> I will gladly help.

Thank you!

First let me reply to Jan: this series is only introducing 1 more call
to evtchn_alloc_unbound, which is to allocate the special xenstore event
channel, the one configured via
d->arch.hvm.params[HVM_PARAM_STORE_EVTCHN].

It is not meant to be a generic function, and it is not meant to be
called more than once. It could (should?) be __init.

The existing XSM check in evtchn_alloc_unbound cannot work and should
not work: it is based on the current domain having enough privileges to
create the event channel. In this case, we have no current domain at
all. The current domain is Xen itself.

For these reasons, given [1], also not to subvert the security
framework as Daniel pointed out, I think I should go back to my own
implementation [2][3] based on get_free_port. That is my preference
because:

- the Xen codebase doesn't gain much by reusing evtchn_alloc_unbound
- adding skip_xsm introduces a component of risk (unless we make it
  __init maybe?)
- using get_free_port is trivial and doesn't pose the same issues


Let's find all an agreement on how to move forward on this.


[1] https://marc.info/?l=xen-devel&m=164194128922838
[2] https://marc.info/?l=xen-devel&m=164203543615114
[3] https://marc.info/?l=xen-devel&m=164203544615129