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From: Ian Jackson <ian.jackson@citrix.com>
To: Paul Durrant <pdurrant@amazon.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>,
	Julien Grall <julien@xen.org>, Wei Liu <wl@xen.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	Andrew Cooper <Andrew.Cooper3@citrix.com>,
	"jandryuk@gmail.com" <jandryuk@gmail.com>,
	George Dunlap <George.Dunlap@citrix.com>,
	Jan Beulich <jbeulich@suse.com>,
	Anthony Perard <anthony.perard@citrix.com>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Subject: Re: [Xen-devel] [PATCH v3 4/6] libxl: allow creation of domains with a specified or random domid
Date: Thu, 16 Jan 2020 18:36:07 +0000
Message-ID: <24096.44311.441643.264877@mariner.uk.xensource.com> (raw)
In-Reply-To: <20200116093602.4203-5-pdurrant@amazon.com>

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.

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...

Thanks,
Ian.


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  parent reply index

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-16  9:35 [Xen-devel] [PATCH v3 0/6] xl/libxl: domid allocation/preservation changes Paul Durrant
2020-01-16  9:35 ` [Xen-devel] [PATCH v3 1/6] libxl: add definition of INVALID_DOMID to the API Paul Durrant
2020-01-16  9:35 ` [Xen-devel] [PATCH v3 2/6] libxl_create: make 'soft reset' explicit Paul Durrant
2020-01-16  9:35 ` [Xen-devel] [PATCH v3 3/6] libxl: add infrastructure to track and query 'retired' domids Paul Durrant
2020-01-16 18:27   ` Ian Jackson
2020-01-17  9:26     ` Durrant, Paul
2020-01-17 11:31       ` Ian Jackson
2020-01-16  9:36 ` [Xen-devel] [PATCH v3 4/6] libxl: allow creation of domains with a specified or random domid Paul Durrant
2020-01-16  9:40   ` Jan Beulich
2020-01-16  9:46     ` Durrant, Paul
2020-01-16  9:59       ` Jan Beulich
2020-01-16 15:53   ` Jason Andryuk
2020-01-16 18:36   ` Ian Jackson [this message]
2020-01-17  9:37     ` Durrant, Paul
2020-01-17 11:35       ` Ian Jackson
2020-01-17 12:06         ` Durrant, Paul
2020-01-17 15:30           ` Ian Jackson
2020-01-16  9:36 ` [Xen-devel] [PATCH v3 5/6] xl.conf: introduce 'domid_policy' Paul Durrant
2020-01-16 18:37   ` Ian Jackson
2020-01-16  9:36 ` [Xen-devel] [PATCH v3 6/6] xl: allow domid to be preserved on save/restore or migrate Paul Durrant
2020-01-16 18:39   ` Ian Jackson
2020-01-16 18:43 ` [Xen-devel] [PATCH v3 0/6] xl/libxl: domid allocation/preservation changes Ian Jackson
2020-01-17  9:11   ` Durrant, Paul

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