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 [thread overview]
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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next prev parent reply other threads:[~2020-01-16 18:36 UTC|newest]
Thread overview: 24+ 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
2020-01-20 8:18 ` Durrant, Paul
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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