Re: [Xen-devel] [PATCH v3 4/6] libxl: allow creation of domains with a specified or random domid

["Durrant, Paul" <pdurrant@amazon.co.uk>]

> -----Original Message-----
> From: Ian Jackson <ian.jackson@citrix.com>
> Sent: 16 January 2020 19:36
> To: Durrant, Paul <pdurrant@amazon.co.uk>
> Cc: xen-devel@lists.xenproject.org; Wei Liu <wl@xen.org>; Anthony Perard
> <anthony.perard@citrix.com>; Andrew Cooper <Andrew.Cooper3@citrix.com>;
> George Dunlap <George.Dunlap@citrix.com>; Jan Beulich <jbeulich@suse.com>;
> Julien Grall <julien@xen.org>; Konrad Rzeszutek Wilk
> <konrad.wilk@oracle.com>; Stefano Stabellini <sstabellini@kernel.org>;
> jandryuk@gmail.com
> Subject: Re: [PATCH v3 4/6] libxl: allow creation of domains with a
> specified or random domid
> 
> Hi.  This broadly contains what I expected, but:
> 
> Paul Durrant writes ("[PATCH v3 4/6] libxl: allow creation of domains with
> a specified or random domid"):
> 
> > +        for (;;) {
> > +            if (info->domid == RANDOM_DOMID) {
> > +                uint16_t v;
> > +
> > +                /* Randomize lower order bytes */
> > +                ret = libxl__random_bytes(gc, (void *)&v, sizeof(v));
> > +                if (ret < 0)
> > +                    break;
> > +
> > +                v &= DOMID_MASK;
> > +                if (!libxl_domid_valid_guest(v) ||
> > +                    libxl__is_retired_domid(gc, v))
> > +                    continue;
> > +
> > +                *domid = v;
> > +            }
> > +
> > +            ret = xc_domain_create(ctx->xch, domid, &create);
> > +            if (ret == 0 || errno != EEXIST || info->domid !=
> RANDOM_DOMID)
> > +                break;
> > +        }
> 
> I think this has a race.
> 
>   Thread A, in domain destroy           Thread B, in code above
> 
>                                          choose domid V
>                                          check V in recent domid list
> 
>      add V to recent domid list
>      destroy domain V in Xen
> 
>                                          create domain V in Xen
>                                          continue constructing V
> 
> Thread B improperly constructs a new guest using V, exposing anyone
> who was talking about V a moment ago to bugs.  Some code might even
> fail to spot the interval where V does not exist and carry on talking
> to the new V as if it were the old one...
> 
> I think there are only two possible solutions:
> 
>   - Check the domain's entry in the recent list *after* creating
>     the domain in Xen.  This involves accepting that we will
>     reuse the domid but only for a domain we are in the early
>     stages of constructing, so hopefully without bad consequence?
> 
>   - Take the recent domid lock.
> 

Or take a global file lock in libxl around domain creation and destruction?

> Also, it seems to me that we should check the recent domid list if we
> let Xen choose the domid.  Maybe that can be in a subsequent patch...
> 

Well, we could solve all this, remove the need for a file and all the associated complexity by simply keeping history inside the hypervisor. I don't know how the Xen maintainers will feel about that though, as Xen itself shouldn't have a problem with eager domid re-use.

  Paul

> Thanks,
> Ian.


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